1. We investigated the ability of humans to tactually discriminate the softness of objects, using novel elastic objects with deformable and rigid surfaces. For objects with deformable surfaces, we cast transparent rubber specimens with variable compliances. For objects with rigid surfaces ("spring cells") we fabricated telescoping hollow cylinders with the inner cylinder supported by several springs. To measure the human discriminability and to isolate the associated information-processing mechanisms, we performed psychophysical experiments under three conditions: 1) active touch with the normal finger, where both tactile and kinesthetic information was available to the subject: 2) active touch with local cutaneous anesthesia, so that only kinesthetic information was available; and 3) passive touch, where a computer-controlled mechanical stimulator brought down the compliant specimens onto the passive fingerpad of the subject, who therefore had only tactile information. 2. We first characterized the mechanical behavior of the human fingerpad and the test objects by determining the relationship between the depth and force of indentation during constant-velocity indentations by a rigid probe. The fingerpad exhibited a pronounced nonlinear behavior in the indentation depth versus force trace such that compliance, as indicated by the local slope of the trace, decreased with increases in indentation depth. The traces for all the rubber specimens were approximately linear, indicating a constant but distinct value of compliance for each specimen. The fingerpad was more compliant than each of the rubber specimens. 3. All the human subjects showed excellent softness discriminability in ranking the rubber specimens by active touch, and the subjective perception of softness correlated one-to-one with the objectively measured compliance. The ability of subjects to discriminate the compliance of spring cells was consistently poorer compared with that of the rubber specimens. 4. For pairwise discrimination of a selected set of rubber specimens, kinesthetic information alone was insufficient. However, tactile information alone was sufficient, even when the velocities and forces of specimen application were randomized. In contrast, for discriminating pairs of spring cells, tactile information alone was insufficient, and both tactile and kinesthetic information were found to be necessary. 5. The differences in the sufficiency of tactile information for the discrimination of the two types of objects can be explained by the mechanics of contact of the fingerpad and its effect on tactile information. For objects with deformable surfaces, the spatial pressure distribution within the contact region depends on both the force applied and the specimen compliance.(ABSTRACT TRUNCATED AT 250 WORDS)

The capacities of monkeys and humans to discriminate between mechanical sinusoids differing in amplitude or frequency were measured in a two-alternative, forced-choice task. The difference limen for amplitude discrimination for both species remained constant near 10% of the standard amplitude over the range of 17-30 dB, relative to detection threshold. Equal subjective intensity curves in the 20-40 Hz range were determined at 20 and 29 dB, relative to detection threshold. These curves followed the threshold curve and were identical for the two species. The difference limen for frequency discrimination averaged 1.8 Hz for humans and 2.7 Hz for monkeys; the range of values for the two species overlapped nearly completely. The small sizes of these difference limens indicate, we believe, the capacity of highly trained individuals of either species to ascertain small differences in the temporal order of somesthetic stimuli and of the neural events evoked by them. In one series of experiments we demonstrated that subjects of both species possess two threshold for two different aspects of flutter-vibration which are displaced from each other along the intensive continuum. For either species, the minimum level of stimulus amplitude required for threshold frequency discrimination is about 8 dB above that sufficient for detection. This difference in amplitude is called the atonal interval and matches that observed between absolute and tuning thresholds for quickly adapting, mechanoreceptor afferents (the Meissner afferents) which innervate the glabrous skin of the monkey hand. These and previous findings have permitted a number of direct correlations between behavioral and neural events as regards the sense of flutter. The neural codes for the intensity and frequency of flutter appear to be different. The capacity to detect the presence of a mechanical sinusoid and the capacity to judge its subjective intensity are likely to depend on criterion levels of activity in the total population of Meissner afferents, the former on the appearance of any activity (absolute threshold) in a small population of the most sensitive of these fibers and the latter on the overall size of the active population of neuronal elements at each level of amplitude. The total activity in the relevant neural population elicited by sinusoids of increasing amplitude defines a prothetic continuum along which subjects can judge the magnitude of sensation..

1. The role of the microgeometry of planar surfaces in the detection of sliding of the surfaces on human and monkey fingerpads was investigated. By the use of a servo-controlled tactile stimulator to press and stroke glass plates on passive fingerpads of human subjects, the ability of humans to discriminate the direction of skin stretch caused by friction and to detect the sliding motion (slip) of the plates with or without micrometer-sized surface features was determined. To identify the associated peripheral neural codes, evoked responses to the same stimuli were recorded from single, low-threshold mechanoreceptive afferent fibers innervating the fingerpads of anesthetized macaque monkeys. 2. Humans could not detect the slip of a smooth glass plate on the fingerpad. However, the direction of skin stretch was perceived based on the information conveyed by the slowly adapting afferents that respond differentially to the stretch directions. Whereas the direction of skin stretch signaled the direction of impending slip, the perception of relative motion between the plate and the finger required the existence of detectable surface features. 3. Barely detectable micrometer-sized protrusions on smooth surfaces led to the detection of slip of these surfaces, because of the exclusive activation of rapidly adapting fibers of either the Meissner (RA) or the Pacinian (PC) type to specific geometries of the microfeatures. The motion of a smooth plate with a very small single raised dot (4 microns high, 550 microns diam) caused the sequential activation of neighboring RAs along the dot path, thus providing a reliable spatiotemporal code. The stroking of the plate with a fine homogeneous texture composed of a matrix of dots (1 microns high, 50 microns diam, and spaced at 100 microns center-to-center) induced vibrations in the fingerpad that activated only the PCs and resulted in an intensive code. 4. The results show that surprisingly small features on smooth surfaces are detected by humans and lead to the detection of slip of these surfaces, with the geometry of the microfeatures governing the associated neural codes. When the surface features are of sizes greater than the response thresholds of all the receptors, redundant spatiotemporal and intensive information is available for the detection of slip.

