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The Sense of Touch: Performance and Peripheral Neural Processes

Ian Darian‐Smith

10.1002/cphy.cp010317

Source: Supplement 3: Handbook of Physiology, The Nervous System, Sensory Processes

Originally published: 1984

Published online: January 2011

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Evolution of Primate Hand
2 Structure of Nerve Terminals in Skin and Underlying Tissues
2.1 Morphological Classification
2.2 Innervation of Ridged Glabrous Skin in Primate
2.3 Innervation of Hairy Skin
2.4 Innervation of Mucocutaneous Regions of Skin
2.5 Innervation of Joints in Primate and Nonprimate Mammals
3 Functional Characterization of Mechanoreceptor Fibers Innervating Skin, Joints, and Other Somatic Tissues
3.1 Taxonomy of Somatic Mechanoreceptive Fibers
3.2 Mechanoreceptive Fibers Innervating Ridged Glabrous Skin in Primate
3.3 Mechanoreceptive Fibers Innervating Hairy Skin
3.4 Mechanoreceptive Fibers Innervating Joints
4 Transfer of Sensory Information from Somatic Tissues to Central Nervous System
4.1 Peripheral Neural Representation of Indentation of Skin
4.2 Neural Representation of Cutaneous Vibratory Stimuli
4.3 Neural Representation of Textured Surfaces
4.4 Sensing Position and Movement of Limb Parts: Peripheral Neural Mechanisms
5 Conclusion
Figure 1.

Hands of a series of living primates illustrating stages in functional organization considered to approximate important phases of phylogenetic development. Clawed hand of tree shrew is similar to forepaw of many nonprimate mammals. In prehensile hand of lemur and tarsier the thumb and fingerpads, backed by flattened nails, have become differentiated. In macaque (Macaca nemestrina shown) the thumb has become fully opposable and the fingerpads assume an important tactile role in identification of different surfaces. In ape and man the individual fingers can now be moved independently, further increasing the precision with which handheld objects may be manipulated and examined with fingerpads. Note, however, specialized shortening of thumb in ape, which limits opposability to some extent.
Figure 2.

Diagram of skin of primate fingerpad illustrating different types of specialized nerve fiber terminals and their locations within epidermis and dermis. Also shown are glandular, adhesive, and cross ridges at interface between epidermis and dermis, and dermal plexuses of nerve fibers.
Figure 3.

Diagram of Merkel disk nerve terminal and associated Merkel cell in hairy skin of cat. A, myelinated axon; BM, basement membrane; D, desmosome; E, epithelial cell nucleus; G, granular vesicles in Merkel cell near junctional zone with Merkel disk (NP); GO, Golgi apparatus; GY, glycogen; L, lamellae beneath Merkel disk; P, cytoplasmic processes from Merkel cell.

From Iggo and Muir 120
Figure 4.

Meissner's corpuscle in ridged glabrous skin in primate. A: Masson‐stained section of corpuscle in human fingerpad. B: diagram of corpuscle, ax, Myelinated fibers terminating in corpuscle; ra, branching axon terminals within corpuscle; SC, Schwann cells; pn, cup‐shaped perineural sheath. C: reconstruction illustrating discoidal form of nerve terminals within Meissner's corpuscle in glabrous skin of vervet monkey.

A and B: from Andres and During 7; C: from Castano and Ventura 40
Figure 5.

Diagram of ridged digital skin in rhesus monkey. Top: vertical cross section. 1, Glandular epidermal ridges; 2, adhesive ridges; 3, cross ridges; 4, Meissner's corpuscles. Bottom: horizontal section; cutting plane at level of arrows in upper figure. Meissner's corpuscles, 4, are located within dermal papillae.

From Halata 97
Figure 6.

Diagram illustrating similar structure of Ruffini corpuscle and Golgi tendon organ. In each representation outer capsule has a window cut to expose fluid‐filled subcapsular space and core of nerve fiber terminals, Schwann cells, and collagen fibrils.

From Bannister 10, adapted from Chambers et al. 51 and Schoultz and Swett 208
Figure 7.

Section of Pacinian corpuscle in digital skin of rhesus monkey. Nerve fiber terminal in inner core of capsule. Subcapsular space separating core from capsule proper is indicated by arrows. × 900.

From Halata 97
Figure 8.

Distribution of Pacinian corpuscles (open circles) in radial half of index finger. Reconstruction based on serial sections of 7‐mo fetal finger.

From Cauna and Mannan 49
Figure 9.

Distribution of Pacinian corpuscles in hand of 7‐mo fetus. Corpuscles, black dots, are concentrated in thenar and hypothenar eminences, with relatively few corpuscles occurring in central part of palm or in dorsum of hand.

From Bushong 35
Figure 10.

Diagram of Merkel touch spot based on light microscopy. A, single myelinated fiber; AA, nonmyelinated nerve fiber; E, thickened epidermis; T, Merkel cells and associated Merkel disks; CF and FF, collagen fibers.

From Iggo and Muir 120
Figure 11.

Diagram of innervation of pelage hair. Left: basal lamina separating epidermal, 1, and dermal components of follicle indicated by arrows; 2, lanciform nerve endings invested with Schwann cell lamella, 3, Right: a, cross section of hair follicle; b, detail of innervation of follicle.

From Halata 97
Figure 12.

Diagram of innervation of sinus hair follicle. Three types of nerve terminals may be identified: 1, Merkel endings; 2, lanciform nerve endings; 4, paciniform Golgi‐Mazzoni corpuscles. 3 and 5, myelinated fibers innervating sinus follicle. Insets: a, detail of longitudinal section; b, detail of cross‐section.

From Halata 97
Figure 13.

A: diagram of innervation of human metacarpophalangeal joint. Joint viewed from palmar aspect with flexor tendon sheath opened and tendon dissected away. Articular capsule has been incised on dorsal aspect and opened out as two flaps (CAP.). COLL., collateral ligament of the joint; stippled area, palmar accessory ligament; AR., articular branch of palmar digital nerve. Ruffini, Pacinian, and free nerve endings are shown schematically. B: diagram of innervation of metacarpophalangeal (M‐P) and interphalangeal (I‐P) joint viewed from lateral aspect. D.D., dorsal digital nerve; P.P., palmar digital nerve; COM., communicating branch between dorsal and palmar digital nerves; AR., articular branches; FL., branch indicating flexor tendon sheath. Ruffini, Pacinian, and free nerve endings are indicated diagrammatically.

From Stilwell 221
Figure 14.

A: frequency histograms of axon diameters in sural nerve in boy aged 15 yr. Shaded zone defines distribution for unmyelinated fibers; unshaded zone defines bimodal distribution for myelinated fibers. B: comparison of distribution of myelinated fiber diameters in palmar digital nerve (radial side of index finger of macaque) with distribution of low‐threshold mechanoreceptive fibers in same nerve. Conversion factor used to match fiber diameter and fiber conduction velocity was 6.0 115. Peak of distribution of conduction velocities was equated with peak of distribution of large myelinated fibers (A‐β‐fibers) to assist with visual matching of distributions.

A: adapted from Ochoa and Mair 189; B: from Darian‐Smith and Kenins 64
Figure 15.

Responses of Meissner's (QA) and Pacinian fiber, each innervating macaque fingerpad, to indentation and vibratory stimulation of skin. Stimulus probe was 2 mm in diameter and positioned over central part of fiber's receptive field; stepwise indentation of 550 μ lasting 1,400 ms; superimposed sinusoidal stimulus lasting approximately 1,000 ms. Stimulus profile indicated by upper trace of each pair. For QA fiber, frequency of vibratory stimulus was 40 Hz with peak‐to‐peak amplitudes of 14 and 16 μm. For Pacinian fiber, frequency of sinusoid was 150 Hz with amplitudes of 16 and 19 μm. Vertical bars in second of each pair of records indicate occurrence of action potential.
Figure 16.

