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UNDERWATER VIBROTACTILE FREQUENCY DETECTION IN HUMAN HAIRY AND GLABROUS SKIN BY JORDAN MARTIN A Thesis Submitted to the Division of Psychology New College of Florida In partial fulfillment of the requirements for the degree Bachelor of Arts Un der the sponsorship of Dr. Gordon Bauer Sarasota, Florida May, 2011
UNDERWATER FREQUENCY DETECTION i i Acknowledgements Thank you to Dr. Gordon Bauer, my thesis sponsor, for the direction and support that helped make this thesis possible. Thank you also to Dr. Michelle Barton and Dr. He idi Harley, whose work as my committee members and professors helped to sharpen my research and presentation skills during my time at New College. Also, to Joe Gaspard and the interns of Mote Marine Laboratory who assisted with and participated in this stu dy, providing patience and humor throughout test sessions. Finally, I'd like to thank my family and friends for providing the constant mental and emotional support that matched the tremendous academic support provided by my professors. This manuscript was also supported by the National Science Foundation Grant IOS 090022, and the New College Council of Academic Affairs Research Grant.
UNDERWATER FREQUENCY DETECTION iii Table of Contents ACKNOWLEDGEMENTS ii TABLE OF CONTENTS iii LIST OF TABLES iv LIST OF FIGURES v ABSTRACT vii INTRODUCTION 1 Part One: Biology of the skin 1 Part Two: Testing sensitivity in the hand 6 MAIN STUDY 14 METHOD 15 RESULTS 19 DISCUSSION 22 REFERENCES 31 TABLES 35 FIGURES 39
UNDERWATER FREQUENCY DETECTION iv List of Tables TABLE 1: Number of Warm Up Trials 35 TABLE 2: Number of Test Trials 36 TABLE 3: False Alarm Rates 37 TABLE 4: Acceleration, Velocity, and Displacement Thresholds 38
UNDERWATER FREQUENCY DETECTION v List of Figures FIGURE 1: Layers of the Skin 39 FIGURE 2: Dipole Shaker 40 FIGURE 3: Participant Seating Arrangement 41 FIGURE 4: Participant View of Lasers 42 FIGURE 5: Participant View of LED Trial Indicator 43 FIGURE 6: Lasers Flanking Dipole Shaker 44 FIGURE 7: Dorsal Hand Alignment with Lasers 45 FIGURE 8: Volar Palm Alignment with Lasers 46 FIGURE 9: Mean Acceleration Thresholds for Glabrous Skin by Method 47 FIGURE 10: Mean Velocity Thresholds for Glabrous Skin by Method 48 FIGURE 11: Mean Displacement Thresholds for Glabrous Skin by Method 49 FIGURE 12:Mean Acceleration Thresholds for Hairy Skin by Method 50 FIGURE 13: Mean Velocity Thresholds for Hairy Sk in by Method 51 FIGURE 14: Mean Displacement Thresholds for Hairy Skin by Method 52 FIGURE 15: Mean Acceleration Thresholds by Skin Type 53 FIGURE 16: Mean Velocity Thresholds by Skin Type 54 FIGURE 17: Mean Displacement Thresholds by Skin Type 55 FIGURE 1 8: Mean Acceleration Thresholds for Hairy Skin by Phase 56 FIGURE 19: Mean Velocity Thresholds for Hairy Skin by Phase 57 FIGURE 20: Mean Displacement Thresholds for Hairy Skin by Phase 58 FIGURE 21: Individual Acceleration Thresholds by Phase for Particip ant 3 59 FIGURE 22: Individual Velocity Thresholds by Phase for Participant 3 60 FIGURE 23: Individual Displacement Thresholds by Phase for Participant 3 61
UNDERWATER FREQUENCY DETECTION vi FIGURE 24: Displacement Thresholds of Mahns et al. (2006) Compared to the Present Study 62
UNDERWATER FREQUENCY DETECTION vii UNDER WATER VIBROTACTILE FREQUENCY DETECTION IN HUMAN HAIRY AND GLABROUS SKIN Jordan Martin New College of Florida, 2011 Abstract The sensitivity of human vibrotactile frequency detection was compared for the hairy and glabrous skin of the hand by means of a st aircase method psychophysical procedure with five subjects. Sinusoidal vibratory stimuli were presented in an underwater setting at five standard frequencies, 10, 25, 50, 100, and 150 Hz. In contrast to previous studies with mechanical contact stimuli, vib rations underwater were used to stimulate receptors sensitive to indentation of the skin and those sensitive to movement of the hairs. Results indicated that with such a stimulus, detection thresholds do not vary between hairy and glabrous skin. The hair o n the dorsal hand was then depilated to determine the contribution of hair follicle afferent fibers to vibration detection. Pre depilation skin was not significantly more sensitive than post depilation skin on measures of velocity or displacement. Pre and post depilation acceleration thresholds showed a reliable but not significant difference. In a follow up measure for one participant 3 weeks post depilation, sensitivity to acceleration of the depilated skin recovered and surpassed that of pre depilated s kin. When compared to reports of in air contact vibrator studies, the results of this study indicated that hairy skin is as sensitive to vibration as the fingertip. These findings with an underwater stimulus provide the first example of a tactile stimulus that is equally effective in stimulating receptors in glabrous and hairy skin. The results
UNDERWATER FREQUENCY DETECTION viii suggest that research on vibrotactile sensitivity in human skin would benefit from further consideration of the mechanical characteristics of stimulus presentations highlighting the importance of hair movement when comparing hairy skin to thresholds of glabrous skin. Implications for vibrotactile studies of marine animals and the development of the skin in utero are discussed. ___________________________ Dr. Gor don Bauer Division of Social Sciences
UNDERWATER FREQUENCY DETECTION 1 Underwater Vibrotactile Frequency Detection in Human Hairy and Glabrous Skin The ability to detect vibrations on human skin has been well documented and cutaneous receptor contributions to vibration sensitivity have b een explored (Gescheider, Gl, Sexton, Karalunas, & Fontana, 2005; Mahns, Perkins, Sahai, Robinson, & Rowe, 2005; Morioka & Griffin, 2005; Morioka, Whitehouse, & Griffin, 2008; Verrillo & Bolanowski, 1986; Verrillo & Bolanowski, 2003; Verrillo, Bolanowsk i, Baran, & Smith, 1996) Much of the physiological research surrounding skin sensitivity has concentrated on detection in the hand with metal or wooden vibrators in contact with the skin. These studies have indicated that sensitivity varies significantly across different skin locations and depends on characteristics of the stimulus ( Mahns et al., 2005; Morioka & Griffin, 2005; Morioka et al., 2008 ). The experiment presented here was conducted to explore how underwater vibration detection in the hand compar es to reports of sensitivity in air. This exploration is intended to prompt a greater understanding of tactile sensitivity in the hand and further consideration for research into the development of human skin. Part One: Biology of the Skin The skin is the first of the human sensory systems to become functional in the embryo ( Wyburn, Pickford, & Hirst, 1964 ). The epithelium and the nervous system derive from the same embryonic tissue known as the ectoderm. Skin, or external epithelium, sends sensory informat ion to the spinal cord where reflex connections are made to muscles, internal organs, and blood vessels (Hendrickson, 2002). Many sensory receptors also surround the joints and supply each muscle. The layers of hairy and glabrous skin, its sensory receptor s and thresholds, and connections to the central nervous
UNDERWATER FREQUENCY DETECTION 2 system from the somatic sensory nerves are explained in this section. Human skin generally functions as a method of temperature regulation, defense against pathogens, evaporation control, water and lipid storage, Vitamin B and D synthesis, pheromone excretion, and chemical absorption (Madison, 2003; Proksch, Brandner, & Jensen, 2008; Stcker, Struk, Altmeyer, Herde, Baumgrtl, & Lbbers, 2002). The general layers of the skin (Figure 1) from most exte rnal inward are the epidermis, dermis, and subcutaneous layers (Cholewiak & Collins, 1991). The epidermis is composed of the corneum and epidermis proper, followed by a layer of nutritive and connective tissue. The epidermis serves as the initial pathogen defense for the body. The corneum is the most surface layer and is made up of dead cell bodies and keratinized cells. The second main layer, the dermis, is the location of cutaneous end organs. Sweat glands, hair (except in glabrous skin), erector pylori ( hair raising muscles), and free nerve endings (unspecialized afferent nerve endings) can also be found in the dermis. Subcutaneous skin, located below the dermis, is made up of superficial fascia and subcutaneous fat. Afferent nerves that extend from the p ostcentral gyrus of the parietal lobe, or somatosensory area, to the subcutaneous skin, supply the cutaneous end organs. Sensory information travels from the receptor end organs through stimulated afferent nerve fibers and to the dorsal root ganglion, a c ollection of nerve fiber cell bodies (Cholewiak & Collins, 1991). Over half a million sensory fibers from the skin enter the spinal cord through the dorsal root ganglion (Montagu, 1986). In the vertebrates, nerve fibers group into bands known as dermatomes representing specific skin areas. Within the spinal cord, fibers split into two systems on the way to the brain. The spinothalamic system, which carries information from the dorsal horn in the spine to the
