The influence of rate of skin indentation on threshold and suprathreshold tactile sensations

Relative posture between head and finger determines perceived tactile direction of motion

The hand explores the environment for obtaining tactile information that can be fruitfully integrated with other functions, such as vision, audition, and movement. In theory, somatosensory signals gathered by the hand are accurately mapped in the world-centered (allocentric) reference frame such that the multi-modal information signals, whether visual-tactile or motor-tactile, are perfectly aligned. However, an accumulating body of evidence indicates that the perceived tactile orientation or direction is inaccurate; yielding a surprisingly large perceptual bias. To investigate such perceptual bias, this study presented tactile motion stimuli to healthy adult participants in a variety of finger and head postures, and requested the participants to report the perceived direction of motion mapped on a video screen placed on the frontoparallel plane in front of the eyes. Experimental results showed that the perceptual bias could be divided into systematic and nonsystematic biases. Systematic bias, defined as the mean difference between the perceived and veridical directions, correlated linearly with the relative posture between the finger and the head. By contrast, nonsystematic bias, defined as minor difference in bias for different stimulus directions, was highly individualized, phase-locked to stimulus orientation presented on the skin. Overall, the present findings on systematic bias indicate that the transformation bias among the reference frames is dominated by the finger-to-head posture. Moreover, the highly individualized nature of nonsystematic bias reflects how information is obtained by the orientation-selective units in the S1 cortex.

Temporal factors in tactile spatial acuity: evidence for RA interference in fine spatial processing

We investigated the extent to which subjects' ability to perceive the fine spatial structure of a stimulus depends on its temporal properties (namely the frequency at which it vibrates). Subjects were presented with static or vibrating gratings that varied in spatial period (1-8 mm) and vibratory frequency (5-80 Hz) and judged the orientation of the gratings, presented either parallel or perpendicular to the long axis of the finger. We found that the grating orientation threshold (GOT)-the spatial period at which subjects can reliably discriminate the orientation of the grating-increased as the vibratory frequency of the gratings increased. As the spatial modulation of SA1 and RA afferent fibers has been found to be independent of vibratory frequency, the frequency dependence of spatial acuity cannot be attributed to changes in the quality of the peripheral signal. Furthermore, we found GOTs to be relatively independent of stimulus amplitude, so the low spatial acuity at high flutter frequencies does not appear to be due to an inadequacy in the strength of the afferent response at those frequencies. We hypothesized that the RA signal, the strength of which increases with vibratory frequency, interfered with the spatially modulated signal conveyed by SA1 fibers. Consistent with this hypothesis, we found that adapting RA afferent fibers improved spatial acuity, as gauged by GOTs, at the high flutter frequencies.

SA1 and RA receptive fields, response variability, and population responses 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 mm below the point of initial skin contact (the background indentation) before delivering single-probe indentations. SA1 and RA responses differed in several ways. 1) SA1 RF boundaries were affected much less by indentation depth than were RA boundaries, and the SA1 RF areas were much more uniform in size. The mean SA1 RF area grew from 5.1 to 8.8 mm2 as the indentation depth increased from 50 to 500 microm; the mean RA RF area grew from 5.5 to 22.4 mm2 over the same intensity range. 2) SA1 RFs were more elongated than RA RFs. Elongated RFs were oriented in all directions relative to the skin ridges and the finger axis. 3) SA1 impulse rates were linear functions of indentation depth at all probe locations in the RF; RA responses tended toward saturation beginning at 100 microm indentation depth when the probe was over the HS. Similarities between SA1 and RA responses were that 1) both were extremely repeatable with SDs < 1 impulse per trial and 2) both had population responses (number of impulses) that were nearly linear functions of indentation depth. However, SA1s represented increasing indentation depth by increasing impulse rates in a small, relatively constant group of afferents, whereas the RAs represented increasing indentation depth predominantly by the recruitment of new afferents at a distance.

Relationships between touch sensations and estimated population responses of peripheral afferent mechanoreceptors

Trapezoidal indentations of the skin by a 0.5-mm-diameter probe were presented at different rates and loads (forces) to the human fingertip, in order to compare estimates of population responses of cutaneous mechanoreceptors with the quality and magnitude of tactile sensations. The subjects were first trained to attend to and evaluate variations in the magnitude of touch sensations associated with the onset ramp, the plateau period, and the offset ramp. They examined a series of line drawings that illustrated a variety of temporal profiles for sensation magnitude. The line drawings provided a straight-forward means of describing temporal fluctuations of sensation intensity, which corresponded well to psychophysical ratios that were determined subsequently with a matching procedure. Influences of ramp rate on qualities of touch sensations were evaluated by tabulating verbal descriptions of sensory experiences. Each of three rate conditions generated a different quality of sensation during the dynamic portions of stimulation. Onsets and offsets at 100 g/s were described as "taps". During ramps at 10 g/s the quality was described as "rolling" or "moving". At 1 g/s no sense of motion was detected; instead, a "pressure" sensation was identified. Touch sensations during the plateau were always described as a pressure. The subjective magnitudes of touch sensations associated with the onset, plateau, and offset were equated by comparing different components of paired stimuli. At 100 g/s, when subjects matched the offset sensation from the first of a pair of stimuli with the onset sensation from the second, the force of the stimulus producing the offset sensation was 1.3 times greater than the intensity of the stimulus that produced the onset sensation. Matching of the plateau sensation (evaluated during the last 1.5 s of the 2.5-s plateau period) with the onset sensation required a plateau stimulus that was 1.7 times greater in force than the stimulus which produced the onset. Comparison of stimulus intensities producing a match of plateau and offset sensations with stimulus intensities predicted from the previous matches (onset versus offset and onset versus plateau) demonstrated a mean within-subject error of 4%. The mean ratio of plateau to offset forces that produced a match was 1.8:1.3. In a matching procedure in which subjects compared the subjective magnitudes of plateau sensations following onset ramps of different rates, onset ramp rate significantly influenced the magnitude of pressure sensations. The ratios of plateau forces which produced equal magnitudes of sensation following 1, 10, and 100 g/s ramps were 1.6:1.3:1.0.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

