Rapidly Adapting Afferents Research Articles

Discriminative and affective touch: sensing and feeling.

The multimodal properties of the human somatosensory system continue to be unravelled. There is mounting evidence that one of these submodalities-touch-has another dimension, providing not only its well-recognized discriminative input to the brain, but also an affective input. It has long been recognized that touch plays an important role in many forms of social communication and a number of theories have been proposed to explain observations and beliefs about the "power of touch." Here, we propose that a class of low-threshold mechanosensitive C fibers that innervate the hairy skin represent the neurobiological substrate for the affective and rewarding properties of touch.

The Organization of Submodality-Specific Touch Afferent Inputs in the Vibrissa Column

The rodent tactile vibrissae are innervated by several different types of touch sensory neurons. The central afferents of all touch neurons from one vibrissa collectively project to a columnar structure called a barrelette in the brainstem. Delineating how distinct types of sensors connect to second-order neurons within each barrelette is critical for understanding tactile information coding and processing. Using genetic and viral techniques, we labeled slowly adapting (SA) mechanosensory neurons, rapidly adapting (RA) mechanosensory neurons, afferent synapses, and second-order projection neurons with four different fluorescent markers to examine their connectivity. We discovered that within each vibrissa column, individual sensory neurons project collaterals to multiply distributed locations, inputs from SA and RA afferents are spatially intermixed without any discernible stereotypy or topography, and second-order projection neurons receive convergent SA and RA inputs. Our findings reveal a"one-to-many and many-to-one" connectivity scheme and the circuit architecture for tactile information processing at the first-order synapses.

Convergence of Submodality-Specific Input Onto Neurons in Primary Somatosensory Cortex

At the somatosensory periphery, slowly adapting type 1 (SA1) and rapidly adapting (RA) afferents respond very differently to step indentations: SA1 afferents respond throughout the entire stimulus interval (sustained response), whereas RA afferents respond only at stimulus onset (on response) and offset (off response). We recorded the responses of cortical neurons to step indentations and found many neurons in areas 3b and 1 to exhibit properties that are intermediate between these two extremes: These neurons responded during the sustained portion of the stimulus and also at the offset of the stimulus. Several lines of evidence indicate that these neurons, which exist in large proportions even at these early stages of somatosensory cortical processing, receive input from both populations of afferents. First, we show that many cortical neurons have both a significant sustained response and a significant off response. Second, the strength of the off response is uncorrelated with that of the sustained response, which is to be expected if sustained and off responses stem from different populations of afferent fibers. Third, the bulk of the variance in cortical responses to step indentations can be accounted for using a linear combination of both SA1 and RA responses. Finally, we show that the off response in cortical neurons does not reflect rebound from inhibition. We conclude that the convergence of modality specific input onto individual neurons is common in primary somatosensory cortex and discuss how this conclusion might be reconciled with previous findings.

Representation of Object Size in the Somatosensory System

In this study we investigate the haptic perception of object size. We report the results from four psychophysical experiments. In the first, we ask subjects to discriminate the size of objects that vary in surface curvature and compliance while changing contact force. We show that objects exhibit size constancy such that perception of object size using haptics does not change with changes in contact force. Based on these results, we hypothesize that size perception depends on the degree of spread between the digits at initial contact with objects. In the second experiment, we test this hypothesis by having subjects continuously contact an object that changes dynamically in size. We show that size perception takes into account the compliance of the object. In the third and fourth experiments we attempt to separate the individual contributions of proprioceptive and cutaneous input. In the third, we test the ability of subjects to perceive object size after altering the sensitivity of cutaneous receptors with adapting vibratory stimuli. The results from this experiment suggest that initial contact is signaled by the cutaneous slowly adapting type 1 afferents (SA1) and/or the rapidly adapting afferents (RA). In the last experiment, we block cutaneous input at the site of contact by anesthetizing the digital nerves and show that proprioceptive information alone provides only a rough estimate of object size. We conclude that the perception of object size depends on inputs from SA1 and possibly RA afferents, combined with inputs from proprioceptive afferents that signal the spread between digits.

Surround suppression in the responses of primate SA1 and RA mechanoreceptive afferents mapped with a probe array.