UNDERWATER FREQUENCY DETECTION 3 ventroposterolateral (VPL) nucleus of the thalam us, consists of small fibers that relay pain and temperature information. The dorsal column medial leminiscal system, which follows a similar path from the dorsal nuclei projecting into the VPL, is comprised of large fibers carrying mechanoreceptor informa tion. These systems transmit information that corresponds to each part of the body to the somatosensory area topographically. There are four categories of information relayed to the cortex: cutaneous, proprioceptive, nociceptive (pain), and visceral. Cutan eous nerves are further divided into thermoreceptors (temperature), chemoreceptors (oxygen and acid base balance), mechanoreceptors (mechanical stimulation), and free nerve endings. The dorsal root ganglion fibers project from the various layers of the ski n and muscle to the spinal cord. From the spinal cord, these ascending axons lose their myelin and split into fine branches that terminate in mechanoreceptor end organs (Fain, 2003). Mechanoreceptors are those primarily responsible for pressure and vibrato ry information received through the physical distortion of the end organ. Pacinian corpuscles (PC), Ruffini corpuscles, Merkel discs, and Meissner corpuscles are four named mechanoreceptors. Mechanoreceptors fall into two categories, fast/rapidly adapting (FA) and slowly adapting (SA) receptors. Meissner corpuscles (FA Type I) are found in the dermal papillae, though rarely appearing in hairy skin, and are sensitive to low frequency vibration and light touch. The encapsulated corpuscle consists of a nerve e nding that sits around horizontal lamellae surrounded by connective tissue. Pacinian corpuscles (FA Type II) are nerve endings encapsulated in a thick oval layered with lamellae that are separated by gelatinous fluid and surrounded by connective tissue. PC receptors are relatively large in the skin (1 mm), are found in the dermal and deep
UNDERWATER FREQUENCY DETECTION 4 subcutaneous layers, and are sensitive to gross pressure change and vibrations. The physiological structures of the rapidly adapting end organs make them most sensitive to vibratory stimuli. Merkel discs (SA Type I) are found in the lower epidermis and upper dermis, are sensitive to low frequency vibration and texture discrimination, and have a sustained response to pressure. Merkel discs are free of a surrounding structur e, unlike the other encapsulated mechanoreceptors. In hairy skin, these tactile discs occur in clusters at the base of skin elevations (Fain, 2003). Ruffini corpuscles (SA Type II) can be found in the dermal and subcutaneous layers and are sensitive to ski n stretch and sustained pressure. In the Ruffini structure, the nerve ending splits into numerous fine processes that meander through the collagen filled corpuscle (Fain, 2003). In addition to these mechanoreceptors, sensitivity in the skin can also be at tributed to the many free nerve endings that appear throughout it. Most forms of these un encapsulated endings are sensitive to cold, warm, and noxious stimulation. However, some respond to very strong mechanical stimulation (Brown & Deffenbacher, 1979). F ree nerve endings, originating from numerous dorsal root ganglion cell axons, also cluster around the neck of the hair follicle, wrapping around in parallel and encircling layers ( Boudreau & Tsuchitani, 1973; Granit, 1955) This network of afferent fibers forms the root hair plexus and is most sensitive to velocity of hair displacement (Brown & Deffenbacher, 1979; Fain, 2003). Because different types of mechanoreceptors can be sensitive to the same types of stimuli, measurement of isolated receptors is impo rtant in understanding the particular characteristics that are unique to each type. Gescheider et al. (2005) isolated the Pacinian corpuscles from other
UNDERWATER FREQUENCY DETECTION 5 mechanoreceptors by using a 300 Hz burst vibration, a frequency above non Pacinian channel activation. They tested the contribution of probability summation and neural integration to spatial summation. Spatial summation is a process in which sensitivity increases as the size of the stimulated surface increases. Mechanoreceptors of the same type are known to vary in individual sensitivities. But as the area of stimulated skin increases, the likelihood that receptors of greater sensitivity are stimulated also increases, an effect known as probability summation. With neural integration, as more receptors are st imulated the stimulus responses summate. When a larger contact surround 1 was used around a probe stimulus, greater sensitivity was observed and the probability of stimulation increased as the size of the contactor increased, indicating probability summatio n. Though local variations in sensitivity affect detection, this can be eliminated if the contact area is sufficiently large. The researchers found that lowest thresholds measured with the large contactor were lower than the lowest measured with the small contactor. As the number of activated highly sensitive receptors increased, the threshold decreased, indicating neural integration. Spatial summation is an important process to consider when investigating sensory thresholds of the skin using different stim ulus conditions. Similarly, because spatial summation depends on a particular number of receptors being present in a given area, the location and density of certain receptors needs to be considered. The distribution of mechanoreceptors appears to differ bo th locally and between different types of skin. The distribution of receptors in the palm of the hand varies with a 1 A metal ring intended to isolate vibratory stimuli to an area of a specified diame ter from the probe stimulator within the ring.
UNDERWATER FREQUENCY DETECTION 6 greater density of FAI, FAII, and SAI receptors increasing from the proximal wrist to the distal fingertip ( Morioka et al., 2008). The derma l layer of hairy skin lacks the receptors associated with FAI and FAII channels, though PC receptors are located in the deep subcutaneous tissue. Root hair plexuses, or hair follicle afferent (HFA) fibers, mediate the rapidly adapting fibers present therei n and might account for sensitivity to low frequency stimulation ( Morioka et al., 2008) The differences in distribution and density of mechanoreceptors is also linked to the difference in sensitivity to mechanical stimulation across different parts of the body ( Mahns et al., 2005; Morioka et al., 2008 ). Part Two: Testing Sensitivity in the Human Hand The human hand accounts for a sizable amount of sensory representation in the cortex (Shoham & Grinvald, 2001). Given the importance of the hand in learning about our tactile world, most research on tactile sensitivity focuses on the skin of the hand. Research of the tactile receptors in the palm is also commonly compared to that of the dorsal side of the hand, since the two skin types (hairy dorsal side and g labrous palm) vary in receptor density and distribution. The sensory characteristics of hairy and glabrous skin types and the methods with which sensitivity is compared between them are discussed in this section. In glabrous skin, vibrotactile frequency de tection of the finger and palm varies depending on contact vibrator conditions. In a series of three experiments, tactile thresholds of frequencies ranging from 8 500 Hz were measured for 12 males (Morioka & Griffin, 2005). In the first experiment, three c ontact conditions (PALM: palm flat on a wooden contact plate; GRIP: right hand grasping a wooden handle; and FINGERTIP: middle fingertip in contact with a 6 mm diameter probe contactor with surround) were
UNDERWATER FREQUENCY DETECTION 7 tested for differences in a 2 interval 2 alternativ e forced choice design. Participants were presented with two periods with the stimulus present in one of the two and were asked to identify which period had the stimulus. Intensity decreased by 2 dB 2 (no reference level provided by Morioka & Griffin) afte r three consecutive correct responses and increased by 2 dB after one incorrect response. Results indicated that thresholds were dependent on the frequency of vibration and contact location. Thresholds in all conditions decreased as frequency increased. Th resholds in the PALM and GRIP conditions did not differ significantly from one another. Thresholds in the FINGERTIP condition were significantly lower than in the PALM and GRIP conditions at frequencies below 31.5 Hz and significantly higher than the other two at frequencies higher than 31.5 Hz. These results indicate greater mediation of the PC channel, which is most sensitive to high frequency stimulation, assuming spatial summation is involved. Experiment 2 examined the effects of varying the contact are a, location, and inclusion of a contact surround, with 8 conditions in total: 1) fingertip with 6 mm contactor with surround, 2) fingertip with 6 mm contactor without surround, 3) fingertip with 6mm contactor without surround using a different vibrator, 4) fingertip with 35 mm contactor without surround, 5) partial finger with 35 mm contactor without surround, 6) whole finger on wooden plate, 7) four whole fingers on wooden plate, and 8) whole hand on wooden plate. When the surround was present at stimulati on of the fingertip, thresholds decreased at 16 Hz and 31.5 Hz, but increased at 125 Hz. Increasing the contact area from the fingertip to the whole hand over the conditions decreased thresholds at frequencies up to 125 Hz. These data demonstrated that spa tial summation 2 dB = Decibel; a logarithmic unit that indicates intensity.
