Monkey Hand Research Articles

Cross-correlation reveals facilitation of single motor units in thenar muscles by single corticospinal neurones in the conscious monkey

The functional connections between corticospinal neurones and motor units of the monkey's hand muscles were investigated by constructing cross-correlograms of activity recorded from pyramidal tract neurones and from single motor units in the contralateral thenar muscles during performance of a precision grip between thumb and index finger. Only those neurones which produced postspike facilitation of the surface-recorded electromyogram (EMG) of these muscles were selected for analysis. Positive correlations were observed for 11 15 selected neurones, and the form of the correlation was suggestive of monosynaptic action. Corticospinal cells which produced a correlation peak often did so with all concurrently-sampled motor units.

Effect of mechanical stimulus spread across glabrous skin of raccoon and squirrel monkey hand on tactile primary afferent fiber discharge.

The role of spread of skin deformation in activating cutaneous mechanoreceptors at a distance from their threshold receptive fields (RFs) was examined in glabrous skin of the North American raccoon and the squirrel monkey. One feedback-controlled mechanical stimulus probe was used to indent the skin to a controlled depth at a constant velocity, at varying distances from a second probe, which was used to monitor vertical displacement depth and velocity at this distant site. In many instances, the monitor probe was positioned over the RF of a cutaneous mechanoreceptor, and single-unit action potentials were simultaneously recorded from individual fibers of the median or ulnar nerve. With distance from the site of stimulation, there was a systematic, monotonic decline in indentation depth and velocity; velocity fell off with distance more rapidly than depth. The degree of diminution with distance varied with the size, shape, and curvature of the digital or palm pad stimulated. Spread of indentation was more restricted on digital than on palm pads, and was more restricted across monkey skin than across raccoon skin. Spread was less with higher-velocity than with lower-velocity indentations, but was seemingly unaffected by indentation depth. As expected from the findings noted above, the number of spikes discharged by slowly adapting mechanoreceptive afferent fibers declined more rapidly with distance between stimulus site and RF for digital than for palmar RFs, in squirrel monkey than in raccoon skin, and with higher-velocity than with lower-velocity stimuli. Furthermore, the number of spikes occurring during either ramp or early static indentation phases of stimulation dropped to zero more rapidly with distance than did either vertical indentation depth or velocity. Decreases with distance in both indentation depth and velocity acted to restrict the size of suprathreshold RFs. For most units, horizontal components of mechanical stimulation subtracted from the effects of vertical components. It is suggested, on the basis of this and other studies, that many neural and perceptual phenomena usually attributed to central mechanisms of afferent inhibition may be attributable, at least in part, to mechanical properties of the skin. In addition, the present data suggest that regional variations in the two-point limen may be associated with variations in spread of mechanical deformation. The conclusion that glabrous skin and subjacent soft tissues act as a low-pass filter system provides a mechanical basis for the relative efficacy of high-frequency vibratory stimuli in tactile pattern perception.(ABSTRACT TRUNCATED AT 400 WORDS)

Cerebral cortical somatosensory evoked responses, multiple unit activity and current source-densities: their interrelationships and significance to somatic sensation as revealed by stimulation of the awake monkey's hand.

In awake monkeys, electrical pulse stimuli which mimic touch stimulation were delivered to the thenar eminence while electrophysiologic recordings were made from surface to depth in postcentral gyrus. Cortical depth profiles of somatosensory evoked responses (SEPs), multiple unit activity (MUA) and current source-densities (CSDs) were analyzed to gain insight into the neural process underlying the SEP and somatic sensation. The following was found: The thenar stimulus evoked four main SEP components which were seen over wide regions of postcentral gyrus: P1 at 12 ms, the primary evoked response; P1a, near 20 ms; N1, near 50 ms; and P2, around 120 ms after the stimulus. MUA was observed during the P1, P1a and N1 temporal intervals whose vigor changed as a function of the respective SEP component's amplitude. CSD analysis showed that during P1 a current sink and source appeared within the middle and superficial cortical layers, respectively. During P1a, a sink just above that of P1 and a superficial source became evident. During N1, a large superficial sink and one or two deep sources appeared. Evoked MUA during P1 and P1a was most prominent at the level of their current sinks while MUA during N1 appeared at the level of the current sources, in general. When stimulation was moved from the thenar portion of the hand to a region which most closely matched the receptive field of the cortical recording site, P1 and P1a increased amplitude while N1 both increased in amplitude and decreased in peak latency. Also, MUA activity during the early temporal intervals become more vigorous. These changes were similar to those observed with thenar stimuli as the recording site approached the thenar cortical representation. Over repeated trials at a single stimulus intensity, the spontaneous changes in SEP amplitude were found to be directly correlated with MUA and CSD measures within the same temporal interval. In contrast, SEP, MUA or CSD measures within the early temporal intervals (i.e., P1 or P1a) were uncorrelated or less frequently, inversely correlated with the same measures in the late temporal interval (i.e. N1). The multiple measures complemented one another which led to a descriptive model of the neural process underlying the evoked cortical response in postcentral gyrus of awake monkey.