Twenty-four slowly adapting type 1 (SA1) and 26 rapidly adapting (RA) cutaneous mechanoreceptive afferents in the rhesus monkey were studied with an array of independently controlled, punctate probes that covered an entire fingerpad. Each afferent had a receptive field (RF) on a single fingerpad and was studied at 73 skin sites (50 mm2). The entire array was lowered to 1.6 to 3.0 mm below the point of initial skin contact (the background indentation) before delivering indentations with one to seven probes. Indentations were generally limited to 100 microm to minimize gross mechanical interactions. There were two major, new findings. 1) The discharge rates of both SA1 and RA afferents were strongly affected by the number of probes indenting the RF simultaneously. The effect was exponential. Each increase in probe number reduced the response by 24% in SA1 and 12% in RA afferents on average. When seven probes indented the skin simultaneously, the impulse rates in SA1 and RA afferents were reduced to 20 and 40% of the rates evoked by a single probe at the hot spot (all indentations were 100 microm). This shows that before any synaptic interaction in the CNS there is already a mechanism analogous to surround inhibition that suppresses an afferent's responses to uniform indentation and makes it especially sensitive to deviations from spatial uniformity. 2) The responses of both SA1 and RA afferents were independent of background array depth over the range from 1.6 to 3 mm below the point of initial skin contact. This shows that the neural responses to elements raised above a background are independent of the applied force over a wide range of forces. To relate the background depths to indentation force and to compare humans and monkeys, we studied the biomechanics of indentation with a uniform surface. A remarkable result is that the force-displacement relationships in humans and monkeys were the same; the skin is highly compliant for the first 2-3 mm of indentation and then becomes much stiffer. The results were the same in alert humans and monkeys and in monkeys anesthetized with pentobarbital. Ketamine anesthesia made the skin much stiffer and reduced the compliant range substantially.

Responses of rapidly adapting afferent neurons to dynamic stretch of rat hairy skin.

Twenty-four rapidly adapting (RA) cutaneous afferents were recorded from a preparation of isolated, innervated hairy skin from the rat hindlimb for the purpose of identifying the mechanical variables associated with the initiation of afferent discharge. Neurons were studied while the skin was stretched dynamically along a single direction with the use of a linear actuator and a feedback controller. Input signals were load- or displacement-controlled stretches that followed either periodic or pseudorandom Gaussian noise control signals. When the tissue was actuated, loads and displacements were measured along the direction of stretch and neuronal responses were recorded. All RA afferents were activated by dynamic stretching. None had a sustained response to static stretch. Cross-correlation products, calculated between neuronal responses and either stress- or strain-related variables observed at the time of the spike, revealed a strong relationship between neuronal responses and tensile stress. Neuronal responses were observed at rates of change of stress between +1,000 and -800 kPa/s. Neuronal responses were poorly related to skin strain. Two loading conditions were used along the direction transverse to the stretch. In one condition the sides were unconstrained, so that on axial loading there was zero stress and negative strain along the transverse axis. In the other condition the sides were constrained so that when the tissue was loaded axially there was zero strain and positive stress along the transverse axis. In these two conditions the same level of axial stress was associated with two levels of axial strain. The neuronal responses were determined by the stress and not the strain. Neuronal responses were observed at stresses >5 kPa. It appears that RA afferents make little contribution to signaling limb movements or position in rat hindlimb on the basis of the behavior of rat hindlimb skin, as observed when the limb is rotated.

Monkey cutaneous SAI and RA responses to raised and depressed scanned patterns: effects of width, height, orientation, and a raised surround.