UNDERWATER FREQUENCY DETECTION 8 was present for low and high frequencies over the whole hand; as area of stimulation increased across conditions, thresholds decreased. Experiment 3 tested whether the decrease in thresholds observed in Experiment 2 were due to spatial summ ation activating points of higher sensitivity in the hand by measuring how the sensitivity of the Pacinian and non Pacinian channels vary across different locations. A total of 8 locations across the hand were tested at 16, 31.5, 63, and 125 Hz using the v on Bekesy tracking method and a 6mm diameter probe contactor with surround. The von Bekesy method allows the participant, rather than the experimenter, to control the initiation of increase and decrease in stimulus intensity by requiring them to push or re lease a button at reversal of stimulus detection. Thresholds, defined the average all reversals in a 45 s window, at the distal fingertips, at the distal palm, or at the proximal palm did not differ within each location. The distal fingertips were most sen sitive to frequencies less than 63 Hz. There was no difference in sensitivity of the proximal palm and distal palm locations below 63 Hz. These data suggested that sensitivity to vibration reflected an increased density of FAI fibers at the distal areas. A t 125 Hz, the distal fingertip did not differ from the distal palm, though distal palm thresholds were significantly lower than those at proximal palm locations. Taken together, the results of the three experiments indicate that vibrotactile frequency dete ction in the hand varies with contact conditions of area, location, and the use of a surround. Morioka et al. (2008) further investigated the difference in sensitivity to vibrotactile stimuli across body locations. Vibrotactile thresholds for 16 males were measured at 8, 16, 31.5, 63, 125 and 250 Hz with a 1 mm and 6 mm diameter contact vibrator in four locations: the index finger, forearm, large toe, and heel. Thresholds were
UNDERWATER FREQUENCY DETECTION 9 measured with a continuous magnitude increase of 5 dB per second until response. Subjects responded by pressing a button once they perceived a vibration, at which point magnitude decreased by 3 dB per second until response by releasing the button. Thresholds were defined as the average of reversals (button operations). Results indicate d that thresholds for both contactors at all frequencies vary from low to high for fingertips, large toe, heel, and forearm; greater sensitivity was indicated in the fingertips and lower sensitivity in the forearm. The data indicated spatial summation for the heel and toe at all frequencies, the fingertip at frequencies at and above 63 Hz, and the forearm at frequencies at and above 125 Hz. High frequency thresholds were presumably lower with the larger contactor due to the increased excitation of the P ch annel (Pacinian corpuscles). Low frequency thresholds, presumably determined by the non Pacinian I channel (Meissner corpuscles), were lowest on the fingertip. The greater density of Meissner corpuscles in the fingertip might account for the greater sensit ivities at low frequencies in the fingertip than any other location. The reduced sensitivity in the forearm might similarly be accounted for by the difference in organization of sensory channels. Pacinian corpuscles in hairy skin are in deeper tissue and f ewer in number than in glabrous skin. However, if PC activation were the only response mediating detection, a lower sensitivity in hairy skin would be expected than was observed. Therefore it is possible that the hair follicle afferent fibers might mediate the thresholds in hairy skin at low frequencies. In order to better understand the contribution of HFA fibers, Mahns et al. (2006) measured thresholds for a vibrotactile stimulus presented at 20, 50, 100, and 200 Hz to the forearm and fingertip with a 4mm diameter probe. Results of the detection task
UNDERWATER FREQUENCY DETECTION 10 indicated that particle displacement threshold values for hairy skin (~150 !m at 20 Hz) 3 were higher than those of glabrous skin (<50 !m at 20 Hz), with both skin type thresholds decreasing as frequency incre ased. When superficial anesthesia was applied to hairy skin, selectively blocking the contribution of HFA receptors and therefore isolating any contribution to the deep subcutaneous PC receptors, detection thresholds rose significantly, but only for 20 and 50 Hz, indicating the involvement of HFA fibers in detection over the low frequency range. The lack of significant difference above 50 Hz when HFA receptors were blocked might indicate deep PC receptors as main contributors to detection sensitivity in tha t range. Differences between thresholds in the hairy and glabrous skin over the high frequency range can possibly be explained by the differential proximity of the PC receptors, which are found in fewer numbers and much deeper in the hairy skin of the fore arm as compared to glabrous skin. Therefore, when measuring sensitivity with contact vibrators on hairy skin, detection of low frequencies (less than ~80 Hz) depends on HFA fibers, whereas, at high frequencies (great than ~80 Hz), detection may depend on d eep FAII Pacinian corpuscles. In glabrous skin, detection sensitivity to low frequencies (< 80 Hz) depends on FAI Meissner corpuscles, whereas, at high frequencies (greater than ~80 Hz), detection depends on FAII Pacinian corpuscles (Mahns et al., 2006). T he sensitivity of HFA fibers to frequency detection may be underestimated because the receptor may not be adequately stimulated with contact vibration. The use of contact vibrators is a method that appears to selectively stimulate the PC receptor, which is sensitive to discontinuous displacement of the skin. HFA fibers innervate the root plexus encompassing the bud of 3 1 m = 0.001mm
UNDERWATER FREQUENCY DETECTION 11 the hair follicle. Sensitivity of hairy skin is localized around these buds and the spiral sensory nerves respond to displacement of the hair s (Wyburn et al., 1964). Therefore, in order to adequately compare glabrous skin sensitivity to that of hairy skin, a stimulus that both PC receptors and HFA fibers are sensitive to must be presented. Such mechanical characteristics can be found in an air puff stimulus. In a study by Hamalainen, Warren, and Gardner (1985) air puffs of 10 msec duration at high (peak force 0.016 N) 4 and low (peak force 0.008 N) intensities were presented at either one or three (15 mm apart) simultaneous locations 5 mm above the dorsal and volar sides of the hand; mean reaction times were measured. In contrast to contact stimuli, an air puff stimulus is able to both indent the skin and move the hair without a probe being in physical contact with the skin. In four of the six Ss detection of air puffs on hairy skin had lower reaction times than on glabrous skin. The two remaining Ss showed no significant difference in reaction time. Overall, when three locations were stimulated, hairy skin was more sensitive than glabrous. Hairy skin was significantly more sensitive at three locations for high and low intensities for five of six Ss. Overall, when one location was stimulated, hairy skin was about as sensitive as glabrous. Hairy skin was significantly more sensitive for single loca tion puffs for three of five Ss at high intensity and two of five Ss at low intensity. Spatial summation, defined as significantly shorter reaction times to 3 point as compared to 1 point air puffs, was observed for both volar and dorsal sides, but more so on hairy skin. Inhibition, as defined as significantly longer reaction times to 3 point stimulation, was observed in 25% of sessions testing low intensity air puffs on glabrous skin. Hairy skin demonstrated high sensitivity to multi 4 N = 1 kg m/s 2 ; Newton, unit of force.
