POPULATION ESTIMATES FOR RESPONSES OF CUTANEOUS MECHANORECEPTORS TO AVERTICALLY INDENTING PROBE ON THE GLABROUS SKIN OF MONKEYS

Recordings were obtained from low-threshold mechanoreceptive afferents during stimulation with a o.5-mm-diameter probe at the receptive fiel d (RF) center and at different distances from the point of maximal sen sitivity. 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. T he 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 stimul ation away from the RF centers revealed that RF size for RAs was prima rily dependent upon ramp rate, and for SAIs the size of the RF was pri marily dependent upon load (force). The action potentials from individ ual afferents during stimulation at each location were binned in time and assigned to spatial segments of 1 mm. These responses were multipl ied by: (A) an annular area of the receptive field and (B) the innerva tion density for the afferent type and skin region. The calculations p rovided estimates of overall rates of activity among the population of cutaneous afferents that respond to indentation by a small probe. Imp ortant differences were obtained between the responses of the populati on of afferents activated by the trapezoidal stimulus and the response s of afferents stimulated only at the RF center. Populations of tactil e afferents provide more information for rate and intensity (force) di scriminations than is available from units stimulated at the RF center . For RA afferents, the exponent of the power function describing rela tionships between stimulus rate and the population discharge (in impul ses per second) was 0.3 times greater than the exponent for responses to on-center stimulation. For SAI mechanoreceptors, the exponent of th e power functions for static responses to force was 0.22 times greater for the population responses than for on-center activation. Populatio n 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 popu lation extends the range of tactile discriminability. The slope and ra nge of stimulus-response functions for populations was enhanced relati ve to responses to on-center stimulation. This occurs because of recru itment of afferents with RF centers adjacent to and remote from the st imulus, depending upon thresholds and receptive field sizes for differ ent stimulus parameters. With stimulation at increasing rates and forc es, there is a progressive spatial recruitment of receptors. Over 90% of the activity elicited by suprathreshold punctate stimuli originated from mechanoreceptors with RF centers 1 mm or more away from the stim ulus site. When the population response of SA afferents was calculated for different intensities of plateau stimulation, ranging from 1 to 4 g, the slope of the power function corresponded well to psychophysica l estimates in the literature on the growth of touch intensity. Recrui tment of afferents stimulated off the RF center shaped the temporal pa ttern of discharge. For RA afferents, the population response reached peak rates toward later portions of the onset and offset response than for on-center stimulation. For SAI afferents, the population discharg e during slow onsets accelerated more positively than the responses to on-center stimulation. Variations in the rate, amplitude, and duratio n of stimulation were demonstrated to be useful in assessing the contr ibution of SAI and RA afferents to different tactile sensations. At ve ry slow rates of stimulus application, the RA response was so minimal that the population response can be considered to arise from SAI affer ents. At high stimulus rates, the population response was greatly acce ntuated during the onset (indentation) and offset (removal) of a trape zoidal ramp-and-hold stimulus, relative to firing rates during maintai ned indentation. Ratios of dynamic to static discharge were 3-4 times greater for the population than for on-center stimulation, reaching va lues as high as 60.2:1. The ratios of dynamic to static population res ponses were greatest for stimuli presented to the palm and were least for stimuli presented to the base and middle phalanges of the fingers. Therefore, the relative magnitudes of onset, offset, and steady-state sensations elicited by stimulation at different rates and locations s hould vary systematically, according to the absolute and relative dens ities of each receptor type.