The peripheral neuronal correlates of heat pain elicited from normal skin and from skin made hyperalgesic following a mild heat injury were studied by simultaneously recording, in humans, evoked responses in C mechanoheat (CMH) nociceptors and the magnitude estimations of pain obtained from the same subjects. Subjects made continuous magnitude ratings of pain elicited by short-duration stimuli of 39-51 degrees C delivered to the hairy skin of the calf or foot before and at varying intervals of time after a heat injury induced by a conditioning stimulus (CS) of 50 degrees C, 100 s or 48 degrees C, 360 s. The stimuli were applied with a thermode pressed against the nociceptor's receptive field. For heat stimulations of normal skin, that is, uninjured skin, pain thresholds in 14 experiments with nine subjects ranged from 41 to 49 degrees C, whereas response thresholds for most of the 14 CMH nociceptors were 41 degrees C (in two cases, 43 degrees C). The latter suggested that spatial summation of input from many nociceptors was necessary at pain threshold. An intensity-response function was obtained for each CMH by relating the total number of nerve impulses evoked per stimulus to stimulus temperature. A corresponding magnitude scaling function for pain was obtained by relating the maximum rating of pain elicited by each stimulus to stimulus temperature. The relation between the subject's scaling function and the intensity-response function of his CMH nociceptor varied somewhat from one experiment to the next, regardless of whether the results were obtained from the same or from different subjects. However, when averages were computed for all 14 tests, there was a near linear relationship between the mean number of impulses elicited in the CMHs and the median ratings of pain, over the range of 45-51 degrees C. It was concluded that the magnitude of heat pain sensation was more closely related to the magnitude of response in a population of CMH nociceptors than in any individual nociceptor. At 0.5 min after the CS, the pain thresholds of most subjects were elevated, and the magnitude ratings of pain elicited by supra-threshold stimuli were lower than pre-CS values (hypoalgesia). Corresponding changes were seen in the increased thresholds and decreased responses (fatigue) of most CMHs. By 5-10 min after the CS, the pain thresholds of most subjects were lower, and their magnitude ratings of suprathreshold stimuli were greater than pre-CS values (hyperalgesia).(ABSTRACT TRUNCATED AT 400 WORDS)

The representation of shape in the responses of monkey cutaneous mechanoreceptive afferents to steps of varying shape vertically indented into the fingerpad was studied. A series of flat plates was used, each with a step change in thickness in the middle so that one-half of the plate was thicker than the other. The cross-sectional shape of the step approximated that of a half-cycle sinusoid, 0.5 mm high, that was varied in half-cycle wavelength (step width) and hence in steepness and curvature. The steps fell into 2 categories, characterized as "steep" and "'gradual." Evoked action potentials were recorded from single, slowly adapting and from rapidly adapting Meissner corpuscle mechanoreceptive afferent fibers (SA and RA, respectively) innervating the fingerpad of the anesthetized monkey while each step was indented at a succession of lateral positions across the fiber's receptive field. The responses of each SA provided a spatial response profile (number of evoked impulses as a function of step position) that was directly related to the variation in curvature across the step. The rate of discharge was greatest under the sharpest (convex) portion of the step, least under the adjacent concave portion, and intermediate under the flat portions of the steps. The results indicated an exquisite sensitivity of the SA, even during the ramp phase of vertical indentation, to the changes in skin curvature. The spatial response profile remained relatively undistorted over time during the ensuing steady phase, while the contrast between the peak and the minimum response improved. RAs responded only during the ramp phase and with fewer responses, and gave rise to a poorly modulated spatial response profile, even though half of the RAs tested showed limited sensitivity to the amount or rate of change of skin curvature. It was hypothesized that RA responses are predominantly influenced by the vertical velocity of the most sensitive spot in the receptive field. When the same step stimuli were applied to the human fingerpad, the capacities of humans to discriminate differences in step shape were found to correlate with the discriminability of SAs, as opposed to the considerably poorer discriminability of RAs. It is concluded that information concerning the local curvature and hence the shape of objects indenting the skin is primarily coded by the SAs. In the 2 preceding papers (LaMotte and Srinivasan, 1987a, b), we investigated the responses of SAs and RAs to the same sinusoidal steps stroked back and forth across the fingerpad under constant compressional force.(ABSTRACT TRUNCATED AT 400 WORDS)