Receptive‐field sensitivity maps for QA fiber (A); SAI fiber (B); SAII fiber (C); and Pacinian fiber (D). Each fiber was isolated in human median nerve. Each receptive field was located, as shown, on digital or palmar glabrous skin. Isosensitivity lines define contour map of sensitivity within each receptive field. Below each contour map a section of the map (corresponding to the horizontal dotted line across the contour map) has been plotted; relative changes in sensitivity are shown. With QA fiber (A) and SAI fiber (B) the receptive field has a sharply defined border. By contrast SAII fiber (C) and Pacinian fiber (D) are both highly responsive to indentation over larger area of skin, and boundary of receptive field is poorly defined. In A and B the roughly parallel lines traversing the receptive fields indicate grooves between papillary ridges.

Data adapted from Johansson 123
Figure 17.

Responses of QA fiber innervating fingerpad in macaque to sinusoidal vibratory stimulus with frequency of 40 Hz; stimulus probe was 2 mm in diameter and located over most sensitive zone in receptive field. Intensity function relates mean number of impulses occurring within each cycle of stimulus to peak‐to‐peak amplitude of vibratory stimulus. As each segment of function was linear, relationship was fully defined by threshold measurements I0, I1, I2, and I3.

From Johnson 129
Figure 18.

Interval and cycle histograms for responses of QA fiber innervating glabrous skin of macaque hand. A sinusoidal vibratory stimulus with frequency of 40 Hz was applied to most sensitive region of fiber's receptive field with probe 2 mm in diameter. Number to right of each vertical scale indicates peak‐to‐peak amplitude of vibratory stimulus evoking response. At all stimulus intensities fiber discharge was phase locked to vibratory stimulus; this was so even when fiber responded to less than half of cycles in stimulus.

Adapted from Talbot, Darian‐Smith, et al. 226
Figure 19.

Tuning curves of QA (upper graph) and Pacinian (lower graph) fibers, illustrating their responses to sinusoidal vibratory stimuli of different frequencies. Threshold measurement used was I1, as shown in Fig. 17, which is lowest peak‐to‐peak amplitude of vibratory stimulus at which fiber responds once during every cycle of stimulus. As shown, all QA fibers have similar U‐shaped tuning curves responding optimally to vibratory stimuli with frequencies of 30-40 Hz. Likewise all Pacinian fibers have similar tuning curves, responding most readily in stimulus frequency range 250-350 Hz.

Adapted from Talbot, Darian‐Smith, et al. 226
Figure 20.

Interval (left) and cycle (right) histograms for responses of Pacinian fiber innervating macaque palmar skin. Vibratory stimulus with frequency of 200 Hz presented at most sensitive part of receptive field with probe 2 mm in diameter. Each histogram is labeled with the amplitude in μm of sinusoidal stimulus evoking response. All impulses evoked by vibratory stimulus were strictly phase locked, even when interimpulse interval was 7 or 8 times longer than cycle period of stimulus.

Adapted from Talbot, Darian‐Smith, et al. 226
Figure 21.

Responses of SA fibers innervating middle finger in macaque to stepwise indentation. Lower graph: poststimulus time histogram for single response is plotted illustrating onset transient (dynamic phase) and subsequent rapid adaptation of response during first 1.0 s of indentation. Rise time of indenting stimulus, 50 ms; amplitude of indentation, 500 μm. Upper graph: linear relationship between amplitude of indentation and cumulative impulse count over period of 2.0 s of stimulation for a second SA fiber. With stimuli of shorter duration, intensity function was also linear. Insets: location of fiber's receptive field and individual discharges of fiber.
Figure 22.

Diagram of location and sensitivity to skin stretch of SAII fibers isolated in median nerve in humans. A and B: dots indicate center of region of skin in which indentation alone elicited discharge. Solid arrows indicate direction of skin stretch that excited fibers and approximate extent of skin zone over which stretch was an effective stimulus. Stretch in direction of dotted arrows reduced spontaneous discharge in fiber. C: receptive fields activated by pressure on nail; arrows indicate direction in which pressure applied to nail was most effective.

From Johansson 123
Figure 23.

Static functional properties of sample of low‐threshold mechanoreceptive fibers isolated in radial palmar digital nerve of index finger in macaque. Left: distribution of conduction velocities of Pacinian, QA, and SA fibers in sample; receptive fields of all fibers were on ridged glabrous skin of terminal, middle, or (with Pacinian fibers) both phalanges. Right: innervation densities for skin of distal and middle phalanges for three mechanoreceptive fiber types. QA fibers had highest and Pacinian fibers lowest innervation densities. For QA and SA fibers innervation density was substantially higher for fingerpad than for skin of middle phalanx; no such gradient for Pacinian fiber was observed.

From data of Darian‐Smith and Kenins 64
Figure 24.

Comparison of human sensitivity to vibratory stimuli of different frequencies applied to hairy skin of forearm with tuning curves of large‐diameter (alpha) and small‐diameter (delta) QA fibers innervating hairy skin in macaque. Heavy line is mean vibratory threshold curve for 7 human subjects. Tuning curves of fiber indicate for each stimulus frequency minimal stimulus amplitude eliciting one impulse per cycle of sinusoidal stimulus. Delta QA fibers were far more responsive to the vibratory stimuli than alpha QA fibers.

From Merzenich and Harrington 176
Figure 25.

Response of profile of joint afferent innervating posterior part of capsule of knee joint in macaque. This fiber responded only when joint was fully extended. The stimulus evoking discharge was a ramp movement into full extension of joint (from 165° to 185° as shown in the bottom trace). Instantaneous rate of discharge in responding fiber is plotted in top record and coincident change in tension within capsular tissue is shown in middle record.

From Grigg and Greenspan 94
Figure 26.

Responses of joint afferent innervating posterior part of capsule of knee joint in macaque with joint held in extension. Left: frequency of ongoing discharge of fiber is plotted against steady angle of rotation (155°-175°). Right: fiber's discharge frequency is plotted against torque applied to knee joint.

From Grigg and Greenspan 94
Figure 27.

Responses in joint afferent innervating posterior part of capsule of cat's knee joint. Fiber normally responded only with full extension of knee joint and not with full flexion. If, in addition to full flexion, joint was abducted and externally rotated, however, fiber discharged as shown in graph. Response in flexion was elicited only when torque applied to capsule was intense and potentially noxious. Inset: relationship of femur and tibia during the experiment.

From Grigg et al. 93
Figure 28.

Top: pressure sensitivity thresholds for body surface. Threshold estimated as force applied using von Frey filament. Middle: 2‐point discrimination threshold for body surface. Calipers used, with contact area for the two tips 3.2 mm2. Bottom: point localization thresholds. Measure plotted is estimated error in localization.

From Weinstein 240
Figure 29.

Human observer's scaling of amplitude of stepwise indenting stimulus applied to fingerpad. Stimuli were 900 ms in duration and delivered every 8 s. Stimulus amplitude was varied in pseudo‐random sequence. Stimulus probe 2 mm in diameter. Three different procedures were used for scaling subject's estimate of stimulus amplitude: open triangles, scaling estimated amplitude into 1 of 15 categories; open squares, scaling estimated amplitude into one of 30 categories; and filled circles, allowing subject to select numbers corresponding to estimated amplitude relative to other stimuli in sequence. Scaling procedure did not alter form of function. Plotted data were average of observations on 10 subjects. Each subject's scaling was normalized relative to his or her mean estimate of most intense stimulus presented.