Relations between stimulus force, skin displacement, and discharge characteristics of slowly adapting type I cutaneous mechanoreceptors in glabrous skin of squirrel monkey hand.

The contributions of viscoelastic properties of squirrel monkey glabrous skin to slowly adapting Type I (SAI) mechanoreceptive afferent fiber discharge were examined in the present study. Individual fibers of the median and ulnar nerves were isolated by microdissection in six monkeys anesthetized with pentobarbital sodium. Utilizing mechanical stimulation and data analysis techniques identical to those of a previous study of raccoon glabrous skin and its mechanoreceptors (Pubols, 1982a; Pubols and Maliniak, 1984), we studied and compared responses to punctate mechanical stimuli controlled with respect to force or displacement. Squirrel monkey glabrous skin was found to be more compliant than raccoon glabrous skin, in that a given force applied to either a digital or a palmar skin pad produced a greater displacement of squirrel monkey skin. Skin displacement increased approximately linearly with increasing forces at the beginning of static stimulation, but over time (at least up to 20 sec), the relationship became negatively accelerated. Absolute-force thresholds of individual SAI units were significantly lower in squirrel monkey (mean = 122 mg, range = 48-340 mg) than in raccoon (mean = 484 mg, range = 70-1,290 mg). However, absolute-displacement thresholds were insignificantly lower (squirrel monkey: mean = 17.24 microns, range = 5-30 microns; raccoon: mean = 30 microns, range = 5-185 microns). Application of suprathreshold forces (range = 1-20 g) and displacements (range = 500-1,000 microns) revealed greater interunit variability in response to maintained stimulation than previously found in raccoon. In 8 out of 15 fibers, the rate of adaptation was significantly greater during constant-displacement than during constant-force stimulation; in 4 cases there was no significant difference; and in 3 cases the rate of adaptation was significantly greater during constant-force stimulation. Potential sources of interunit variability include surface topography of the hand, properties of cutaneous and subcutaneous tissues in the vicinity of the receptor, and experimental variables such as stimulus amplitude and rate of stimulus onset. It is suggested that both regional and species differences in functional properties of cutaneous mechanoreceptors are more likely attributable to differences in mechanical properties of skin and subjacent tissues than to any inherent differences in receptor properties.

Microbicidal activity of octenidine hydrochloride, a new alkanediylbis[pyridine] germicidal agent.

The potential of octenidine hydrochloride (WIN 41464-2) as a topical microbicide was measured both by in vitro death kinetics and reductions in numbers of bacteria on the skin of cynomolgus monkeys. Semilogarithmic survival curves were plotted to measure the microbicidal activity of various concentrations of octenidine against Staphylococcus aureus. The microbicidal activity of octenidine was also determined for Staphylococcus epidermidis, Proteus mirabilis, Streptococcus pyogenes, Klebsiella pneumoniae, Escherichia coli, Pseudomonas aeruginosa, Serratia marcescens, and Candida albicans. Death rates for the same microbial strains were compared with those obtained by using chlorhexidine gluconate. Octenidine concentrations of less than 1.5 microM (0.94 microgram/ml) caused a greater than 99% reduction of each microbial population within 15 min. Staphylococcus epidermidis was the most susceptible of the test organisms, and E. coli and C. albicans were the least susceptible. Octenidine was more active than chlorhexidine against each test strain. Skin-degerming activities of aqueous and formulated octenidine and formulated chlorhexidine were compared in single and multiple applications of these agents to the hand and foot surfaces of monkeys by using a glove-juice extraction procedure to measure the skin microflora. Aqueous octenidine, at a concentration of 0.2 to 1.6% reduced resident microflora populations from 90 to 99.98%, depending on the concentration and number of applications. Octenidine formulated at 2% in a surfactant-based vehicle exhibited significantly better skin-degerming activity than did either a nonmedicated vehicle or the Hibiclens brand of 4% chlorhexidine gluconate.