UNDERWATER FREQUENCY DETECTION 12 point stimulation of b oth high and low intensities. Glabrous skin demonstrated high sensitivity to single point stimulation of high intensity. Thus, spatial summation is less apparent on glabrous skin than on hairy skin. However, it may be the case that hairy skin, or HFA fibe rs, selectively summates to the specific stimulation provided by the air puff (movement in the airstream), whereas glabrous skin, or PC receptors, selectively summates to the specific stimulation provided by the contact vibrator, as discussed in the previo us studies. When the hair of the dorsal hand was depilated (chemically dissolved), the increase in reaction time to the air puff stimulation on the dorsal side was so significant that glabrous skin was superior in sensitivity. This decrease in sensitivity on hairy skin following depilation occurred in three of five Ss. One of the remaining subjects showed no change in sensitivity, but also demonstrated equal reaction times for one and three puff stimuli on hairy and glabrous skin pre depilation. The other subject with no observed effect of depilation first demonstrated higher sensitivity post depilation, but later demonstrated lower sensitivity post depilation when retested several months later. Spatial summation was again observed in depilated skin, where all Ss displayed significantly shorter reaction times to 3 point as compared to 1 point air puffs (Hamalainen at al., 1985). The results of this study indicate spatial summation in hairy skin and that sensitivity in the skin is dependent on specific mechan ical properties of the tactile stimulus. The characteristics of controlled air puff stimuli offer preferential stimulation to the hair follicle by both indenting the skin and causing the hairs to move in the airstream, granting hairy skin greater or equal sensitivity as compared to glabrous skin.
UNDERWATER FREQUENCY DETECTION 13 When testing tactile sensitivity, the methods and equipment a researcher employs must adequately measure the intended stimulation. However, receptors can differ in the properties of stimuli they are most sensitive to. In addition to the mechanical properties indicated in the air puff study, receptors vary particularly in their frequency detection thresholds to vibration stimuli ( Morioka & Griffin, 2005 ; Morioka et al., 2008 ). FAII receptors are sensitive to frequenc ies in the 40 400 Hz range and FAI receptors have the lowest thresholds between 5 and 50 Hz. Slowly adapting Ruffini endings and Merkel discs have greatest sensitivity to frequencies below 8 Hz. Because the receptors selectively respond to vibration in a c ertain frequency range, we are able to isolate them and investigate the properties by which sensitivity varies. In the studies described thus far, both psychophysical and non psychophysical methods were employed to determine sensitivity to varying frequenc ies or intensities. In Hamalainen et al. (1985), using an air puff stimulus limited the researchers in the type of response they could use to define sensitivity to reaction time. In Morioka and Griffin (2005), the researchers used an adjusted method of lim its, or staircase method, in which participants were asked to discriminate which of two periods (one with stimulus present, one with stimulus absent) contained a vibration stimulus. Correct responses resulted in a decrease of intensity and incorrect respon ses resulted in an increase of intensity, with threshold being defined as the average of six reversals. Mahns et al. (2006) also used the method of limits with ascending and descending trials, defining the threshold as the average of six reversals. Morioka et al. (2008) chose the von Bekesy tracking method, a more refined form of the method of limits in which the intensity increase and decrease is
UNDERWATER FREQUENCY DETECTION 14 initiated by the participant rather than the experimenter, defining threshold as the average all reversals in a 45 s window. The current study sought to employ a method with which receptors in both hairy and glabrous skin could be equally stimulated and where participant thresholds could be calculated in units comparable to those of contact vibrator studies. Provi ding close proximity vibrotactile stimulation in an underwater environment to the dorsal and volar sides of the hand allowed for both indentation of the skin and movement of the hairs in the water stream, similar to movement observed in the air stream by H amalainen et al. (1985). Since stimulation in the form of sinusoidal vibrations is possible with this method, psychophysical methods can be utilized to determine threshold sensitivities like those in the contact studies. This study also included a phase in which the hair of the dorsal side of the hand was depilated in order to determine the contribution of the hair to tactile sensitivity. In order to theoretically compare results to those of contact studies in air, the staircase method of limits was used. This method is an adaptive form of the method of limits in which participants are initially presented a high intensity frequency that is gradually decreased in intensity after correct responses and increased after incorrect responses, allowing for a gradua l narrowing toward threshold. To verify the thresholds obtained with this method, the more precise psychophysical method of constant stimuli was also used. It was hypothesized that the two psychophysical methods would yield similar results. Given that when hair was effectively stimulated by an air puff, hairy and glabrous skin were similar in sensitivities (Hamalainen et al., 1985), it was also expected that
UNDERWATER FREQUENCY DETECTION 15 thresholds of hairy and glabrous skin would not differ significantly in an underwater environment. A ssuming that the HFA fibers are responding to the movement of the hair in the follicle and that sensitivity is dependent on this response, sensitivity pre depilation was expected to be greater than post depilation. When the hair was dissolved, the HFA fibe rs are not expected to be contributing to sensitivity. This decrease in sensitivity was expected to recover after three weeks pass between post depilation and a follow up session. Given the smaller distribution of Pacinian Corpuscles in hairy skin, it was hypothesized that the sensitivity observed in glabrous skin would be greater than the under stimulated post depilation hairy skin, and that as the hair grew back, HFA fibers would again be stimulated, and sensitivity would recover. Finally, it was expected that mean displacement thresholds obtained in this medium would resemble the psychometric curves of those of contact studies, with greater sensitivity in the high frequency range than low frequency range. Method Participants Seven individuals (1 male, 6 female) were recruited from a marine research laboratory and an undergraduate college. Participants ranged in age from 21 to 24 years old with a median age of 23 years old. They were informed that they would be participating in a passive touch study in w hich their detection of vibration underwater would be assessed. Any individual with a history of hair growth disorders, nervous system disorders, or hand paralysis, burns, chronic paresthesia, or surgery was excluded during recruitment. As compensation, pa rticipants received $10 for participation in the sessions measuring detection thresholds prior to hair depilation; $5 for participation in the
UNDERWATER FREQUENCY DETECTION 16 session just after hair depilation; and $5 for participation in the three week follow up after depilation. Materi als A dipole shaker (Data Physics Signal Force, Model V4) with a 5.08 cm diameter plastic sphere on a stainless steel extension (Coombs, 1994) was used to generate the stimuli (Figure 2). The dipole shaker generates a localized, controlled, and calibrat ed sinusoidal flow that decreases in amplitude as 1/distance 3 and allowed determination of sensitivity on the volar and dorsal side of the hand. For calibration, a 3 dimensional accelerometer was mounted to a neutrally buoyant frame to measure the motion received at the location of the participant's hand. The shaker was placed in a shallow tank of water that averaged 22 C. Figures 3 8 demonstrate the experimental set up of the dipole shaker and a participant's hand in the tank. Two lasers flanking the ste el frame were used to approximate the distance between the hand and the oscillating sphere. Light beams from each laser were positioned to converge at the desired distance. The participants sat in a chair next to the tank with their arms submerged in the w ater. A sheet was positioned in front of the participant such that they were unable to see the shaker but able to see the converging light beams for hand placement. Procedures Participants were instructed to submerge their right hand into the shallow tank of water and were positioned at the appropriate distance from the instrument where the two light beams converge (approximately 40 cm deep and 20 or 10 cm distal). Brown noise (random noise with more energy at lower frequencies, decreasing in power by 6 dB per octave) was continuously presented to each participant via noise canceling headphones