From LaMotte 152
Figure 30.

Graphs describing response profile of population of SA fibers responding to stepwise indentation of skin of macaque's fingerpad with 1‐mm probe. Indentation lasted 0.8 s. Response measures based on data from 10 SA fibers. Upper graph: surface representing average discharge rate in SA fiber as a function of intensity and time after onset of indentation. Responses to stimuli with intensities between 200 and 1,000 μm are shown. Response decay with time was not exponential. Lower graph: surface describing mean response rate within population of fibers beneath and surrounding 1‐mm probe during first 0.8 s of indentation. Amplitude of stimulus was 1,000 μm. Shape of surface illustrates that immediately after onset of indentation SA fibers were engaged over a relatively large area of skin. This zone decayed rapidly, however, so that by 0.8 s after onset of stimulus, responding SA fibers were restricted to skin immediately below probe and to a small surround. Initially many SA fibers were recruited, but many responding fibers fell off rapidly. Decay of response in fibers at a fixed interval, after stimulus onset at increasing distances from edge of indenting probe, was exponential.

From Phillips 192
Figure 31.

Response profile in time of SA fibers excited by stepwise indentation of skin of macaque's fingerpad. Indenting probe was 1 mm in diameter and amplitude of stimulus was 1,000 μm. Profile on inset: response of single fiber (data averaged from 10 SA fibers) to stimulus: probe was centrally located in fiber's receptive field. Profile on left: time course of discharge in population of SA fibers responding to same stimulus. Dynamic phase of response in SA fibers population relative to subsequent, more steady discharge was much greater than that in single fibers, illustrating, as in Fig. 30, large initial recruitment of fibers in skin surrounding probe and subsequent rapid constriction of zone containing responding fibers.

From Phillips 192
Figure 32.

Responses of SA fiber innervating macaque fingerpad to indentation by bar 3 mm wide. Indentation was stepwise, 1,000 μm in amplitude and 1,000 ms in duration. Each vertical column of dots represents discharge of fiber during indentation, each dot indicating occurrence of action potential. Successive responses were evoked after bar was translated laterally by 200 μm across fiber's receptive field. Lower histogram: response profile in space for dynamic phase of response (mean discharge rate in interval 40-70 ms after onset of indentation). Upper histogram: response profile in space for later, more static phase of response (mean discharge rate in interval 600-950 ms after stimulus onset).

From Phillips and Johnson 194
Figure 33.

Discharge profile in SA fiber innervating macaque fingerpad, responding to stepwise indentation of skin with aperiodic grating. Response profile was determined experimentally as in Fig. 32—response measure being in static phase 600-950 ms after onset of indentation, and grating being incrementally moved across fiber's receptive field between successive indentations. Three response profiles shown were generated by aperiodic gratings with bar widths of 0.5, 0.75, and 1.00 mm: responses to a 3‐mm bar were also shown at right end of each sequence. Shaded profiles, discharge profile predicted from model of mechanics of skin in which compressive strain generated within skin by indenting stimulus was considered to be mechanical determinant of fiber's response: receptor terminal of fiber was assumed to be located 0.8 mm below surface of skin; response sensitivity of fiber to be 230 impulses/s per unit of compressive strain; and threshold of fiber to be 0.1 unit of compressive strain. With these parameters experimentally determined response profile and predicted discharge matched well.

From Phillips and Johnson 195
Figure 34.

Composite figure comparing frequency‐threshold function (heavy line with vertical bars, 2 × SEM) detecting sinusoidal vibratory stimuli applied to glabrous skin of hand in macaque with I1 threshold (see Fig. 17) for individual QA and Pacinian fibers innervating same region of skin. Each dot indicates single estimate of I1 on QA or Pacinian fiber. The 2 thin lines bound response zones for QA and Pacinian fibers, respectively. Monkey's capacity to detect vibratory stimuli over frequency range 2-400 Hz depends on responses of both QA and Pacinian fiber populations: QA fibers alone determine performance in frequency range 2-60 Hz, whereas Pacinian fibers alone account for detecting vibratory stimuli in frequency range 80-400 Hz.

From Mountcastle et al. 179
Figure 35.

Comparison of detection and frequency discrimination of vibratory stimulus applied to palmar skin of hand with responses of QA fibers innervating skin. Frequencies of sinusoidal stimuli used were centered about 30 Hz. Detection of occurrence of 30‐Hz stimulus in both man and macaque is defined by psychometric functions on left. Curves on right relate “net correct score” for frequency discrimination to stimulus amplitude. Shaded area is stimulus range bounded by I0 and I1 thresholds as defined in Fig. 17 for sample of QA fibers innervating monkey's palmar skin. Horizontal dashed line defines psychophysical threshold for both detection and discrimination tasks (50% correct scores). Stimulus amplitudes defined relative to psychophysical detection threshold.

From LaMotte and Mountcastle 153
Figure 36.

Relation of human observer's estimate of magnitude of sinusoidal vibratory stimulus to actual amplitude of stimulus. Vibratory stimulus applied to fingerpad of middle finger with probe 2 mm in diameter; frequency was 40 Hz, and peak‐to‐peak amplitude of stimulus was varied from 0-400 μm. Vertical bars, 6 × SEM of normalized subjective estimate. Dashed line is best‐fitting linear regression function.

From Talbot, Darian‐Smith, et al. 226
Figure 37.

Profiles of mean impulse frequency in QA fibers innervating palmar skin of macaque in response to sinusoidal vibratory stimulus with frequency of 40 Hz and successively increasing amplitude of 30, 60, 100, and 200 μm. Fiber discharge frequency averaged over interval of 25 ms. Maximum discharge frequencies occurred in fibers when most sensitive zone in receptive fields was directly beneath probe tip (diam 2 mm). There was rapid falloff in the discharge frequency in fibers innervating skin surrounding immediate contact zone with oscillating probe. Curves illustrate both intensive and spatial recruitment of QA fiber population.

From Johnson 129
Figure 38.

Profile of mean impulse frequency in QA fibers innervating macaque's palmar skin in response to sinusoidal vibratory stimulus of 40 Hz. Tip of stimulus probe, 2 mm; amplitude of sinusoid, 200 μm. Response frequency estimated over integration interval of 1 ms (cf. Fig. 37). Each profile illustrates relative discharge frequencies in fibers with receptive fields successively distant from center of vibrating probe. Consecutive profiles trace changes in response during single cycle of sinusoidal stimulus: phase increment between consecutive profiles was 30°. Averaging of these instantaneous profiles yields the 200‐μm curve of Fig. 37. As seen from development of 2nd response peak at phase angles greater than 180°, some fibers in averaged sample discharged twice during each cycle of stimulus.

From Johnson 129
Figure 39.

Profile of averaged discharge frequency in population of QA fibers innervating macaque's palmar skin to pair of vibrating probes. Diameter of each probe, 1 mm; separation of probes, 1 mm between edges; frequency of sinusoidal vibration, 40 Hz; amplitude of sinusoid, 47 μm. Coordinates of centers of probes were (0, −1) and (0, +1). Average response frequency at 2 peaks was 39 impulses/s and at the trough was 15 impulses/s.

From Goodwin et al. 84
Figure 40.