The neural signal for the intensity of a tactile stimulus

The effect of indenting the skin at different rates on the perceived intensity of the stimulus was studied by indenting the skin of the fingertip with two triangular waveforms, given as a pair. The subjects were asked to judge which member of the pair was more intense. Perceived intensity was found to increase both with the depth and the speed of the indentation. In contrast, changes in the rate of skin indentation had little influence on perceived skin indentation depth. This suggests that intensity and depth are different attributes of tactile sensibility. Since the skin is viscous, a rapid indentation is more forceful than a slow indentation of the same depth, raising the possibility that perceived intensity is related to stimulus force. Even though intensity judgments were more closely correlated with the force of a stimulus than with the indentation it produced, a rapidly increasing force was felt as more intense than one that increased more slowly but attained the same final magnitude. When mechanoreceptors in the palmar aspect of the monkey's hand were excited with triangular stimuli like those used in the psychophysical studies, their discharge frequency increased with the rate of skin indentation. However, the receptors were distinctly more rate sensitive than the human judgments of stimulus intensity, suggesting that impulse summation in the central nervous system summates (integrates in the mathematical sense) the receptor input so as to enhance, relatively, the perceived intensity of the slower stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional precursors to language and its lateralization.

An adequate account of language evolution must reconcile the propositions that language is unique but has precursors. The paper of Bellman and Goldberg and that of Tzeng and Wang each neglects one of these propositions. We suggest that the uniqueness of language lies primarily in its dualistic structure which has a frame-content mode of organization: at the phonological level, consonant and vowel elements are inserted into syllabic frames; and at the morphological level, stem forms of content words are inserted into syntactic frames. We suggest that the morphological level evolved from the phonological level and that the frame-content mode of organization in phonology had a precursor in the form of bimanual coordination in which the nonpreferred (frame) hand holds an object operated on by the preferred hand (content). It is argued that lateralization of cortical function evolved first for bimanual coordination, then for language. Old World monkey hand preferences may be consistent with both the putative left-hemisphere specialization for bimanual coordination and the human right-hemisphere specialization for spatial functions.

The neural signal for skin indentation depth. II. Steady indentations

The glabrous skin of the monkey's hand was stimulated with a waveform that indented the skin at a rate of 0.4 mm/sec, held the skin steadily or nearly steadily indented for 12 sec or longer, and then retracted back to the starting position. Recordings were made of activity in single afferent fibers in response to these stimuli. The average discharge frequency of 21 slowly adapting mechanoreceptors declined 38% during the first 12 sec of a steady indentation when the amplitude of the displacement was 0.65 mm and 36% when the displacement was 1.3 mm. When the plateau was not steady but the indentation depth gradually decreased by 15% during the 12-sec plateau period, the average decline was 47% for the 0.65-mm indentation and 46% for the 1.3-mm stimulus. When the indentation depth gradually increased by 15% during the 12-sec plateau, the discharge declined an average of 26% during the 0.65-mm indentation and 22% during the 1.3-mm displacement. To determine the effect of receptor adaptation on the perception of skin indentation depth, 13 human subjects had the skin of their fingertips indented 1 mm with similar trapezoidal waveform and were asked whether the indentation depth increased or decreased during the plateau portion of the stimulus. Ten of the 13 subjects thought that the indentation depth was increasing when the plateau was steady. The method of limits was then used to determine how much the stimulus had to change for the subject to feel the depth during the plateau as unchanging; i.e., a "perceptual zero."(ABSTRACT TRUNCATED AT 250 WORDS)

A quantitative study of muscle spindles and tendon organs in some intrinsic muscles of the hand in the bonnet monkey (Macaca radiata).