Monkey cutaneous SAI and RA responses to raised and depressed scanned patterns: effects of width, height, orientation, and a raised surround. J. Neurophysiol. 78: 2503-2517, 1997. The aim of this study was to examine the slowly adapting type I (SAI) and rapidly adapting (RA) primary afferent representation of raised and depressed surface features. Isolated, raised, and depressed squares and small raised squares with a circular surround were scanned across the receptive fields of SAI and RA mechanoreceptive afferents innervating the distal fingerpads of the rhesus monkey. Pattern height ranged from -620 to +620 micron and width ranged from 0.2 to 7.0 mm. The surround radii ranged from 3.0 to 7.0 mm. Previous combined psychophysical and neurophysiological studies have provided evidence that SAI afferent responses are responsible for the perception of spatial form and texture and that RA afferents are responsible for the detection of stimuli that produce minute skin motion (flutter, slip, microgeometric surface features). Our results strengthen these hypotheses. Response properties shared by both SAI and RA afferent types were that both responded only to the edges of the larger raised and depressed patterns, both responded to falling edges half as vigorously as to rising edges, both responded to rising and falling edges with impulse rates that were proportional to the sine of the angle between the edge and the scanning direction, and both had suppressed responses to a small raised surface feature when a raised surround was closer than 6 mm. Response differences consistent with the hypothesis that SAI afferents are specialized for the representation of form were that SAI responses were confined to areas around the features that evoked them in areas that were 40-50% smaller than the comparable RA response areas, SAI responses were more than four times more sensitive to stimulus height than were RA afferents over the range from 280 to 620 micron, and SAI (but not RA) afferents responded 20-50% more vigorously to corners than to edges. Response differences consistent with the hypothesis that RA afferents are specialized for the detection of minute surfaces features were that only RA afferents responded to very small surface depressions, depressed squares 0.8 mm wide, that were detectable by palpation. Mechanisms underlying the many differences in SAI and RA response properties are discussed.

Neural Coding Mechanisms in Tactile Pattern Recognition: The Relative Contributions of Slowly and Rapidly Adapting Mechanoreceptors to Perceived Roughness

Tactile pattern recognition depends on form and texture perception. A principal dimension of texture perception is roughness, the neural coding of which was the focus of this study. Previous studies have shown that perceived roughness is not based on neural activity in the Pacinian or cutaneous slowly adapting type II (SAII) neural responses or on mean impulse rate or temporal patterning in the cutaneous slowly adapting type I (SAI) or rapidly adapting (RA) discharge evoked by a textured surface. However, those studies found very high correlations between roughness scaling by humans and measures of spatial variation in SAI and RA firing rates. The present study used textured surfaces composed of dots of varying height (280-620 micron) and diameter (0.25-2.5 mm) in psychophysical and neurophysiological experiments. RA responses were affected least by the range of dot diameters and heights that produced the widest variation in perceived roughness, and these responses could not account for the psychophysical data. In contrast, spatial variation in SAI impulse rate was correlated closely with perceived roughness over the whole stimulus range, and a single measure of SAI spatial variation accounts for the psychophysical data in this (0.974 correlation) and two previous studies. Analyses based on the possibility that perceived roughness depends on both afferent types suggest that if the RA response plays a role in roughness perception, it is one of mild inhibition. These data reinforce the hypothesis that SAI afferents are mainly responsible for information about form and texture whereas RA afferents are mainly responsible for information about flutter, slip, and motion across the skin surface.

Population estimates for responses of cutaneous mechanoreceptors to a vertically indenting probe on the glabrous skin of monkeys.

Recordings were obtained from low-threshold mechanoreceptive afferents during stimulation with a 0.5-mm-diameter probe at the receptive field (RF) center and at different distances from the point of maximal sensitivity. At each location, force-controlled stimuli of 0.5-4.0 g were ramped on to a plateau and then off at rates of 1, 10, and 100 g/s. The properties of rapidly adapting (RA) and slowly adapting type I (SAI) mechanoreceptors, when stimulated at the RF center, were similar in many respects to those reported in previous studies. Controlled stimulation away from the RF centers revealed that RF size for RAs was primarily dependent upon ramp rate, and for SAIs the size of the RF was primarily dependent upon load (force). The action potentials from individual afferents during stimulation at each location were binned in time and assigned to spatial segments of 1 mm. These responses were multiplied by: (A) an annular area of the receptive field and (B) the innervation density for the afferent type and skin region. The calculations provided estimates of overall rates of activity among the population of cutaneous afferents that respond to indentation by a small probe. Important differences were obtained between the responses of the population of afferents activated by the trapezoidal stimulus and the responses of afferents stimulated only at the RF center. Populations of tactile afferents provide more information for rate and intensity (force) discriminations than is available from units stimulated at the RF center. For RA afferents, the exponent of the power function describing relationships between stimulus rate and the population discharge (in impulses per second) was 0.3 times greater than the exponent for responses to on-center stimulation. For SAI mechanoreceptors, the exponent of the power functions for static responses to force was 0.22 times greater for the population response than for on-center activation. Population functions for RA responses to the rate of force application and for SAI responses to static load saturated less than comparable responses to stimulation of the RF center. Thus, the coding capacity of the population extends the range of tactile discriminability. The slope and range of stimulus-response functions for populations was enhanced relative to responses to on-center stimulation. This occurs because of recruitment of afferents with RF centers adjacent to and remote from the stimulus, depending upon thresholds and receptive field sizes for different stimulus parameters. With stimulation at increasing rates and forces, there is a progressive spatial recruitment of receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Collateral sprouting of the central terminals of cutaneous primary afferent neurons in the rat spinal cord: Pattern, morphology, and influence of targets