Discrimination by human subject of incremental changes in spatial period of rigid grating with cycle profile similar to that in Fig. 41. Upper graph: data obtained from high‐speed photography of moving finger as subject explored each surface with to‐and‐fro movement. Contact force between fingerpad and surface also fluctuated in phase with finger movement. Lower graph: psychometric functions relate subject's discrimination of incremental (right side) or decremental (left side) changes in spatial period of grating. Subject presented with sets of 3 surfaces, 2 of which were identical, was required to identify odd surface. Mean probability of correctly selecting the different surface is plotted against change in spatial period of grating (expressed as percentage of spatial period of 770 μm). Each datum point is mean of 140 observations (±SEM). Continuous line is psychometric function with observer selecting movement pattern of finger; broken line defines performance when imposed‐movement profile was sinusoidal with peak velocity of 145 mm/s.

From unpublished data of G. Casper, I. Darian‐Smith, and A. W. Goodwin
Figure 41.

Responses of QA fiber innervating fingerpad of index finger in macaque to gratings moving across skin. A: profile of grating and method of presenting moving surface to fingerpad. Revolving drum could be lowered onto and raised from skin automatically: velocity, contact force, and direction of movement across skin could be regulated and a range of gratings presented in rapid succession. B: responses in QA fiber during first 1,500 ms of contact with grating moving across fingerpad at velocity of 32 mm/s: spatial period of grating, 870 μm; contact force, 60 g · wt. Each horizontal sequence of dots indicates succession of action potentials during single passage of surface; 14 successive responses are shown. C: response of QA fiber to different gratings, each moving across fingerpad with velocity of 72 mm/s; contact force 60 g · wt. The spatial period of each grating is indicated at right. Response display for each surface was generated as in B, but only an 80‐ms segment of response is shown. At left the close correspondence between temporal period of stimulus and mean interspike interval of the fiber response is apparent.

Data from Darian‐Smith and Oke 65
Figure 42.

Responses of QA fiber to 3 different gratings (spatial period of 1,025, 790, and 540 μm) moving across receptive field at 3 different velocities (22, 66, and 142 mm/s). Fiber's receptive field on fingerpad of index finger. Radial force, 60 g · wt; contact area, approximately 5 × 5 mm. Each response block is segment of response beginning approximately 500 ms after onset of stimulation. Stimulus temporal frequency indicated by vertical bars above each response block; its numerical value stated below the block. Response frequency accurately reflects stimulus frequency in range 64-140 Hz. At frequencies <64 Hz, stimulus temporal frequency was represented in modulation of discharge but not in mean discharge frequency; at stimulus temporal frequencies >140 Hz, although response was phase‐locked to stimulus, fiber did not respond to each successive cycle of stimulus and hence mean discharge frequency did not equal stimulus temporal frequency.

From Darian‐Smith and Oke 65
Figure 43.

Relationship of response modulation pattern of stimulus temporal frequency for 3 fiber types (SA, QA, and Pacinian fiber) innervating ridged glabrous skin of macaque's fingerpad. Contact force was 60 g · wt. Each response was first categorized in terms of whether stimulus temporal frequency was identifiably represented in response modulation present. When this relationship was absent response was classified as nonmodulated (black). Modulated responses in which stimulus temporal frequency was identifiable from modulation frequency in evoked response were grouped according to number of impulses phase‐locked to each stimulus period. Simplest pattern was when 1 impulse occurred in phase with each stimulus temporal cycle (1:1, no shading). Other categories: 2 impulses per stimulation cycle (2:1, vertical lines); more than 2 impulses per stimulation cycle (>2:1, dots); less than 1 impulse per cycle (<1:1, horizontal lines). Nonmodulated responses were split into two groups, depending on whether mean discharge rate was greater or less than stimulus temporal frequency: the former subgroup, when presented, is located in upper left corner of each diagram and the latter in lower right corner. Because the total responses examined with each stimulus frequency category varied greatly, percentage estimates of responses are approximate; nevertheless, trends in response patterns are clearly demonstrated.

From Darian‐Smith and Oke 65
Figure 44.

Representation in responding population of digital nerve fibers of spatial and temporal features of grating moving across skin with fixed velocity and fixed contact force. Skin surface is horizontal plane above moving grating. Receptive fields of individual fibers are shown schematically: commonly more than one ridge of grating would lie within limits of fiber's receptive field at any one instant. Each dot indicates occurrence of action potential: impulses along any vertical axis represent successive impulses in discharge of an individual fiber. Impulses in neighboring fibers evoked by moving grating generate response planes or traveling waves of activity that sweep across receptors sheet with velocity of grating. Successive response planes are separated by a mean time interval equal to temporal period of stimulus. In illustration all fibers are assumed functionally identical. In natural population of QA fibers, for example, this would not be so: response plane would be statistically defined with finite uncertainty.

From Darian‐Smith and Oke 65
Figure 45.

Diagram of dot patterns differentiated by touching with fingerpad. Test surface in each experiment was square pattern of elevated dots, each 0.67 mm in diameter with centers 2.00 mm apart. Expt. A: surfaces to be differentiated from test surface differed only in spacing of dots along direction of movement of finger. Expt. B: incremental change in spacing of dots was at right angles to direction of movement of finger. Experiments were done both with direction of movement parallel to axis of finger (as shown) and at right angles to axis. Subjects could correctly identify incremental changes in spacing of dots with probability of 0.75 if change were 2%-4% of spacing between dots, i.e., in range 40-80 μm.

From G. D. Lamb, unpublished observations
Figure 46.

Experimental procedure for specifying response of single cutaneous mechanoreceptive fiber to 2‐dimensional surface moving across fiber's receptive field. Surface traversed skin field n times in all. With each traverse the instantaneous position of moving surface relative to skin was defined and could be directly related to occurrence of each action potential in fiber's discharge. Successive traverses of surface across fiber's receptive field were identical, except that between successive scans nyloprint surface was displaced laterally relative to axis of movement by increment of 0.125 mm. Stimulus succession is illustrated schematically on left. Display of fiber responses to each successive stimulus is shown on right: each dot indicates occurrence of single action potential.

From Darian‐Smith et al. 61
Figure 47.

Series of response patterns of a single slowly adapting fiber to 7 different dot patterns (in rows) moving across skin at 3 velocities of 40, 80, and 150 mm/s, respectively (in columns). Contact force between moving surface and underlying skin was 60 g · wt. Neural response pattern for each stimulus combination was constructed as shown in Fig. 45. Abscissa of each response block was relative to moving surface: hence with an increasing velocity of moving surface, time scale was correspondingly expanded.

From Darian‐Smith et al. 61
Figure 48.

Diagram of representation in population of uniformly responding digital nerve fibers of spatial features of dot pattern moving across skin. Surface is moving to right at constant velocity. Skin surface is horizontal plane above patterned surface. Large open circles, receptive fields of individual fibers; vertical axis, dimension of time; each small black dot, occurrence of single action potential in fiber. Each dot in moving surface engages assembly of fibers whose discharge defines response cylinder with both spatial and temporal dimensions. Slope of cylinder depends solely on velocity with which surface sweeps across skin. Separation in time of adjacent response cylinders depends on spatial separation of dots on surface and velocity of moving surface.

From Darian‐Smith et al. 61
Figure 49.

Series of response patterns in SA, QA, and Pacinian fiber, each innervating macaque fingerpad, to passage of Braille‐like pattern of dots across skin. Braille‐dot patterns on left; diameter of dot, 1.2 mm, with separation drawn to scale. Response patterns generated as illustrated in Fig. 45, with direction of movement of surface from bottom to top of figure. Velocity of moving surface, 40 mm/s. Contact forces of 60 g · wt for SA fiber and 20 g · wt for QA and Pacinian fibers were selected for optimum pattern resolution.

From Johnson and Lamb 134
Figure 50.