The number and density of muscle spindles and tendon organs have been determined in the following intrinsic muscles of the hand of bonnet monkeys: I lumbrical, II lumbrical, abductor digiti minimi, adductor pollicis, and I dorsal interosseous. All these muscles were found to be very rich in muscle spindles (17.6 to 42.31 per gram wet weight) but relatively poor in tendon organs (0.606 to 10.06 per gram wet weight). The lumbricals have very few tendon organs. The possible functional significance of these findings has been discussed.

Recovery of normal topography in the somatosensory cortex of monkeys after nerve crush and regeneration.

After median nerve fibers to glabrous skin on the hands of monkeys were crushed and allowed to regenerate, normal topographical organization was recovered in the representation of the hand in primary somatosensory cortex. Similar recovery of normal cortical organization may underlie the sensory restoration that usually follows nerve crush injury in humans.

The neural signal for skin indentation depth. I. Changing indentations

Psychophysical tests on human subjects showed that judgments of skin indentation depth made when the fingertip was indented at rates from 0.2 to 16 mm/sec were quite insensitive to changes in indentation velocity. Similar results were obtained on the forearm at indentation velocities of 0.4 to 16 mm/sec. Recordings were made from mechanoreceptors in the monkey's hand that were able to respond over the same range of velocities and at comparable depths to determine how skin indentation depth might be signaled (coded) at the receptor level and to examine the rate sensitivity of the possible depth codes. It was found that most of the receptors with foci under the stimulator were recruited relatively early during an indentation, especially at velocities of 1.6 mm/sec and higher, making it improbable that the full range of indentation depths is signaled by the "subsurface" recruitment of different receptors at different indentation depths. A subsurface recruitment code involving subcutaneous receptors is not likely since subjects could feel virtually none of the stimuli after skin anesthesia. Progressive recruitment with depth of receptors whose foci lie further and further away from the stimulator ("lateral" recruitment) was considered an unlikely depth code because changing the area of the stimulator had little effect on its perceived depth. Also, it was shown that subjects could sense the curvature of the indentation (the profile of the depth at right angles to the skin surface), which requires information about the depth of individual patches of skin beneath the stimulator. There is no obvious way that a lateral recruitment code can provide this information. Thus it is probable that the discharge rate of some or all of the receptors excited by the indentation is involved in indicating its depth. Both impulse frequency and receptor recruitment at any given depth increased as the velocity of the indentation increased. The demonstrated reliability of information about skin indentation depth in humans indicates that the central neural circuitry responsible for judgments of skin indentation depth is able to compensate for the rate-sensitive receptor signals.

The myelinated fibers of the deep branch of the ulnar nerve at the wrist in bonnet monkeys (Macaca radiata) and of some of its branches to the hand

In order to estimate the peripheral nervous correlates concerning the motor performance of the hand, the myelinated fibers of the deep ulnar nerve and some of its branches to the intrinsic muscles of the monkey (Macaca radiata) hand have been enumerated and their caliber spectra plotted. The content of the myelinated sensory fibers in the deep ulnar nerve was found to be 70% of the total. This high figure is attributable to the fact that 25-50% of the myelinated fibers in the deep ulnar nerve are destined to innervate the joints of the hand. Approximately 50% of the myelinated fibers in nerves to the intrinsic muscles of the hand were sensory.

Hand preferences in the rhesus monkey. Implications for the study of cerebral dominance.

• Cerebral dominance is reflected in man by a seemingly inborn preference for the right hand in 80% to 90% of mature individuals. However, not all of the factors that govern human cerebral dominance are clearly understood. We studied rhesus monkeys to determine if hand preference in that primate also reflected a form of cerebral dominance and hence could be used to elucidate cerebral hemispheric asymmetry as it appears in man. Shortterm experiments provided positive evidence that experiential and environmental factors were sufficient to explain the originally observed hand preference in individual and groups of monkeys. We then demonstrated that training readily induced new hand preferences, further suggesting that monkey hand preference is primarily shaped by experience. Our evidence failed to support previous conclusions of right or left preference as a characteristic of the species. Although handedness based on inborn morphological and functional hemispheric asymmetry as seen in man was not demonstrated, the preferences induced by training in our monkeys were systematically altered by contralateral association cortical removals, suggesting that asymmetries of cerebral function may be acquired in monkeys.