UNDERWATER FREQUENCY DETECTION 17 throughout each run to reduce the possibility of auditory cues. An LED light signaled the beginning of each trial. At all frequencies, participants were required to f ocus on the LED signaling light. A 1 s presentation of the LED light initiated the trial, followed by .5 s of no light, and 3 s stimulus presentations. Participants were given 3 s from stimulus offset to verbally respond to the stimulus with "yes" to dete cting its presence and "no" to its absence. Trials were separated with a 3 s inter trial interval. A set of trials composed a single frequency run. Five total frequencies were measured per session in a random order: 10, 25, 50, 100, and 150 Hz. Within a se ssion, participants were afforded a brief rest period between frequency measures for recovery in order to decrease effects of adaptation and paresthesia due to restricted blood flow. The study consisted of three phases: (1) detection thresholds measured f or both hairy and glabrous skin; (2) detection thresholds measured for hairy skin post depilation; and (3) detection thresholds measured for hairy skin three weeks after depilation. Phase One consisted of four test sessions in which thresholds for both vol ar and dorsal sides of the hand were measured utilizing the staircase method of limits and the method of constant stimuli. Sessions were divided over the course of three days in order to reduce the amount of time a participant had his or her hand submerged underwater, which may lead to habituation effects. Thresholds for both sides of the hand were measured with the staircase method in the first two sessions (Day 1). Thresholds measured with the constant stimuli method were divided so that each side of the hand was measured on a different day (Day 2 and 3). Phase Two consisted of one test session in which the staircase method was used to measure the threshold of the dorsal side of the hand. This phase required
UNDERWATER FREQUENCY DETECTION 18 participants to have the hairy side of their han d, excluding fingers, chemically depilated. Hair was dissolved using a commercial chemical depilator (Veet). Phase Three consisted of one session in which thresholds for the dorsal side of the hand were measured with the staircase method three weeks post d epilation. Psychophysical Methods Staircase method of limits A set of at least four warm up trials was presented to familiarize the participant with the study and ensure detection of the test frequency (Table 1). A stimulus was presented in half of the tr ials; stimulus present and stimulus absent trials were presented in randomized order. If a participant was unable to detect the stimulus at the initial distance of 20 cm, her hand was repositioned to 10 cm distal from the dipole shaker and presented a seco nd set of warm up trials. If she was still unable to detect the stimulus, that frequency was not tested for that subject. After being presented 4 8 warm up trials in which a stimulus was presented half the time, the participant moved on to a series of test trials (Table 2). Test trials were arranged in blocks of 16, 8 signal present and 8 signal absent trials. Presentation of signal present vs. signal absent trials were counterbalanced and controlled for human bias using quasi random schedules. Each frequen cy started at a standardized 0 dB (re 1 Pa) and dropped in 6 dB increments, until the first reversal. After the first reversal, intensity rose by 6 dB and then dropped or rose in 3 dB increments if the participant responded correctly or incorrectly. Thres holds were defined as the average of the attenuations for six reversals, discounting the first pair. Method of constant stimuli This method was used as a more narrow measurement of detection thresholds than the staircase method. Five attenuations per freq uency were calculated based on results of the previous method. The staircase
UNDERWATER FREQUENCY DETECTION 19 threshold attenuation, and stimuli of +/ 1 dB and +/ 3 dB of this attenuation were utilized. These 5 stimuli were randomly presented 10 times each at the same distance and with the same response criteria as the previous method. Thresholds were defined by the attenuation at which participants responded correctly more than 50% of the time, or for more than 5 trials at a particular test frequency. Stimulus Calibration Threshold valu es from both staircase and constant methods were mathematically transformed into acceleration, velocity, and displacement thresholds. Staircase method values were calculated as the average intensity (dB) for six reversals; constant stimuli method values we re determined by the intensity (dB) at which participants responded correctly over 50% of the time. Stimulus values were measured at the end of all sessions in the absence of participants' hands. An accelerometer was placed at the location of participants' hands to determine the motion received at the location of stimulation. Because the signals are sinusoidal, the calibrated acceleration was used to calculate velocity by dividing by 2* *F; displacement was calculated by dividing velocity by 2* *F. Results Tables 1 and 2 depict frequencies of warm up and test trials for each participant for the staircase method of limits. All participants experienced 50 60 trials per frequency in the constant stimuli method. Table 3 shows false alarm rates for each particip ant for staircase method of limits. False alarms are errors of commission; the participant reports a signal present when in fact the signal is absent. Individual participants' frequency runs in which false alarm rates were above 10% were eliminated from fi nal data analyses. In
UNDERWATER FREQUENCY DETECTION 20 addition, due to consistently high false alarm rates, Participant 2's data were eliminated completely. Data from Participant 5, the only male, was dropped due to scheduling conflicts before having his thresholds measured with the method of constant stimuli during the first phase of the study. Ref er to Table 4 for mean acceleration, velocity, and displacement thresholds for each frequency and phase of the study. Note that cells under the follow up phase represent true threshold values for Participant 3, only. Staircase Method of Limits and Method of Constant Stimuli Overall, measurements of the more precise constant stimuli method confirm thresholds obtained by the staircase method of limits (Figures 9 14). Subsequent comparisons were made solely with the staircase method of limits. Hairy and Glabr ous Skin In order to determine if the glabrous skin of the palm differs in sensitivity to the hairy skin on the top of the hand, mean acceleration, velocity, and displacement thresholds for each skin type was plotted for comparisons. Acceleration, velocit y, and displacement thresholds for hairy skin were similar to those of glabrous skin (Figures 15 17). Upon further analysis of data obtained at 150 Hz, an outlier was discovered for Participant 3's hairy post depilation threshold of 188.1645 mm/s 2 After removing this point, mean acceleration decreased from 141.5846 mm/s 2 ( SD =40.3555) to 118.2946 mm/s 2 ( SD =1.6082). Matched pairs t tests determined that hairy and glabrous thresholds did not differ significantly from one another on measures of acceleration, velocity, and displacement ( p > 0.10). Effects of Depilation
UNDERWATER FREQUENCY DETECTION 21 The importance of the hair present on the dorsal side of the palm was determined by measuring thresholds after depilating the hair from the skin. Pre and post depilation thresholds of the dors al side of the hand were compared for sensitivity to acceleration, velocity, and displacement (Figures 18 20). Matched pairs t tests determined that pre depilation and post depilation thresholds did not differ significantly from one another on measures of velocity, and displacement ( p > 0.10). Pre depilation and post depilation acceleration thresholds showed a reliable but not significant difference, t (13) = 1.759, p = 0.102. Mean acceleration thresholds were higher post depilation for all frequencies, ex cept at 150 Hz where pre depilation was less sensitive than post depilation skin. The differences between mean acceleration thresholds pre and post depilation per frequencies were: 7.8 mm/s 2 at 10Hz, 5.3 mm/s 2 at 25 Hz, 4.2 mm/s 2 at 50 Hz, 3.6 mm/s 2 at 10 0 Hz, and 2.9 mm/s 2 at 150 Hz. Thresholds obtained from Participant 3 were compared for differences between pre depilation, post depilation, and at a three week follow up. Acceleration threshold values indicated that hairy skin was more sensitive pre depi lation than post depilation and most sensitive at the time of follow up measurements (Figure 21). Velocity and displacement threshold values indicated that sensitivity remained stable over time (Figures 22 and 23). Displacement Curves in Air Versus Underwa ter In Morioka and Griffin (2005) and Mahns et al. (2006), displacement thresholds in the skin decreased as a function of frequency, with greater sensitivity observed in the higher frequency range. The present study resulted in the same relationship, but varied from Mahns et al. (2006), in which hairy and glabrous skin were compared (Figure 24).