The capacity of the central terminals of primary afferents to sprout into denervated areas of neonatal spinal cord and the morphology of any novel terminals has been investigated. In rats which had undergone sciatic nerve section on the day of birth, 12 of 18 physiologically characterized intact saphenous hair follicle afferents (HFAs) were labelled intra-axonally with horseradish peroxidase (HRP) were shown to sprout up to 2,000 microns into the deafferented sciatic terminal field. The morphology of these sprouts depended on which area of the sciatic nerve territory was invaded by the afferent sprouts. Six HFAs sprouted into areas normally innervated by glabrous skin afferents and the morphology of the collateral sprouts in this region resembled that of rapidly adapting (RA) afferents. The other six saphenous HFAs had sprouted into sciatic "hairy" skin areas and the morphology of these sprouts, although abnormal, was flame shaped. In rats whose sural, saphenous, and superficial peroneal nerves were cut at birth, 4 of 7 single HRP labelled RA afferents had central terminals that had sprouted into regions of cord normally devoted to "hairy" input. These showed clear signs of HFA morphology despite their peripheral receptive fields remaining in the glabrous skin. The results show collateral sprouting of single cutaneous sensory afferent axons into adjacent inappropriate central target regions following neonatal deafferentation. Such plasticity may provide some compensation following neonatal injury. The morphology of the sprouted terminals is appropriate to the new target area rather than to its functional class and is also independent of the peripheral receptive field location providing an example of central rather than peripheral control over afferent growth patterns.

Simulation of motion on the skin. III. Mechanisms used by rapidly adapting cutaneous mechanoreceptors in the primate hand for spatiotemporal resolution and two-point discrimination

1. The contribution of rapidly adapting (RA) mechanoreceptors to two-point discrimination has been evaluated by examining their ability to resolve the spacing of grating patterns shifted across the skin. The experiments test two different neural coding mechanisms that have been proposed to underlie resolution of spatial detail on the hand: 1) a rate-intensity code in which the spacing of surface features is encoded by the average frequency of firing of individual sensory afferents, and 2) an isomorphic representation of shape in which variations in the firing patterns of individual afferents reflect the spatiotemporal profile of skin indentation. 2. To measure the spatial acuity of RA mechanoreceptors innervating the hands of macaque monkeys, we displayed pairs of horizontal bars spaced 1-13 mm apart on a computer-controlled OPTACON stimulator placed over glabrous skin. Two-point resolution was measured by simultaneously pulsing pairs of rows at rates of 100, 50 and 25 Hz; each pair was shifted in tandem across the hand to simulate lateral motion. Single-fiber recordings were made from physiologically identified RA afferents in anesthetized monkeys. 3. Receptive field diameter appears to be the critical determinant of spatial resolution of gaps between two bars. RAs fire continuously if bar spacing is less than the field diameter but do not summate inputs when both active rows are contained within the field. Response profiles evoked by two bars spaced less than 4.8 mm apart can be predicted from the single-bar profiles, assuming occlusion between overlapping inputs with the strongest member dominating axonal output. Two-thirds of the RAs tested discharge 1 spike/pulse as bar patterns cross the field, yielding a uniform spike train whose frequency reflects stimulus pulse rates but fails to indicate gaps between bars. An additional 17% fire 2 spikes/pulse when the bars contact or straddle the field center, but also fail to differentiate individual stripes spaced less than 3.6 mm apart. 4. Only 17% of RAs represent gaps narrower than the field diameter. These fibers show double-peaked response profiles to bar patterns spaced at least 2.4 mm apart, firing 2 spikes/pulse as first one, and then the second stripe crosses the field center. Timing between peaks corresponds to bar spacing. Responses are reduced in amplitude when adjacent bars straddle the field center, as occlusion between simultaneous inputs prevents summation of inputs from the two stimuli. Fifteen of 16 RAs failed to resolve bars spaced 1.2 mm apart, as double-spike responses were evoked only by the leading stripe.(ABSTRACT TRUNCATED AT 400 WORDS)