Influences of cerebellar hemispherectomy on slow potentials in the motor cortex preceding self-paced hand movements in the monkey

With chronically implanted electrodes, surface negative and deep positive, slowly increasing potentials were recorded in the forelimb area of the motor cortex prior to self-paced movements of the contralateral hand in monkeys. The slow premovement potentials were markedly reduced in size after ablation of the cerebellar hemisphere on the contralateral side to the motor cortex under recording. It was suggested that the cerebellar hemisphere (neocerebellum) participates in preparing the activity of the motor cortex prior to voluntary movements.

Warm fibers innervating palmar and digital skin of the monkey: responses to thermal stimuli.

1. Three hundred fourteen warm fibers innervating the glabrous skin of the monkey's hand were isolated by dissection in the median and ulnar nerves in two species, Macaca mulatta and M. nemestrina. Fiber samples in the two species were functionally similar and uniform in their properties. Their mean conduction velocity of 1.2 m/s (SD 0.5; n = 50) implies that these warm fibers were all unmyelinated. 2. A parametric study of the responses of warm fibers to near-rectangular warming and cooling pulses applied to glabrous skin was completed using 104 fibers. At a steady base-line skin temperature (T-base) of 34 degrees C all these warm fibers responded to warming pulses in the intensity range 0--8 degrees C with a simple, uniform discharge, which reached a peak rate of 1.5--4.0 s after the onset of stimulation; subsequent decay in this discharge rate had a time constant of 5--12 s and was virtually independent of the intensity of the warm pulse. The intensity function was linear for most fibers when the interstimulus interval was 60 s or longer. At a T-base of 29 degrees C, warm fibers were less responsive, but the temporal profile of the response was similar to that at a T-base of 34 degrees C in the intensity range 4--8 degrees C, and the intensity function was again linear. 3. At a T-base of 39 degrees C the intensity function of each warm fiber was complex. Most fibers responded briskly to warming pulses of 2--4 degrees C: the response to more intense warming pulses, particularly when the skin temperature rose above 45 degrees C, was structured and reproducible, but varied greatly among different fibers. With some the discharge evoked was of very high frequency for a few seconds, and then ceased. More than 80% of the sample of warm fibers did not discharge at all in response to warming pulses, which raised the skin temperature to 50 degrees C or above. 4. The responsiveness of warm fibers to warming pulses was dependent on previous stimulation when the interstimulus interval was less than 60 s. This temporal suppression was precisely structured and was examined quantitatively for trains of warming pulses, each lasting 4.0 s and presented every 10 s. The pattern of suppressive interaction was similar in form to that previously reported for cold fibers innervating palmar skin. 5. A quantitative study of the receptive fields of individual warm fibers demonstrated a spatiotemporal response pattern, which is best described in terms of a focal receptor zone less than 1 mm in diameter surrounded by thermally conducting skin. The skin's thermal conductivity is paramount in determining the warm-fiber's receptive-field characteristics. 6. The responses of warm fibers to cooling pulses and to warming ramps are described.

Coding of incremental changes in skin temperature by single warm fibers in the monkey.