UNDERWATER FREQUENCY DETECTION 22 In air, thresholds at the fingertip were lower than those at the forearm. In an underwater setting, thresholds between the volar and dorsal sides of the hand were equal and fell in the range of fingertip sensitivity seen in air. Discussion Hairy skin appears to be equal to glabrous skin in vibrotactile sensitivity to acceleration, velocity, and displacement. Velocity and displacement curves indicated that the unde rwater setting provided equal stimulation for receptor populations of both skin types. However, acceleration detection on the two sides of the hand was not as consistent across frequencies. At 10, 25, and 150 Hz, glabrous skin was less sensitive than hairy skin. Possible explanations for this finding include that HFA fibers mediate detection below 80 Hz, making hairy skin more sensitive (but superior sensitivity was not apparent at 50 Hz); PC activation is greater in glabrous skin, thus there is greater sen sitivity above 40 Hz in glabrous skin (but hairy skin was more sensitive at 150 Hz); Meissner corpuscle presence in glabrous skin provides more sensitivity between 2 40Hz (but hairy skin was more sensitive at 10 and 25 Hz); or SA fibers are contributing to vibration sensitivity, the tactile discs in hairy skin may be more stimulated than the Merkel discs in glabrous, granting superior sensitivity at 10 Hz. Because PC receptors and HFA fibers are most sensitive to acceleration, it is likely that HFA fibers m ediate sensitivity at 10 and 25 Hz in favor of hairy skin and, with greater activation above 40 Hz, PC receptors enhance glabrous sensitivity. Contrary to my hypothesis, hairy skin thresholds pre depilation equaled that of post depilation in sensitivity to velocity and displacement. Thresholds for acceleration indicated slightly higher sensitivity pre depilation than post depilation across frequencies
UNDERWATER FREQUENCY DETECTION 23 below 150 Hz. With the results of pre depilation hairy and glabrous comparisons, sensitivity in hairy skin appears to be dependent on the presence of hair for vibrations below 100 Hz. Above 100 Hz, deep PC activation might mediate the detection of vibration in hairy skin. Glabrous and hairy skin appear similar in acceleration sensitivity, but around 50 Hz, the greater density of PC receptors in glabrous skin might grant it greater sensitivity than hairy skin. The hypothesis that decreased sensitivity post depilation would recover in three week follow up was confirmed in acceleration analyses. Velocity and displa cement thresholds were equal through all phases from frequencies above 10 Hz. At 10 Hz, follow up was more sensitive than pre or post depilation. For acceleration, sensitivity was greater at follow up for the included 10, 25, 50, and 150 Hz data points tha n pre or post depilation. At follow up measurements, the hair had grown vertically and perpendicular to the skin. This is a similar position to that of the manatee vibrissae during feedings and active exploration with the muzzle (Marshall, Clark, & Reep, 1998). This placement in relation to the skin might have made the hairs move to smaller changes in particle displacement in the water than when full length, activating the HFA fibers more readily. The expectation that displacement curves of air would resem ble underwater curves was also confirmed depicting a relationship in which sensitivity was greater at higher frequencies. Underwater, both hairy and glabrous sensitivity fell in the most sensitive in air contact study range, that of the glabrous fingerti p, despite the lack of PC density in hairy skin, indicating HFA fibers in low frequency detection and a possible switching to deep PC stimulation somewhere above 50 Hz. Mechanoreceptors of the same kind have been shown to differ in the highest frequency se nsitivity depending on
UNDERWATER FREQUENCY DETECTION 24 location. Moriaka et al. (2008) showed that Meissner corpuscles were most sensitive below 20 Hz on the sole of the foot and below 40 Hz at the fingertip, presumably due to the spatial summation of a greater number of stimulated recep tors in the fingertip. In the underwater study, there may have been spatial summation of PC receptors on the palm and of the HFA fibers stimulated by multiple hairs moving on the dorsal side of the hand. As previously discussed, spatial summation was obser ved in hairy skin when multi point air puffs were presented, yielding greater sensitivity than single point air puffs (Hamalainen et al., 1985 ). Because more follicles are being stimulated in the underwater setting, HFA fibers might be mediating sensitivi ty up to a higher frequency of 100 Hz, unlike in air contact studies that concluded that PC activation in hairy skin might occur around 50 Hz (Mahns et al., 2006). It is also possible that HFA fibers are mediating the entire range for hairy skin, as observ ed in a study of the effects of temperature on displacement thresholds on the forearm (Verrillo & Bolanowski, 1986). The researchers suggest that the HFA fibers might be the rapidly adapting system involved in the smooth and uniform response to temperature changes observed in hairy skin. Cues and Confounds Though attempts were made to eliminate audio and visual cues, which were presumably successful since no participant said they could hear or see the stimulus when asked, there were tactile cues that were n ot eliminated. Because of the size of the dipole shaker and its necessary placement in the tank, participants were required to straddle or sit sidesaddle to the tank of water, allowing their legs to be in contact with the tank. In addition, a participant's upper arm was resting on the side of the tank in order to reach his or her hand deep enough to meet the intersecting beams of light. Being in contact with
UNDERWATER FREQUENCY DETECTION 25 the tank may have provided tactile cues to high intensity vibrations. Towels were placed between the tank and participant's legs and arm where in contact with the tank. When asked about the stimulation to these locations, participants noted that they were able to tune that sensation out within a few high intensity runs, and as they approached threshold f or the higher frequencies, intensity was so low that they claimed to only feel stimulation in their hands. Tactile stimulation experienced at any point other than the intended site may have served as an unintentional masking stimulus. A masking stimulus is one that is presented before, during, or after the test stimulus and effectively inhibits perception of the target stimulus. In this case, this tactile cue on the legs or upper arm might either serve to mask detection in the hand or participants may be un able to ignore the stimulation and respond to detection in the hand based on this additional stimulation through the tank. However, the measurements of sensitivity are intended to obtain a threshold, and participants noted that this additional stimulation was undetectable at low intensities. Thus, at threshold measurements, it is likely that sensitivity was not due to extraneous stimulation. Additional confounds may be present in this study, such as temperature effects or tissue changes. Bolanowski and Verr illo (1986) measured temperature differences in sensitivity to displacement with a .08 mm contact vibrator with 1 mm surround, in order to isolate non Pacinian receptors, testing frequencies from 12 500 Hz. When temperature was raised from 15 C to 30 C o n the volar forearm and thenar eminence (the base of the thumb) there was a gradual improvement of sensitivity. Verrillo and Bolanowski (2003) investigated the effect of temperatures of 15, 20, and 40 ¡ C on judgments of subjective magnitude to 15, 250, and 400 Hz vibration stimuli presented to the thenar eminence of
UNDERWATER FREQUENCY DETECTION 26 the right hand. Using a 15 Hz stimulus, the non Pacinian I (Meissner corpuscle) and non Pacinian III (Merkel disc) channels were selectively stimulated. When stimulated, these channels showed no difference in judgments between the different temperatures, showing that Meissner corpuscles and Merkel cells are not responsive to variations in temperature. However, results of the Pacinian channel stimulation with a 250 Hz stimulus indicated that it is dependent on temperature, with perceived magnitude increasing as temperature increases. Theoretically, receptor populations of both the glabrous and hairy sides of the hand in the present study were affected by temperature. Across the days of testing, the temperature of the water in the tank varied due to an unstable room temperature in the lab; the temperature ranged from 20 24 C. However, because thresholds were obtained at a low temperature, averaging 22 C, it can be concluded that thresholds presente d are underestimating the potential sensitivity of both glabrous and hairy skin in this underwater setting. Physical tissue changes due to water absorption and tactile illusions due to restricted blood flow are also potential confounds in this study. When the skin is saturated for long enough, the outer layers thicken and the change in distribution of water could have an effect on the mechanoreceptor populations. However, Verrillo et al. (1996) found that after 20 min of submersion in a seawater mixture, d isplacement thresholds on the thenar eminence and volar forearm obtained prior to submersion did not significantly differ from those of saturated skin at frequencies between 10 250 Hz. Threshold sensitivity of the receptors in the dermis and subdermal tiss ues are not affected by changes in saturation of the skin. Participants in the present study were required to submerge their hands for no more than 10 minutes for each frequency. However, in one
UNDERWATER FREQUENCY DETECTION 27 session, their hands were underwater for a total time of up t o 60 min. Any effects of saturation would presumably have been observed with a decrease or increase of threshold over time. Because frequencies were presented in a random order, and results indicated the same trend in all participants toward lower threshol ds at high frequencies, any potential effects of saturation can be considered negligible. When an appendage is held still for a period of time, the onset of paresthesia produces a tingling sensation, also known as "pins and needles." This sensation is due to aberrant spontaneous activity of mechanoreceptors ( Ochoa & Torebjork, 1980) and could theoretically interfere with vibrotactile detection. Using natural plant derived compounds (sanshool), Lennertz, Tsunozaki, Bautista, and Stucky (2010) simulated the e xperience of paresthesia to determine the receptor correlates to the tingling sensation using ex vivo mouse skin. The results indicated that sanshool activates rapidly adapting fibers more than slowly adapting fibers. Sanshool sensitive fibers included slo wly conducting C fibers, rapid D hair afferents, and guard hair afferents of the mouse. Without getting bogged down in physiology, since the present study was unable to measure precise activation of afferent fibers or physically isolated receptors, the fol lowing logic is sufficient: there exists several classifications of afferent fibers innervating the skin. The D hair afferents in mice resemble the root hair plexus structure formed by the free nerve endings that wrap around the human hair follicle. The C fiber is also a classification associated with both RA and SA receptor populations of human hairy skin. Guard hairs in mice and cats are similar to the vibrissae found in marine mammals, but are absent in human skin.