Performance of human subject detecting 10° displacement of distal interphalangeal joint of middle finger at various angular velocities. Scores indicate number of displacements correctly identified in 10 observations. Three conditions were examined: the first 2 were averaged responses for 7 subjects, with SE indicated by vertical bars; filled circles, normal movement of joint, with input to central nervous system from joint, skin, and muscle afferents; open circles, with input from joint and skin afferents only. The 3rd condition, crosses, were individual responses with input from muscle afferents only, following digital nerve block with local anesthesia. Individual responses illustrate the great variability in performance. No one set of afferents could account for normal performance.

From Gandevia and McCloskey 76


Figure 1.

Hands of a series of living primates illustrating stages in functional organization considered to approximate important phases of phylogenetic development. Clawed hand of tree shrew is similar to forepaw of many nonprimate mammals. In prehensile hand of lemur and tarsier the thumb and fingerpads, backed by flattened nails, have become differentiated. In macaque (Macaca nemestrina shown) the thumb has become fully opposable and the fingerpads assume an important tactile role in identification of different surfaces. In ape and man the individual fingers can now be moved independently, further increasing the precision with which handheld objects may be manipulated and examined with fingerpads. Note, however, specialized shortening of thumb in ape, which limits opposability to some extent.



Figure 2.

Diagram of skin of primate fingerpad illustrating different types of specialized nerve fiber terminals and their locations within epidermis and dermis. Also shown are glandular, adhesive, and cross ridges at interface between epidermis and dermis, and dermal plexuses of nerve fibers.



Figure 3.

Diagram of Merkel disk nerve terminal and associated Merkel cell in hairy skin of cat. A, myelinated axon; BM, basement membrane; D, desmosome; E, epithelial cell nucleus; G, granular vesicles in Merkel cell near junctional zone with Merkel disk (NP); GO, Golgi apparatus; GY, glycogen; L, lamellae beneath Merkel disk; P, cytoplasmic processes from Merkel cell.

From Iggo and Muir 120


Figure 4.

Meissner's corpuscle in ridged glabrous skin in primate. A: Masson‐stained section of corpuscle in human fingerpad. B: diagram of corpuscle, ax, Myelinated fibers terminating in corpuscle; ra, branching axon terminals within corpuscle; SC, Schwann cells; pn, cup‐shaped perineural sheath. C: reconstruction illustrating discoidal form of nerve terminals within Meissner's corpuscle in glabrous skin of vervet monkey.

A and B: from Andres and During 7; C: from Castano and Ventura 40


Figure 5.

Diagram of ridged digital skin in rhesus monkey. Top: vertical cross section. 1, Glandular epidermal ridges; 2, adhesive ridges; 3, cross ridges; 4, Meissner's corpuscles. Bottom: horizontal section; cutting plane at level of arrows in upper figure. Meissner's corpuscles, 4, are located within dermal papillae.

From Halata 97


Figure 6.

Diagram illustrating similar structure of Ruffini corpuscle and Golgi tendon organ. In each representation outer capsule has a window cut to expose fluid‐filled subcapsular space and core of nerve fiber terminals, Schwann cells, and collagen fibrils.

From Bannister 10, adapted from Chambers et al. 51 and Schoultz and Swett 208


Figure 7.

Section of Pacinian corpuscle in digital skin of rhesus monkey. Nerve fiber terminal in inner core of capsule. Subcapsular space separating core from capsule proper is indicated by arrows. × 900.

From Halata 97


Figure 8.

Distribution of Pacinian corpuscles (open circles) in radial half of index finger. Reconstruction based on serial sections of 7‐mo fetal finger.

From Cauna and Mannan 49


Figure 9.

Distribution of Pacinian corpuscles in hand of 7‐mo fetus. Corpuscles, black dots, are concentrated in thenar and hypothenar eminences, with relatively few corpuscles occurring in central part of palm or in dorsum of hand.

From Bushong 35


Figure 10.

Diagram of Merkel touch spot based on light microscopy. A, single myelinated fiber; AA, nonmyelinated nerve fiber; E, thickened epidermis; T, Merkel cells and associated Merkel disks; CF and FF, collagen fibers.

From Iggo and Muir 120


Figure 11.

Diagram of innervation of pelage hair. Left: basal lamina separating epidermal, 1, and dermal components of follicle indicated by arrows; 2, lanciform nerve endings invested with Schwann cell lamella, 3, Right: a, cross section of hair follicle; b, detail of innervation of follicle.

From Halata 97


Figure 12.

Diagram of innervation of sinus hair follicle. Three types of nerve terminals may be identified: 1, Merkel endings; 2, lanciform nerve endings; 4, paciniform Golgi‐Mazzoni corpuscles. 3 and 5, myelinated fibers innervating sinus follicle. Insets: a, detail of longitudinal section; b, detail of cross‐section.

From Halata 97


Figure 13.

A: diagram of innervation of human metacarpophalangeal joint. Joint viewed from palmar aspect with flexor tendon sheath opened and tendon dissected away. Articular capsule has been incised on dorsal aspect and opened out as two flaps (CAP.). COLL., collateral ligament of the joint; stippled area, palmar accessory ligament; AR., articular branch of palmar digital nerve. Ruffini, Pacinian, and free nerve endings are shown schematically. B: diagram of innervation of metacarpophalangeal (M‐P) and interphalangeal (I‐P) joint viewed from lateral aspect. D.D., dorsal digital nerve; P.P., palmar digital nerve; COM., communicating branch between dorsal and palmar digital nerves; AR., articular branches; FL., branch indicating flexor tendon sheath. Ruffini, Pacinian, and free nerve endings are indicated diagrammatically.

From Stilwell 221


Figure 14.

A: frequency histograms of axon diameters in sural nerve in boy aged 15 yr. Shaded zone defines distribution for unmyelinated fibers; unshaded zone defines bimodal distribution for myelinated fibers. B: comparison of distribution of myelinated fiber diameters in palmar digital nerve (radial side of index finger of macaque) with distribution of low‐threshold mechanoreceptive fibers in same nerve. Conversion factor used to match fiber diameter and fiber conduction velocity was 6.0 115. Peak of distribution of conduction velocities was equated with peak of distribution of large myelinated fibers (A‐β‐fibers) to assist with visual matching of distributions.

A: adapted from Ochoa and Mair 189; B: from Darian‐Smith and Kenins 64


Figure 15.

Responses of Meissner's (QA) and Pacinian fiber, each innervating macaque fingerpad, to indentation and vibratory stimulation of skin. Stimulus probe was 2 mm in diameter and positioned over central part of fiber's receptive field; stepwise indentation of 550 μ lasting 1,400 ms; superimposed sinusoidal stimulus lasting approximately 1,000 ms. Stimulus profile indicated by upper trace of each pair. For QA fiber, frequency of vibratory stimulus was 40 Hz with peak‐to‐peak amplitudes of 14 and 16 μm. For Pacinian fiber, frequency of sinusoid was 150 Hz with amplitudes of 16 and 19 μm. Vertical bars in second of each pair of records indicate occurrence of action potential.



Figure 16.

Receptive‐field sensitivity maps for QA fiber (A); SAI fiber (B); SAII fiber (C); and Pacinian fiber (D). Each fiber was isolated in human median nerve. Each receptive field was located, as shown, on digital or palmar glabrous skin. Isosensitivity lines define contour map of sensitivity within each receptive field. Below each contour map a section of the map (corresponding to the horizontal dotted line across the contour map) has been plotted; relative changes in sensitivity are shown. With QA fiber (A) and SAI fiber (B) the receptive field has a sharply defined border. By contrast SAII fiber (C) and Pacinian fiber (D) are both highly responsive to indentation over larger area of skin, and boundary of receptive field is poorly defined. In A and B the roughly parallel lines traversing the receptive fields indicate grooves between papillary ridges.