Simulation of motion on the skin. I. Receptive fields and temporal frequency coding by cutaneous mechanoreceptors of OPTACON pulses delivered to the hand.

1. Tactile discrimination of form requires motion of the hand across the object scanned. To dissociate lateral distortion of the skin from neuronal processing mechanisms involving multiple receptor classes and parallel central networks, we have simulated motion of bar patterns across the fingers and palm by the use of a computer-controlled grid of sequentially activated probes (OPTACON stimulator). Horizontal bar patterns have been swept across the hand at speeds of 30-120 mm/s to quantitatively characterize responses of cutaneous mechanoreceptive afferents recorded in the median and ulnar nerves. 2. Mechanoreceptors with phasic responses to pressure are activated by spatial patterns on the OPTACON, whereas those with tonic pressure responses are not; moving-bar patterns strongly excite both Meissner's afferents [rapidly adapting (RA) mechanoreceptors] and Pacinian corpuscles (PCs) but fail to excite slowly adapting (SA) afferents. OPTACON-type stimulators thus allow selective activation of phasic mechanoreceptor channels with spatially complex stimuli. 3. RA afferents respond in an all-or-none fashion to activation of two to five adjacent rows spanning 1-5 mm on the finger, with nearly identical latencies on all trials; response profiles are remarkable for their regularity and reproducibility. PCs have larger fields (4-13 rows) and stronger but more irregular responses than RAs. 4. Uniform sensitivity throughout the receptive field is a consistent feature of RA responses. Individual mechanoreceptor terminals appear to have equal access to the spike initiation zone and provide the same amplitude input as the fiber discharges 1 spike/pulse at each field location in 75% of the RAs tested. Uniform sensitivity allows each afferent to transmit a repetitive signal of the parameter of interest such as object speed, contour, or texture. 5. One-quarter of RAs fire two spikes to probe indentation and retraction at the field center. Such graded responses are usually observed in only one direction of motion, reflecting a preferred sequence of receptor activation rather than a specific location on the skin. PCs fire bursts of two to four spikes throughout most of their receptive fields; sensitivity is broadly distributed rather than peaked. Thus phasic mechanoreceptors fail to provide a precise signal of stimulus location; localization at the level of individual papillary ridges appears to be signaled by a population mechanism involving unique combinations of RA, SA, and PC afferents.(ABSTRACT TRUNCATED AT 250 WORDS)

Simulation of motion on the skin. II. Cutaneous mechanoreceptor coding of the width and texture of bar patterns displaced across the OPTACON.

1. These experiments assay the functional significance of receptive-field architecture for information processing. Rapidly adapting (RA) afferents have been previously shown to converge information from clusters of 14-25 Meissner's corpuscles, whereas afferents innervating Pacinian corpuscles (PCs) have only a single, large receptor terminal. We tested two opposing hypotheses of functional architecture: 1) summation models, in which an afferent integrates signals from all of its terminals, showing monotonic increases in activity as a function of contact area, and 2) winner-take-all models, in which the most strongly activated receptor in the cluster dominates axonal output by cancellation of signals from other branches. 2. Bar and stripe patterns have been swept across the finger or palm of the monkey's hand at speeds of 30-120 mm/s with the use of a computer-controlled grid of sequentially activated miniature probes (OPTACON stimulator). The dense packing of OPTACON probes permits placement of up to five groups of stimulators within an individual receptive field, allowing us to activate one or more clusters of Meissner's corpuscles simultaneously and to stimulate the bulbar corpuscle of PC afferents at different orientations through the skin. Integration of information from moving bar patterns has been tested with two protocols. In the variable width protocol, the total number of activated rows in the pattern is varied from one to five, with a constant spacing of 1.2 mm between pulsed rows. In the variable density protocol, the length of skin stimulated is held constant at 5 mm and the spacing of stimuli varied. 3. RA afferents show no evidence of summation of inputs within their receptive fields. Motion of wide bars across the field increases the duration of firing but not the total spikes evoked by each pulse. Responses to the leading edge of wide bars were found to be identical to those evoked by a single-row bar. Simultaneous activation of two to five rows evokes the same or fewer spikes per pulse than the most effective individual row tested alone. When broad-bar patterns are centered over the field, contacting the maximum number of receptors, RAs follow activity in the dominant branch or terminus, suppressing additional inputs. Lack of summation is observed at all pulse frequencies tested (25-100 Hz). 4. Moving bar patterns evoke responses as long as at least one row stimulates the receptive field; broader patterns evoke longer spike trains whose total number of impulses is proportional to bar width.(ABSTRACT TRUNCATED AT 400 WORDS)