1. Experiments were designed to answer the question: how well does a single warm fiber innervating the glabrous skin of the monkey's hand resolve incremental changes in the intensity of near-rectangular warming pulses applied to the fiber's receptive field? 2. In these experiments the measure of the warm fiber's capacity to resolve incremental changes in the intensity of successive warming pulses was termed the discriminable stimulus increment (DSI). The DSI is defined as that incremental difference in the intensity of a pair of warming pulses that could be resolved correctly, with a probability of 0.75, by comparing the fiber's responses to these two stimuli. In the specified conditions of the experiment, DSI = 0.67 sigma delta tau/(dR/dI) where sigma delta tau is the standard deviation of the difference in responses of the fiber to pairs of stimuli, and dr/dI is the fiber's sensitivity to incremental stimulus change. (dr/dI) was experimentally determined as the mean rate of change of the fiber's responses to incremental changes in the intensity of the warming pulse. 3. The DSI, as defined above, assumes that the basis for differentiating the stimuli in each pair was that the larger response in the fiber was in each instance generated by the more intense stimulus. A more general form of the DSI was also developed and used to examine the effects on intensity resolution of different discrimination rules that the brain might use. 4. In the experimental analysis the response measure of each warm fiber was the cumulative impulse count over successively longer segments of the stimulus period. With short integration intervals the DSI was high (i.e., intensity resolution was poor), but typically the DSI fell to a plateau level within 2.0--2.5 s of the onset of the warming stimulus. 5. The DSI was measured on 23 warm fibers in Macaca nemestrina for warming pulses with intensities of 0, 2, 4, 6, and 8 degrees C, at T-base levels of 29, 34 (near normal temperature of palmar skin), and 39 degrees C. For most observations the intensity resolution possible from the responses of single warm fibers, measured over this wide variety of stimulus conditions, was less than is achieved by the human observer trained to differentiate comparable warming pulses applied to the skin of the thenar eminence.

II. Functional properties of cells in anterolateral part of area 7 associative face area of awake monkeys.

The most anterior part of area 7 of awake, behaving macaque monkeys was investigated using single cell recording technique. Eighty-five cells from three hemispheres of two monkeys were isolated and studied. These cells showed more complex functional properties than the cells in the primary and secondary cortical fields. Of the cells 61% responded to somatosensory (26%) or visual (2%) or both somatosensory and visual (33%) stimulation; 39% of the cells were active only during the monkey's own movements. Most of the cells studied were active while the monkey was bringing an object to the mouth with its hand, when reaching for an object with lips, or while chewing. The neurons responded selectively to, e.g., palpation of the flexors of the arm, a visual stimulus approaching the face, passive movement of the monkey's hand towards the mouth, or they were active only when the monkey was reaching for an object with its lips or was mouthing it. The cellular activity in the anterolateral part of area 7 was prominently related to the stimulation or motor activity of the face (especially the mouth). In this respect, it differed from the more posterior part of area 7 adjacent to it. The results thus indicate that there is a separate and rather extensive mouth (or face) area in the parietal association cortex of the monkey.

Comparison of responses of warm and nociceptive C-fiber afferents in monkey with human judgments of thermal pain

1. Radiant-heat stimuli of different intensities were delivered every 28 s to the thenar eminence of the hand of human subjects and to the receptive fields (RFs) of 58 "mechanothermal nociceptive" and 16 "warm" C-fibers, most of which innervated the glabrous skin of the monkey hand. A CO2 infrared laser under control via a radiometer provided a step increase in skin temperature to a level maintained within +/- 0.1 degrees C over a 7.5-mm-diameter spot. 2. Human subjects categorized the magnitude of warmth and pain sensations evoked by stimuli that ranged in temperature from 40 to 50 degrees C. The scale of subjective thermal intensity constructed from these category estimates showed a monotonically increasing relation between stimulus temperature and the magnitude of warmth and pain sensations. 3. The mechanothermal fibers had a mean RF size of 18.9 +/- 3.2 mm2 (SE), a mean conduction velocity of 0.8 +/- 0.1 m/s, mean thresholds of 43.6 +/- 0.6 degrees C for radiant heat and 5.95 +/- 0.59 bars for mechanical stimulation, and no spontaneous activity. In contrast, warm fibers had punctate RFs, a mean conduction velocity of 1.1 +/- 0.1 m/s, heat thresholds of less than 1 degrees C above skin temperature, no response to mechanical stimulation, and a resting level of activity in warm skin that was suppressed by cooling. 4. The cumulative number of impulses evoked during each stimulation in the nociceptive afferents increased monotonically as a function of stimulus temperature over the range described by humans as increasingly painful (45-50 degrees C). Nociceptive fibers showed little or no response to stimulus temperatures less than 45 degrees C that elicited in humans sensations primarily of warmth but not pain. In contrast, the cumulative impulse count during stimulation of each warm fiber increased monotonically with stimulus temperature over the range of 39-43 degrees C. However, for stimuli of 41-49 degrees C the cumulative impulse count in warm fibers was nonmonotonic with stimulus temperature. Warm-fiber response to stimuli of 45 degrees C or greater usually consisted of a short burst of impulses followed by cessation of activity. 5. The subjective magnitude of warmth and pain sensations in humans and the cumulative impulse count evoked by each stimulus in warm and nociceptive afferents varied inversely with the number, delivery rate, and intensity of preceding stimulations. 6. The results of these experiments suggest the following: a) that activity in the mechanothermal nociceptive C-fibers signals the occurrence of pain evoked by radiant heat, and that the frequency of discharge in these fibers may encode the intensity of painful stimulation; b) that activity in warm fibers may encode the intensity of warmth at lower stimulus temperatures, but is unlikely to provide a peripheral mechanism for encoding the intensity of painful stimulation at higher stimulus temperatures.