Data adapted from Johansson 123


Figure 17.

Responses of QA fiber innervating fingerpad in macaque to sinusoidal vibratory stimulus with frequency of 40 Hz; stimulus probe was 2 mm in diameter and located over most sensitive zone in receptive field. Intensity function relates mean number of impulses occurring within each cycle of stimulus to peak‐to‐peak amplitude of vibratory stimulus. As each segment of function was linear, relationship was fully defined by threshold measurements I0, I1, I2, and I3.

From Johnson 129


Figure 18.

Interval and cycle histograms for responses of QA fiber innervating glabrous skin of macaque hand. A sinusoidal vibratory stimulus with frequency of 40 Hz was applied to most sensitive region of fiber's receptive field with probe 2 mm in diameter. Number to right of each vertical scale indicates peak‐to‐peak amplitude of vibratory stimulus evoking response. At all stimulus intensities fiber discharge was phase locked to vibratory stimulus; this was so even when fiber responded to less than half of cycles in stimulus.

Adapted from Talbot, Darian‐Smith, et al. 226


Figure 19.

Tuning curves of QA (upper graph) and Pacinian (lower graph) fibers, illustrating their responses to sinusoidal vibratory stimuli of different frequencies. Threshold measurement used was I1, as shown in Fig. 17, which is lowest peak‐to‐peak amplitude of vibratory stimulus at which fiber responds once during every cycle of stimulus. As shown, all QA fibers have similar U‐shaped tuning curves responding optimally to vibratory stimuli with frequencies of 30-40 Hz. Likewise all Pacinian fibers have similar tuning curves, responding most readily in stimulus frequency range 250-350 Hz.

Adapted from Talbot, Darian‐Smith, et al. 226


Figure 20.

Interval (left) and cycle (right) histograms for responses of Pacinian fiber innervating macaque palmar skin. Vibratory stimulus with frequency of 200 Hz presented at most sensitive part of receptive field with probe 2 mm in diameter. Each histogram is labeled with the amplitude in μm of sinusoidal stimulus evoking response. All impulses evoked by vibratory stimulus were strictly phase locked, even when interimpulse interval was 7 or 8 times longer than cycle period of stimulus.

Adapted from Talbot, Darian‐Smith, et al. 226


Figure 21.

Responses of SA fibers innervating middle finger in macaque to stepwise indentation. Lower graph: poststimulus time histogram for single response is plotted illustrating onset transient (dynamic phase) and subsequent rapid adaptation of response during first 1.0 s of indentation. Rise time of indenting stimulus, 50 ms; amplitude of indentation, 500 μm. Upper graph: linear relationship between amplitude of indentation and cumulative impulse count over period of 2.0 s of stimulation for a second SA fiber. With stimuli of shorter duration, intensity function was also linear. Insets: location of fiber's receptive field and individual discharges of fiber.



Figure 22.

Diagram of location and sensitivity to skin stretch of SAII fibers isolated in median nerve in humans. A and B: dots indicate center of region of skin in which indentation alone elicited discharge. Solid arrows indicate direction of skin stretch that excited fibers and approximate extent of skin zone over which stretch was an effective stimulus. Stretch in direction of dotted arrows reduced spontaneous discharge in fiber. C: receptive fields activated by pressure on nail; arrows indicate direction in which pressure applied to nail was most effective.

From Johansson 123


Figure 23.

Static functional properties of sample of low‐threshold mechanoreceptive fibers isolated in radial palmar digital nerve of index finger in macaque. Left: distribution of conduction velocities of Pacinian, QA, and SA fibers in sample; receptive fields of all fibers were on ridged glabrous skin of terminal, middle, or (with Pacinian fibers) both phalanges. Right: innervation densities for skin of distal and middle phalanges for three mechanoreceptive fiber types. QA fibers had highest and Pacinian fibers lowest innervation densities. For QA and SA fibers innervation density was substantially higher for fingerpad than for skin of middle phalanx; no such gradient for Pacinian fiber was observed.

From data of Darian‐Smith and Kenins 64


Figure 24.

Comparison of human sensitivity to vibratory stimuli of different frequencies applied to hairy skin of forearm with tuning curves of large‐diameter (alpha) and small‐diameter (delta) QA fibers innervating hairy skin in macaque. Heavy line is mean vibratory threshold curve for 7 human subjects. Tuning curves of fiber indicate for each stimulus frequency minimal stimulus amplitude eliciting one impulse per cycle of sinusoidal stimulus. Delta QA fibers were far more responsive to the vibratory stimuli than alpha QA fibers.

From Merzenich and Harrington 176


Figure 25.

Response of profile of joint afferent innervating posterior part of capsule of knee joint in macaque. This fiber responded only when joint was fully extended. The stimulus evoking discharge was a ramp movement into full extension of joint (from 165° to 185° as shown in the bottom trace). Instantaneous rate of discharge in responding fiber is plotted in top record and coincident change in tension within capsular tissue is shown in middle record.

From Grigg and Greenspan 94


Figure 26.

Responses of joint afferent innervating posterior part of capsule of knee joint in macaque with joint held in extension. Left: frequency of ongoing discharge of fiber is plotted against steady angle of rotation (155°-175°). Right: fiber's discharge frequency is plotted against torque applied to knee joint.

From Grigg and Greenspan 94


Figure 27.

Responses in joint afferent innervating posterior part of capsule of cat's knee joint. Fiber normally responded only with full extension of knee joint and not with full flexion. If, in addition to full flexion, joint was abducted and externally rotated, however, fiber discharged as shown in graph. Response in flexion was elicited only when torque applied to capsule was intense and potentially noxious. Inset: relationship of femur and tibia during the experiment.

From Grigg et al. 93


Figure 28.

Top: pressure sensitivity thresholds for body surface. Threshold estimated as force applied using von Frey filament. Middle: 2‐point discrimination threshold for body surface. Calipers used, with contact area for the two tips 3.2 mm2. Bottom: point localization thresholds. Measure plotted is estimated error in localization.

From Weinstein 240


Figure 29.

Human observer's scaling of amplitude of stepwise indenting stimulus applied to fingerpad. Stimuli were 900 ms in duration and delivered every 8 s. Stimulus amplitude was varied in pseudo‐random sequence. Stimulus probe 2 mm in diameter. Three different procedures were used for scaling subject's estimate of stimulus amplitude: open triangles, scaling estimated amplitude into 1 of 15 categories; open squares, scaling estimated amplitude into one of 30 categories; and filled circles, allowing subject to select numbers corresponding to estimated amplitude relative to other stimuli in sequence. Scaling procedure did not alter form of function. Plotted data were average of observations on 10 subjects. Each subject's scaling was normalized relative to his or her mean estimate of most intense stimulus presented.

From LaMotte 152


Figure 30.

Graphs describing response profile of population of SA fibers responding to stepwise indentation of skin of macaque's fingerpad with 1‐mm probe. Indentation lasted 0.8 s. Response measures based on data from 10 SA fibers. Upper graph: surface representing average discharge rate in SA fiber as a function of intensity and time after onset of indentation. Responses to stimuli with intensities between 200 and 1,000 μm are shown. Response decay with time was not exponential. Lower graph: surface describing mean response rate within population of fibers beneath and surrounding 1‐mm probe during first 0.8 s of indentation. Amplitude of stimulus was 1,000 μm. Shape of surface illustrates that immediately after onset of indentation SA fibers were engaged over a relatively large area of skin. This zone decayed rapidly, however, so that by 0.8 s after onset of stimulus, responding SA fibers were restricted to skin immediately below probe and to a small surround. Initially many SA fibers were recruited, but many responding fibers fell off rapidly. Decay of response in fibers at a fixed interval, after stimulus onset at increasing distances from edge of indenting probe, was exponential.