Perceived roughness of a grating: correlation with responses of mechanoreceptive afferents innervating the monkey's fingerpad

Human subjects scaled gratings of alternating grooves and ridges for perceived roughness. Roughness increased with an increase in groove width and decreased with an increase in ridge width, but the effect of groove width was much greater than the effect of ridge width. In corresponding neurophysiological experiments, the gratings were moved sinusoidally across the receptive fields of single mechano-receptive afferents innervating the fingerpads of anesthetized monkeys. The measure of response used was the mean cyclic discharge rate (averaged over one cycle of the sinusoid). Slowly adapting afferents (SAs), rapidly adapting afferents (RAs), and Pacinian afferents (PCs) all showed a marked increase in response when groove width increased. An increase in ridge width had no consistent effect on the responses of SAs or RAs but resulted in a small decrease in the response of PCs. The response to a smooth surface differed significantly from the responses to the finer gratings only for the RAs. An alternative measure of response (the number of impulses elicited by each spatial cycle of the grating) increased with an increase in ridge width for all 3 fiber types. Thus, the large effect of groove width on perceived roughness can be accounted for by the mean cyclic discharge rate in the active afferent fibers. The smaller effect of ridge width can be accounted for by the number of impulses per spatial cycle of the grating.

Spatial and temporal factors determining afferent fiber responses to a grating moving sinusoidally over the monkey's fingerpad

Gratings of alternating grooves and ridges were moved sinusoidally across the fingerpads of anesthetized monkeys, while responses were recorded from individual slowly adapting afferents (SAs), rapidly adapting afferents (RAs), and Pacinian afferents (PCs) in the median nerve. The stimulus comprised 2 spatial variables, namely, groove width (G) and ridge width (W), and 2 temporal variables, namely, the peak speed of movement (S) and the peak temporal frequency (F) at which successive spatial cycles of the grating pass over a point in the receptive field. The responses of all 3 fiber types were determined by only 1 spatial variable, G, and only 1 temporal variable, F. Changes in W or S affected responses only if there was a concomitant change in either G or F. Responses were phase-locked to the occurrence of successive spatial cycles of the grating, and we have used the number of impulses elicited by a single spatial cycle as the fundamental measure of response. An equation of the form I = cGaexp(-b square root of F) describes the responses of all 3 fiber types. For SAs, the effect of groove width was greater (a = 2.64) than for RAs and PCs (a = 0.924 and 1.05, respectively). The reduction in response with frequency was most marked for SAs (b = 0.262), and greater for PCs (b = 0.167) than for RAs (b = 0.130). From the equation, the instantaneous response during the entire sinusoidal cycle was reconstructed as well as a second measure, the mean cyclic response. These 2 measures behaved differently with changes in the stimulus parameters. The temporal properties of the fibers, as revealed by gratings, may appear to be in conflict with those established by vibratory threshold studies; in fact, they are compatible with suprathreshold responses to vibrating probes.