Coding of mechanical stimulus velocity and indentation depth by squirrel monkey and raccoon glabrous skin mechanoreceptors.

1. A sample of 113 large, myelinated first-order afferent fibers innervating the glabrous skin of the squirrel monkey's hand proved to consist primarily of two basic types. In a sample where the only known source of sampling bias is a greater likelihood to record from larger diameter fibers, 40% of the fibers were rapidly adapting (RA) and 60% were very slowly adapting (VSA). Two units were moderately slowly adapting (MSA), and one had the properties of a Pacinian afferent (Pc). 2. The RA and VSA resemble those in the glabrous skin of other mammalian species in terms of thresholds, receptive-field areas, conduction velocities, and the coding of velocity of mechanical displacement of the skin. Mean instantaneous frequency during ramp stimulation is a power function of ramp velocity for both RA and VSA, with exponents generally less than 1.00. However, ramp discharge patterns differ for RA and VSA. 3. The VSA exhibit a wide range of coefficients of variation (CV) of their interspike-interval distributions, but form a continuous distribution with respect to this statistic. In other respects the VSA are more similar to slowly adapting type I than to slowly adapting type II. They lack spontaneous activity, have restricted receptive fields, and are relatively insensitive to skin stretch. 4. Effects of mechanical stimulus velocity and static indentation depth on static discharge rate were examined in 23 squirrel monkey and 22 raccoon SA units having receptive fields on glabrous skin of the hand. 5. Discharge rate during static indentation is a monotonic, increasing function of identation depth. However, the nature of the best-fitting function (highest r) varies from unit to unit. Using a set of standard conditions (milliseconds 100-500 of static displacements up to 960 mum, following a ramp velocity of 100 mum/ms, interstimulus interval of at least 10 s), the ratio of units for which linear, as opposed to logarithmic, functions provided the best fit was 4:3 for squirrel monkeys and 1:3 for raccoons. Few units had power functions as best fits in either species. Differences between fits for different functions within the same unit, however, were often trivial and insignificant. 6. Response rate during static skin displacement is also strongly influenced by prior stimulus ramp velocity. For at least the first 500 ms, discharge rate is positively related to onset velocity but, in many units, within the first 1 s of static displacement, this relationship reverses itself, and the inverse relationship may persist for at least 5 s.

Coding of steady and transient temperatures by cutaneous ‘cold” fibers serving the hand of monkeys

Much of the neurophysiological data obtained from rapidly cooled skin surfaces can be related to the mean frequency of discharge in single cold fibers; mean frequency data obtained during steady skin temperatures are less informative. The deterioration of the frequency code during steady temperatures parallels the deterioration in psychophysical measurements of sensations of cold. However, small differences of steady skin temperature alter thermoregulatory responses demonstrating the central availability of detailed information about steady temperatures, even though the mean frequency of the individual cold fiber's adapted discharge lacks this information. Another parameter of the single fiber's response, the temporal arrangement of impulses, provides sufficient information during steady temperatures to account for published data on thermoregulatory responses. Thus the organization of the impulse pattern in cold fibers may constitute an additional code capable of conveying information about steady skin temperature centrally.