From Phillips 192


Figure 31.

Response profile in time of SA fibers excited by stepwise indentation of skin of macaque's fingerpad. Indenting probe was 1 mm in diameter and amplitude of stimulus was 1,000 μm. Profile on inset: response of single fiber (data averaged from 10 SA fibers) to stimulus: probe was centrally located in fiber's receptive field. Profile on left: time course of discharge in population of SA fibers responding to same stimulus. Dynamic phase of response in SA fibers population relative to subsequent, more steady discharge was much greater than that in single fibers, illustrating, as in Fig. 30, large initial recruitment of fibers in skin surrounding probe and subsequent rapid constriction of zone containing responding fibers.

From Phillips 192


Figure 32.

Responses of SA fiber innervating macaque fingerpad to indentation by bar 3 mm wide. Indentation was stepwise, 1,000 μm in amplitude and 1,000 ms in duration. Each vertical column of dots represents discharge of fiber during indentation, each dot indicating occurrence of action potential. Successive responses were evoked after bar was translated laterally by 200 μm across fiber's receptive field. Lower histogram: response profile in space for dynamic phase of response (mean discharge rate in interval 40-70 ms after onset of indentation). Upper histogram: response profile in space for later, more static phase of response (mean discharge rate in interval 600-950 ms after stimulus onset).

From Phillips and Johnson 194


Figure 33.

Discharge profile in SA fiber innervating macaque fingerpad, responding to stepwise indentation of skin with aperiodic grating. Response profile was determined experimentally as in Fig. 32—response measure being in static phase 600-950 ms after onset of indentation, and grating being incrementally moved across fiber's receptive field between successive indentations. Three response profiles shown were generated by aperiodic gratings with bar widths of 0.5, 0.75, and 1.00 mm: responses to a 3‐mm bar were also shown at right end of each sequence. Shaded profiles, discharge profile predicted from model of mechanics of skin in which compressive strain generated within skin by indenting stimulus was considered to be mechanical determinant of fiber's response: receptor terminal of fiber was assumed to be located 0.8 mm below surface of skin; response sensitivity of fiber to be 230 impulses/s per unit of compressive strain; and threshold of fiber to be 0.1 unit of compressive strain. With these parameters experimentally determined response profile and predicted discharge matched well.

From Phillips and Johnson 195


Figure 34.

Composite figure comparing frequency‐threshold function (heavy line with vertical bars, 2 × SEM) detecting sinusoidal vibratory stimuli applied to glabrous skin of hand in macaque with I1 threshold (see Fig. 17) for individual QA and Pacinian fibers innervating same region of skin. Each dot indicates single estimate of I1 on QA or Pacinian fiber. The 2 thin lines bound response zones for QA and Pacinian fibers, respectively. Monkey's capacity to detect vibratory stimuli over frequency range 2-400 Hz depends on responses of both QA and Pacinian fiber populations: QA fibers alone determine performance in frequency range 2-60 Hz, whereas Pacinian fibers alone account for detecting vibratory stimuli in frequency range 80-400 Hz.

From Mountcastle et al. 179


Figure 35.

Comparison of detection and frequency discrimination of vibratory stimulus applied to palmar skin of hand with responses of QA fibers innervating skin. Frequencies of sinusoidal stimuli used were centered about 30 Hz. Detection of occurrence of 30‐Hz stimulus in both man and macaque is defined by psychometric functions on left. Curves on right relate “net correct score” for frequency discrimination to stimulus amplitude. Shaded area is stimulus range bounded by I0 and I1 thresholds as defined in Fig. 17 for sample of QA fibers innervating monkey's palmar skin. Horizontal dashed line defines psychophysical threshold for both detection and discrimination tasks (50% correct scores). Stimulus amplitudes defined relative to psychophysical detection threshold.

From LaMotte and Mountcastle 153


Figure 36.

Relation of human observer's estimate of magnitude of sinusoidal vibratory stimulus to actual amplitude of stimulus. Vibratory stimulus applied to fingerpad of middle finger with probe 2 mm in diameter; frequency was 40 Hz, and peak‐to‐peak amplitude of stimulus was varied from 0-400 μm. Vertical bars, 6 × SEM of normalized subjective estimate. Dashed line is best‐fitting linear regression function.

From Talbot, Darian‐Smith, et al. 226


Figure 37.

Profiles of mean impulse frequency in QA fibers innervating palmar skin of macaque in response to sinusoidal vibratory stimulus with frequency of 40 Hz and successively increasing amplitude of 30, 60, 100, and 200 μm. Fiber discharge frequency averaged over interval of 25 ms. Maximum discharge frequencies occurred in fibers when most sensitive zone in receptive fields was directly beneath probe tip (diam 2 mm). There was rapid falloff in the discharge frequency in fibers innervating skin surrounding immediate contact zone with oscillating probe. Curves illustrate both intensive and spatial recruitment of QA fiber population.

From Johnson 129


Figure 38.

Profile of mean impulse frequency in QA fibers innervating macaque's palmar skin in response to sinusoidal vibratory stimulus of 40 Hz. Tip of stimulus probe, 2 mm; amplitude of sinusoid, 200 μm. Response frequency estimated over integration interval of 1 ms (cf. Fig. 37). Each profile illustrates relative discharge frequencies in fibers with receptive fields successively distant from center of vibrating probe. Consecutive profiles trace changes in response during single cycle of sinusoidal stimulus: phase increment between consecutive profiles was 30°. Averaging of these instantaneous profiles yields the 200‐μm curve of Fig. 37. As seen from development of 2nd response peak at phase angles greater than 180°, some fibers in averaged sample discharged twice during each cycle of stimulus.

From Johnson 129


Figure 39.

Profile of averaged discharge frequency in population of QA fibers innervating macaque's palmar skin to pair of vibrating probes. Diameter of each probe, 1 mm; separation of probes, 1 mm between edges; frequency of sinusoidal vibration, 40 Hz; amplitude of sinusoid, 47 μm. Coordinates of centers of probes were (0, −1) and (0, +1). Average response frequency at 2 peaks was 39 impulses/s and at the trough was 15 impulses/s.

From Goodwin et al. 84


Figure 40.

Discrimination by human subject of incremental changes in spatial period of rigid grating with cycle profile similar to that in Fig. 41. Upper graph: data obtained from high‐speed photography of moving finger as subject explored each surface with to‐and‐fro movement. Contact force between fingerpad and surface also fluctuated in phase with finger movement. Lower graph: psychometric functions relate subject's discrimination of incremental (right side) or decremental (left side) changes in spatial period of grating. Subject presented with sets of 3 surfaces, 2 of which were identical, was required to identify odd surface. Mean probability of correctly selecting the different surface is plotted against change in spatial period of grating (expressed as percentage of spatial period of 770 μm). Each datum point is mean of 140 observations (±SEM). Continuous line is psychometric function with observer selecting movement pattern of finger; broken line defines performance when imposed‐movement profile was sinusoidal with peak velocity of 145 mm/s.

From unpublished data of G. Casper, I. Darian‐Smith, and A. W. Goodwin


Figure 41.