Sinusoidal movement of a grating across the monkey's fingerpad: temporal patterns of afferent fiber responses

Responses were recorded from cutaneous afferents innervating mechanoreceptors in the monkey's fingerpad, while gratings of alternating grooves and ridges were moved sinusoidally across their receptive fields. The gratings were specified by their spatial period and the movement by its peak speed: together these determined the peak temporal frequency at which grating ridges passed over the receptive field. During the central 42 degrees of each half cycle of movement, the speed and thus the temporal frequency of the grating ridges remained constant to within 6.6% of their peak values. In this region the responses of all afferents were phase-locked to the temporal sequence of grating ridges. The number of impulses elicited by each grating ridge was a function of the stimulus variables. For all 3 afferent classes--namely, slowly adapting afferents (SAs), rapidly adapting afferents (RAs), and Pacinian afferents (PCs)--the number of impulses per grating ridge increased as the spatial period of the grating increased (while the peak speed of movement was held constant). Similarly, for all 3 classes, the number of impulses per ridge decreased as the peak speed of movement increased (while the spatial period of the grating remained constant). When the peak temporal frequency of the grating ridges was held constant, for SAs and RAs the number of impulses per ridge increased with an increase in the spatial period of the grating and thus with an increase in the peak speed. These phase-locked responses provided information about the peak temporal frequency of the grating ridges independent of the grating spatial period and of the peak speed of movement. The shape of the response profile during a half cycle of movement was different for different afferents. Many of the RA response profiles were close to sinusoidal. The SA and PC profiles tended to have reduced peaks or raised troughs, resulting in flatter profiles. Other departures from sinusoidal profiles were also seen.

Sinusoidal movement of a grating across the monkey's fingerpad: representation of grating and movement features in afferent fiber responses

Gratings of alternating grooves and ridges were moved sinusoidally back and forth across the monkey's fingerpad. Each grating was completely specified by its spatial period and the movement by its peak speed: together these determined the peak temporal frequency at which grating ridges passed over the skin. Responses of cutaneous, mechanoreceptive afferents innervating the fingerpad were characterized in terms of these 3 parameters. Slowly adapting afferents (SAs), rapidly adapting afferents (RAs), and Pacinian afferents (PCs) had different characteristics. The responses (mean cyclic discharge rates) of the SAs increased when the spatial period of the grating increased (and peak speed of movement remained constant) but did not change with changes in the peak speed of the movement (while the spatial period of the grating remained constant). Conversely, the responses of the PCs increased when the peak speed of movement increased (and the spatial period remained constant) but were relatively insensitive to changes in the spatial period of the grating (while the peak speed remained constant). The responses of the RAs increased as the spatial period of the grating increased (and peak speed remained constant) and also increased as the peak speed of movement increased (and the grating spatial period remained constant). When the peak temporal frequency of the grating ridges was held constant, the responses of all 3 afferent groups changed with changes in the grating spatial period or in the peak speed of movement. Information about the spatial features of the grating, independent of the peak speed of movement, was present in the SA population response and in the ratios of the RA and PC population responses. Information about the peak speed of movement, independent of the spatial period of the grating, was present in the PC population response and could be extracted from the RA population response.

Sinusoidal movement of a grating across the monkey's fingerpad: effect of contact angle and force of the grating on afferent fiber responses

Responses were recorded from cutaneous afferents innervating mechanoreceptors in the monkey's fingerpad. When gratings of alternating grooves and ridges were moved sinusoidally back and forth across the receptive field, the responses of the afferent were often not equal for the 2 directions of movement. To investigate this phenomenon, the position of the center of the afferent's receptive field, relative to the contact area between the grating and the finger, was varied systematically. For some afferents, regardless of these relative positions, the response was always greater for a particular direction of movement. For other afferents, regardless of these relative positions, the responses for the 2 directions of movement were always equal. For a minority of afferents, the response was greater for movement in one particular direction for some relative positions and greater for movement in the opposite direction for other relative positions. Slowly adapting afferents (SAs), rapidly adapting afferents (RAs), and Pacinian afferents (PCs) exhibited all 3 types of response patterns. We could not relate these patterns to the afferent type or to the positions, in the fingerpad, of the receptive field center. The contact force between the grating and the finger was varied by varying the contact displacement (indentation). Two grating spatial periods were used. For SAs and PCs the response increased with increasing indentation for both gratings. For RAs the response to the finer grating increased with increasing indentation, but the response to the coarser grating did not.