UNDERWATER FREQUENCY DETECTION 28 Based on the findings of Lennertz et al (2010), paresthesia could interfere with sensation in hairy skin. Participants in the present study were granted time between each frequency run and each test session to reduce the possible experience of paresthesia. They were periodically asked if they experienced any numbing or tingling to confirm that this was ruled out. Even though none mentioned the experience, it is still possible that they were temporally very near experiencing paresthesia. More research on the temporal onset of and effects of pare sthesia on detection thresholds in both hairy and glabrous skin is necessary to understand how spontaneous activation of mechanoreceptors is indicated in vibrotactile threshold measures. Implications The contact studies previously reported have indicated hairy skin as having less sensitivity than glabrous skin. Researchers base their conclusions of behavioral importance off of these results, proposing that glabrous skin provides high acuity tactile information for superior localization and spatial resoluti on (Mahns et al., 2006). However, the stimulus presented in contact to the skin is providing unequal stimulation, favoring the mechanoreceptors in glabrous skin. By not providing a stimulus with mechanical properties that HFA fibers are sensitive to, cont act studies in air present an incomplete picture of the sensitivity of hairy skin. The study conducted underwater serves to provide a greater understanding of the neurosensory processes involved in vibration detection of hairy skin, while highlighting the importance of considering the mechanical properties of the stimulus used in tactile studies. In addition, this was the first study of the sensory capacity of human skin using a non contact stimulus underwater. The dipole shaker with staircase procedure ha s also
UNDERWATER FREQUENCY DETECTION 29 been used in studies of manatee vibrotactile detection underwater (Coombs, 1994; Mann, Bauer, Reep, Gaspard, Dziuk, & Read, 2009 ). Results from two manatees indicated that above 10 Hz they could sense, with their oral disk, displacements of less tha n 1 micron, velocity below .03 mm/s, and acceleration of less than 4 mm/s 2 Compared to the results for human sensation, manatee sensitivity is about 20 times that of humans for low frequencies (10 50 Hz) and 100 times better for high frequencies (150 Hz). Given the environment manatees live in, high sensitivity to vibration underwater is useful for locomotion, eating, and detection of boats. For manatees, sensitivity to vibrations serves an evolutionary purpose to enhance rate of survival. For humans, sens itivity of hairy skin might also serve an evolutionary purpose in emotional learning, beginning in the womb. During gestation, the hair follicles of the fetus produce long unpigmented lanugo hairs. These hairs shed en masse around 7 8 months of gestation ( Tre b & Tobin, 2010 ). The purpose of these hairs is unknown and how they are indicated in tactile sensitivity has not been studied. HFA fiber indication in adult underwater sensitivity of hairy skin might stem from development of the follicle and root hai r plexuses during gestation, the only time humans naturally exist underwater. It is possible that the physical characteristics of lanugo hair, unlike shorter vellus hair and thicker terminal hair, make it more easily moved in the amniotic fluid and contrib ute to the development of the topographical cortical representations in the brain. The development of such connections might serve plasticity in the brain and automatic affective aspects of touch later in life. Lay, Davis, Chen Bee, and Frostig (2010) fou nd that mild rat whisker stimulation up to 1 hr after ischemic stroke onset helped maintain healthy cortical tissue, normal neuronal activity, functional
UNDERWATER FREQUENCY DETECTION 30 representations, and normal sensorimotor related behavior. The authors proposed that stimulation of ar eas of the human body with similarly large cortical representation could be applied to suspected stroke victims as a non invasive early intervention, capitalizing on the plasticity of the brain. A woman with selective loss of myelinated tactile afferents, unable to detect a tactile stimulus on the palm of the hand but able to detect touch on the forearm and dorsal hand, was studied for slowly adapting unmyelinated (C) tactile afferent contributions to light touch (Olausson et al., 2002). fMRI analyses indic ated that CT afferent stimulation of hairy skin is involved in interoception and affective but not discriminative aspects of touch. In a follow up to Lat et al. (2010), it was discovered that direct CT stimulation with a soft artists brush can elicit symp athetic sudomotor (sweating) skin response (Olausson, 2007). The authors highlight a dichotomy of innervations in hairy skin, the fast adapting myenlinated afferents sensitive to vibrations and discrimination and the slowly adapting unmyenlinated afferents sensitive to light affective touch. Though the CT afferents are not involved in glabrous skin, which receptors are being innervated in hairy skin is still unknown. Research into the development of hairy skin and its neurosensory correlates to affective to uch can serve as behavioral support for the finding that hairy skin, under the right mechanical stimulation, can be as sensitive as glabrous skin.
UNDERWATER FREQUENCY DETECTION 31 References Boudreau, J. C. & Tsuchitani, C. (1973). Sensory Neurophysiology New York, NY: Van Nostrand Reinh old Company, pp. 211. Brown, E. L., & Deffenbacher, K. (1979). Perception and the Senses New York, NY: Oxford University Press, pp. 58 60. Cholewiak, R., & Collins, A. (1991). Sensory and physiological bases of touch. In M. Heller & W Schiff (Eds.), The p sychology of touch (pp. 23 60). Hillsdale, NJ England: Lawrence Erlbaum Associates, Inc. Fain, G. L. (2003). Sensory Transduction Sunderland, MA: Sinauer Associates, pp. 111 113. Gescheider, G., Gl, B., Sexton, J., Karalunas, S., & Fontana, A. (2005). Spatial summation in the tactile sensory system: Probability summation and neural integration. Somatosensory and Motor Research 22(4), 255 268. Granit, R. (1955). Receptors and Sensory Perception Newhaven, CT: Yale University Press, pp. 39 43. Hamalainen H. A., Warren, S., & Gardner, E. P. (1985). Differential sensitivity to airpuffs on human hairy and glabrous skin. Somatosensory Research 2 (1), 281 302. Heller, M., & Schiff, W. (1991). The psychology of touch Hillsdale, NJ England: Lawrence Erlbaum As sociates, Inc. Hendrickson, T. (2002). Massage for orthopedic conditions (28 th ed.). Lippincott Williams and Wilkins.
UNDERWATER FREQUENCY DETECTION 32 Lay, C. C., Davis, M. F., Chen Bee, C. H., & Frostig, R. D. (2010). Mild sensory stimulation completely protects the adult rodent cortex from ischemic stroke. PLoS ONE 5 (6): e11270. doi:10.1371/journal.pone.0011270. Lennertz, R. C., Tsunozaki, M., Bautista, D. M., & Stucky, C. L. (2010). Physiological basis of tingling paresthesia evoked by Hydroxy a Sanshool. Journal of Neuroscience 30 (1 2), 4353 4361. Mahns, D., Perkins, N., Sahai, V., Robinson, L., & Rowe, M. (2006). Vibrotactile Frequency Discrimination in Human Hairy Skin Journal of Neurophysiology, 95 (3), 1442 1450. Madison, K. C. (2003). Barrier function of the skin: "la raison d't re" of the epidermis. Journal of Investigative Dermatology 121 (2), 231 241. Mann, D., Bauer, G., Reep, R., Gaspard, J., Dziuk, K., & Read, L. (2009). Auditory and tactile detection by the West Indian manatee. Final Report to Florida Fish and Wildlife Cons ervation Commission. Marshal, C. D., Clark, L. A., & Reep, R. L. (1998). The muscular hydrostat of the Florida manatee ( Trichechus Manatus Latirostris ): A functional morphological model of perioral bristle use. Marine Mammal Science 14 (2), 290 303. Montag u, A. (1986). Touching: The human significance of the skin (3rd ed.). New York: Harper & Row. Morioka, M., Whitehouse, D. J., & Griffin, M. J. (2008). Vibrotactile thresholds at the fingertip, volar forearm, large toe, and heel. Somatosensory and Motor Re search 25 (2), 101 112. Morioka, M. & Griffin, M. J. (2005). Thresholds for the perception of hand transmitted
UNDERWATER FREQUENCY DETECTION 33 vibration: Dependence on contact area and contact location. Somatosensory and Motor Research 22 (4), 281 297. Ochoa, J. L., & Torebjork, H. E. (1 980). Paraesthesiae from ectopic impulse generation in human sensory nerves. Brain 103 (4), 835 853. Olausson, H., Lamarre, Y., Backlund, H., Morin, C., Wallin, B. G., Starck, G., Ekholm, S., Strigo, I., Worsley, K., Vallbo, A. B., & Bushnell, M. C. (2002 ). Unmyelinated tactile afferents signal touch and project to insular cortex. Nature Neuroscience 5 (9), 900 904. Olausson, H., Cole, J., Rylander, K., McGlone, F., Lamarre, Y., Wallin, B. G., Krmer, H., Wessberg, J., Elam, M., Bushnell, M. C., and Vallbo A. (2007). Functional role of unmyelinated tactile afferent in human hairy skin: sympathetic and perceptual localization. Experimental Brain Research 184 135 140. doi: 10.1007/s00221 007 1175 x Proksch, E., Brandner, J. M., & Jensen, J. M. (2008). The skin: An indispensable barrier. Experimental Dermatology 17 (12), 1063 1072. Shoham, D., & Grinvald, A. (2001). The cortical representation of the hand in macaque and human area S I: High resolution optical imaging. Journal of Neuroscience 21 (17), pp. 682 0 6835. Stcker, M., Struk, A., Altmeyer, P., Herde, M., Baumgrtl, H., & Lbbers, D. W. (2002). The cutaneous uptake of atmospheric oxygen contributes significantly to the oxygen supply of human dermis and epidermis. The Journal of Physiology 538 985 99 4.