Coding of stimulus location and intensity in populations of mechanosensitive nerve fibers of the raccoon: I. Single fiber response properties

The primary aim of this study was to determine and compare the receptive field (RF) characteristics and response properties of single mechanosensitive nerve fibers innervating the glabrous skin of the forepaw and hindpaw of the raccoon. The action potentials of 129 median nerve fibers and 61 posterior tibial nerve fibers were recorded in response to punctate mechanical stimuli varying in location and intensity. The stimuli were delivered to six standard test sites on digit 1 and the contiguous pads of each paw. Attempts were made to classify each fiber according to its rate of adaptation to sustained stimulation; the RF of each fiber was mapped using a standard series of stimulus intensities. The results indicated that the response properties of individual fibers were highly complex and depended on the location and intensity of stimulation. 1) The distributions of absolute threshold were not different for the median or tibial nerve fibers or for different classes of fibers based on adaptation rate. A distal to proximal increase in threshold was found for each paw, suggesting a corresponding gradient of sensitivity across the glabrous skin. 2) Threshold RF areas did not vary across either paw nor did they differ between the two paws. Suprathreshold RFs were quite large relative to expected tactile acuity and displayed complex features. 3) Response properties such as adaptation rate, on- and off-responses, were found to vary with both stimulus location and intensity. It was concluded that the responses of individual nerve fibers could not uniquely encode any stimulus parameter tested, and that the properties of single fibers could not account for apparent differences in tactile acuity across each paw or between the two paws.

Linear systems analysis of cutaneous type I mechanoreceptors

Linear system transfer functions of Type I mechanoreceptor domes in hairy skin were extracted for study. The system input, defined in units of indentation depth, was a punctate, 85 Hz bandlimited colored noise indenting stimulus. The system output was a dome's afferent AP event stream. After digitally low-pass filtering the AP event stream to prevent aliasing, transfer and coherence function estimates were generated from spectral and cross spectral estimates of the sampled input and output signals. Transfer function magnitudes (receptor sensitivity) beyond 10 Hz could be well fit by a fractional exponent (0.35 +/- 0.10) of frequency, indicating fractional order differentiation. Transfer function phases were near 120 degrees for frequencies less than 10 Hz and decreased progressively beyond 10 Hz. Coherence function estimates were low as a result of inherent nonlinearities, primarily rectification. Single dome results closely matched those reported by French and Kuster [6] for the tactile spine of the cockroach, suggesting that in terms of spectral sensitivity the receptors have similar transduction mechanisms. However, nonlinear systems modeling techniques are required for a more complete system characterization.

Psychophysical assessment of tactile, pain and thermal sensory functions in burning mouth syndrome

Tactile, two-point discrimination, thermal change detection and heat pain thresholds as well as oral stereognostic ability, warmth scaling and heat pain tolerance were compared in a group of 72 subjects with burning mouth syndrome (BMS) and 43 age- and sex-matched control subjects. No differences were found between the BMS and control subjects for any of the sensory modalities tested except for heat pain tolerance. Pain tolerance was significantly decreased for the BMS subjects at the tongue tip, a site of clinical pain in approximately 85% of the subjects tested in this study, but not at the cutaneous lower lip which was a site of pain only in approximately 17% of the subjects tested in this study. In addition, no differences in heat pain tolerance were found at the cutaneous lower lip between the control subjects and the BMS subjects who reported pain on the mucosal lower lip (approximately 49% of subjects), but heat pain tolerance was significantly decreased at this site for those BMS subjects tested without pain on the mucosal lower lip (approximately 51% of subjects). These findings do not suggest a psychogenic origin for the alteration of heat pain tolerance in the BMS subjects, but suggest instead specific changes in their peripheral or central sensory functions.

Tactile detection threshold determined with single sinusoidal mechanical pulses in the monkey skin

Tactile detection thresholds for single sinusoidal mechanical pulses increased with decreases in the frequency of the stimulus pulse (from 150 to 20 Hz) in a monkey's skin. The results correspond with those of similar electrophysiological studies.