Responses of QA fiber innervating fingerpad of index finger in macaque to gratings moving across skin. A: profile of grating and method of presenting moving surface to fingerpad. Revolving drum could be lowered onto and raised from skin automatically: velocity, contact force, and direction of movement across skin could be regulated and a range of gratings presented in rapid succession. B: responses in QA fiber during first 1,500 ms of contact with grating moving across fingerpad at velocity of 32 mm/s: spatial period of grating, 870 μm; contact force, 60 g · wt. Each horizontal sequence of dots indicates succession of action potentials during single passage of surface; 14 successive responses are shown. C: response of QA fiber to different gratings, each moving across fingerpad with velocity of 72 mm/s; contact force 60 g · wt. The spatial period of each grating is indicated at right. Response display for each surface was generated as in B, but only an 80‐ms segment of response is shown. At left the close correspondence between temporal period of stimulus and mean interspike interval of the fiber response is apparent.

Data from Darian‐Smith and Oke 65


Figure 42.

Responses of QA fiber to 3 different gratings (spatial period of 1,025, 790, and 540 μm) moving across receptive field at 3 different velocities (22, 66, and 142 mm/s). Fiber's receptive field on fingerpad of index finger. Radial force, 60 g · wt; contact area, approximately 5 × 5 mm. Each response block is segment of response beginning approximately 500 ms after onset of stimulation. Stimulus temporal frequency indicated by vertical bars above each response block; its numerical value stated below the block. Response frequency accurately reflects stimulus frequency in range 64-140 Hz. At frequencies <64 Hz, stimulus temporal frequency was represented in modulation of discharge but not in mean discharge frequency; at stimulus temporal frequencies >140 Hz, although response was phase‐locked to stimulus, fiber did not respond to each successive cycle of stimulus and hence mean discharge frequency did not equal stimulus temporal frequency.

From Darian‐Smith and Oke 65


Figure 43.

Relationship of response modulation pattern of stimulus temporal frequency for 3 fiber types (SA, QA, and Pacinian fiber) innervating ridged glabrous skin of macaque's fingerpad. Contact force was 60 g · wt. Each response was first categorized in terms of whether stimulus temporal frequency was identifiably represented in response modulation present. When this relationship was absent response was classified as nonmodulated (black). Modulated responses in which stimulus temporal frequency was identifiable from modulation frequency in evoked response were grouped according to number of impulses phase‐locked to each stimulus period. Simplest pattern was when 1 impulse occurred in phase with each stimulus temporal cycle (1:1, no shading). Other categories: 2 impulses per stimulation cycle (2:1, vertical lines); more than 2 impulses per stimulation cycle (>2:1, dots); less than 1 impulse per cycle (<1:1, horizontal lines). Nonmodulated responses were split into two groups, depending on whether mean discharge rate was greater or less than stimulus temporal frequency: the former subgroup, when presented, is located in upper left corner of each diagram and the latter in lower right corner. Because the total responses examined with each stimulus frequency category varied greatly, percentage estimates of responses are approximate; nevertheless, trends in response patterns are clearly demonstrated.

From Darian‐Smith and Oke 65


Figure 44.

Representation in responding population of digital nerve fibers of spatial and temporal features of grating moving across skin with fixed velocity and fixed contact force. Skin surface is horizontal plane above moving grating. Receptive fields of individual fibers are shown schematically: commonly more than one ridge of grating would lie within limits of fiber's receptive field at any one instant. Each dot indicates occurrence of action potential: impulses along any vertical axis represent successive impulses in discharge of an individual fiber. Impulses in neighboring fibers evoked by moving grating generate response planes or traveling waves of activity that sweep across receptors sheet with velocity of grating. Successive response planes are separated by a mean time interval equal to temporal period of stimulus. In illustration all fibers are assumed functionally identical. In natural population of QA fibers, for example, this would not be so: response plane would be statistically defined with finite uncertainty.

From Darian‐Smith and Oke 65


Figure 45.

Diagram of dot patterns differentiated by touching with fingerpad. Test surface in each experiment was square pattern of elevated dots, each 0.67 mm in diameter with centers 2.00 mm apart. Expt. A: surfaces to be differentiated from test surface differed only in spacing of dots along direction of movement of finger. Expt. B: incremental change in spacing of dots was at right angles to direction of movement of finger. Experiments were done both with direction of movement parallel to axis of finger (as shown) and at right angles to axis. Subjects could correctly identify incremental changes in spacing of dots with probability of 0.75 if change were 2%-4% of spacing between dots, i.e., in range 40-80 μm.

From G. D. Lamb, unpublished observations


Figure 46.

Experimental procedure for specifying response of single cutaneous mechanoreceptive fiber to 2‐dimensional surface moving across fiber's receptive field. Surface traversed skin field n times in all. With each traverse the instantaneous position of moving surface relative to skin was defined and could be directly related to occurrence of each action potential in fiber's discharge. Successive traverses of surface across fiber's receptive field were identical, except that between successive scans nyloprint surface was displaced laterally relative to axis of movement by increment of 0.125 mm. Stimulus succession is illustrated schematically on left. Display of fiber responses to each successive stimulus is shown on right: each dot indicates occurrence of single action potential.

From Darian‐Smith et al. 61


Figure 47.

Series of response patterns of a single slowly adapting fiber to 7 different dot patterns (in rows) moving across skin at 3 velocities of 40, 80, and 150 mm/s, respectively (in columns). Contact force between moving surface and underlying skin was 60 g · wt. Neural response pattern for each stimulus combination was constructed as shown in Fig. 45. Abscissa of each response block was relative to moving surface: hence with an increasing velocity of moving surface, time scale was correspondingly expanded.

From Darian‐Smith et al. 61


Figure 48.

Diagram of representation in population of uniformly responding digital nerve fibers of spatial features of dot pattern moving across skin. Surface is moving to right at constant velocity. Skin surface is horizontal plane above patterned surface. Large open circles, receptive fields of individual fibers; vertical axis, dimension of time; each small black dot, occurrence of single action potential in fiber. Each dot in moving surface engages assembly of fibers whose discharge defines response cylinder with both spatial and temporal dimensions. Slope of cylinder depends solely on velocity with which surface sweeps across skin. Separation in time of adjacent response cylinders depends on spatial separation of dots on surface and velocity of moving surface.

From Darian‐Smith et al. 61


Figure 49.

Series of response patterns in SA, QA, and Pacinian fiber, each innervating macaque fingerpad, to passage of Braille‐like pattern of dots across skin. Braille‐dot patterns on left; diameter of dot, 1.2 mm, with separation drawn to scale. Response patterns generated as illustrated in Fig. 45, with direction of movement of surface from bottom to top of figure. Velocity of moving surface, 40 mm/s. Contact forces of 60 g · wt for SA fiber and 20 g · wt for QA and Pacinian fibers were selected for optimum pattern resolution.

From Johnson and Lamb 134


Figure 50.

Performance of human subject detecting 10° displacement of distal interphalangeal joint of middle finger at various angular velocities. Scores indicate number of displacements correctly identified in 10 observations. Three conditions were examined: the first 2 were averaged responses for 7 subjects, with SE indicated by vertical bars; filled circles, normal movement of joint, with input to central nervous system from joint, skin, and muscle afferents; open circles, with input from joint and skin afferents only. The 3rd condition, crosses, were individual responses with input from muscle afferents only, following digital nerve block with local anesthesia. Individual responses illustrate the great variability in performance. No one set of afferents could account for normal performance.

From Gandevia and McCloskey 76
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Article Title: The Sense of Touch: Performance and Peripheral Neural Processes
 

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Ian Darian‐Smith. The Sense of Touch: Performance and Peripheral Neural Processes. Compr Physiol 2011, Supplement 3: Handbook of Physiology, The Nervous System, Sensory Processes: 739-788. First published in print 1984. doi: 10.1002/cphy.cp010317