UNDERWATER FREQUENCY DETECTION 34 Tre b, R. M., & Tobin, D. J. Eds. (2010). Aging Hair Berlin Heidelberg: Springer Verlag, pp. 1 8. Verrillo, R. T. & Bolanowski, S. J. (1986). The effects of skin temperature on the psychophysical responses to vibration on glabrous and hairy skin. Journ al of Acoustical Society of America 80 (2), 528 532. Verrillo, R., & Bolanowski, S. (2003). Effects of temperature on the subjective magnitude of vibration. Somatosensory & Motor Research 20 (2), 133 137. Verrillo, R. T., Bolanowski, S. J. Baran, F., & Smi th, P. F. (1996). Effects of underwater environmental conditions on vibrotactile thresholds. Journal of Acoustical Society of America 100 (1), 651 658. Wyburn, G. M., Pickford, R. W., & Hirst, R. J. (1964). Human Senses and Perception Canada: University o f Toronto Press, pp. 16 18.
UNDERWATER FREQUENCY DETECTION 35 Table 1 Number of Warm Up Trials per Frequency, Phase, and Participant for Staircase Method of Limits Participant Frequency 1 2 3 4 5 6 7 Phase 1 150 4 8 8 4 8 8 4 Glabrous 100 4 8 8 4 8 4 4 50 4 4 4 4 8 4 4 25 4 8 8 4 4 4 4 10 8 8 4 8 8 4 4 Hairy 150 4 4 8 4 8 4 4 100 4 8 8 4 8 4 4 50 4 8 8 4 8 4 4 25 4 8 8 4 4 4 4 10 4 4 8 4 8 4 8 Phase 2 150 4 8 8 4 100 8 8 8 4 50 4 8 4 4 25 4 8 4 4 10 4 4 8 4 Phase 3 150 4 100 8 5 0 4 25 4 10 4
UNDERWATER FREQUENCY DETECTION 36 Table 2 Number of Test Trials per Frequency, Phase, and Participant for Staircase Method of Limits Participant Frequency 1 2 3 4 5 6 7 Phase 1 150 32 32 27 27 27 32 26 Glabrous 100 28 57 29 18 25 18 20 50 27 3 8 26 29 29 26 24 25 29 49 0 28 25 23 26 10 21 34 18 24 19 24 29 Hairy 150 25 20 29 24 17 27 32 100 19 27 24 19 18 22 21 50 23 26 23 28 25 26 27 25 21 18 27 26 23 27 27 10 18 42 20 23 18 23 30 Phase 2 150 21 36 21 40 100 24 24 21 24 50 24 33 24 20 25 27 24 18 24 10 18 26 24 20 Phase 3 150 24 100 32 50 19 25 26 10 27 Note A value of 0 indicates that the participant was unable to move into test trials due to the unsuccessful detectio n of stimuli during warm up trials.
UNDERWATER FREQUENCY DETECTION 37 Table 3 False Alarm Rates per Frequency, Phase, and Participant for Staircase Method of Limits Participant Frequency 1 2 3 4 5 6 7 Phase 1 150 0% 13% 0% 0% 0% 0% 0% Glabrous 100 0% 43% 0% 0% 0% 0% 0% 50 0% 55% 0% 0% 7% 0% 0% 25 0% 4% 0% 0% 0% 0% 10 0% 19% 0% 0% 0% 24% 13% Hairy 150 0% 0% 0% 0% 0% 0% 0% 100 0% 7% 0% 0% 0% 0% 0% 50 0% 0% 0% 0% 0% 0% 23% 25 0% 0% 0% 0% 0% 0% 15% 10 0% 10% 0% 8% 0% 0% 14% Phase 2 150 0% 21% 0% 5% 100 0% 17% 0% 0% 50 0% 31% 0% 0% 25 0% 17% 0% 0% 10 0% 54% 0% 0% Phase 3 150 8% 100 13% 50 0% 25 8% 10 8%
UNDERWATER FREQUENCY DETECTION 38 Table 4 Acceleration, Velocity, and Displacement Thresholds Skin Types Hairy Glabrous Phase Method Frequency 10 25 50 100 150 10 25 50 100 150 Acceleration (mm/s/s) 128.3200 125.0308 121.2696 119.6546 121.2062 134.3220 126.5879 118.4656 118.6248 126.9449 Pre Depilation Staircase Velocity (mm/s) 2.0423 0.7960 0.3860 0.1904 0.1286 2 .1378 0.8059 0.3771 0.1888 0.1347 ( N = 5) Displacement (microns) 32.5038 5.0673 1.2287 0.3031 0.1365 34.0242 5.1304 1.2003 0.3005 0.1429 Acceleration (mm/s/s) 127.2927 134.6559 125.0317 120.6244 122.8917 135.5453 125.0302 118.2485 119.0715 126.6359 Pre Depilation Constant Velocity (mm/s) 2.0259 0.8572 0.3980 0.1920 0.1304 2.1573 0.7960 0.3764 0.1895 0.1344 ( N = 5) Displacement (microns) 32.2436 5.4574 1.2668 0.3055 0.1384 34.3340 5.0673 1.1981 0.3016 0.1426 Acceleration (mm/s/s) 136.1548 13 0.3661 125.4942 123.2089 118.2946 Post Depilation Staircase Velocity (mm/s) 2.1670 0.8299 0.3995 0.1961 0.1502 ( N = 3) Displacement (microns) 34.4884 5.2835 1.2715 0.3121 0.1594 Acceleration (mm/s/s) 122.1255 121.3624 119.9035 119.4317 Follow Up Staircase Velocity (mm/s) 1.9437 0.7726 0.3817 0.1267 ( N = 1) Displacement (microns) 30.9347 4.9186 1.2149 0.1345 Note Phase 1 and 2 show mean thresholds. Phase 3 shows individual threshold values for Par ticipant 3, only. Acceleration threshold for 150 Hz post depilation is excluding Participant 3's outlier.
3 9 Figure 1 Layers of the Skin. From The psychology of touch (p. 25), by M. Heller and W. Schiff, 1991, Hillsdale, NJ England: Lawrence Erlbaum Associ ates, Inc.
40 Figure 2 Dipole Shaker.
41 Figure 3 Participant Seating Arrangement.
42 Figure 4 Participant View of Lasers.
43 Figure 5 Participant View of LED Trial Indicator.
44 Figure 6 Lasers Flanking Dipole Shaker.
45 Figure 7 Dorsal Hand Align ment with Lasers.
46 Figure 8 Volar Palm Alignment with Lasers.
47 Figure 9 Mean Acceleration Thresholds for Glabrous Skin by Method.
48 Figure 10 Mean Velocity Thresholds for Glabrous Skin by Method.
49 Figure 11 Mean Displacement Thresholds for Gl abrous Skin by Method.
50 Figure 12 Mean Acceleration Thresholds for Hairy Skin by Method.
51 Figure 13 Mean Velocity Thresholds for Hairy Skin by Method.
52 Figure 14 Mean Displacement Thresholds for Hairy Skin by Method.
53 Figure 15 Mean Accelerati on Thresholds by Skin Type.
54 Figure 16 Mean Velocity Thresholds by Skin Type.
55 Figure 17 Mean Displacement Thresholds by Skin Type.
56 Figure 18 Mean Acceleration Thresholds for Hairy Skin by Phase.
57 Figure 19 Mean Velocity Thresholds for Hairy S kin by Phase.
58 Figure 20 Mean Displacement Thresholds for Hairy Skin by Phase.
59 Figure 21 Individual Acceleration Thresholds by Time for Participant 3.
60 Figure 22 Individual Velocity Thresholds by Time for Participant 3.
61 Figure 23 Individua l Displacement Thresholds by Time for Participant 3.
62 Figure 24 Displacement Thresholds of Mahns et al. (2006) Compared to the Present Study.