Human Fingerpad Research Articles

Investigation of vibrotactile sensation of human fingerpads by observation of contact zones

An experimental apparatus has been developed to observe the contact zones between human fingerpads and a smooth flat surface of a total-reflection prism. Using this apparatus, the following experiments have been carried out: (1) measurement of the apparent and real contact areas of fingerpads under a controlled contact load, (2) measurement of the vibrotactile thresholds of fingerpads under a controlled contact load, and (3) investigation of the discrimination of tones by tactile sensation under a controlled contact load. From the third experiment, it is found that human fingerpads can discriminate between tones from the differences in their harmonic components.

Three-dimensional line-field Fourier domain optical coherence tomography for in vivo dermatological investigation

We demonstrate 3-D optical coherence tomography using only 1-D mechanical scanning. This system uses the principle of Fourier domain optical coherence tomography for depth resolution, 1-D imaging for lateral vertical resolution, and mechanical scanning by a galvanometer for lateral horizontal resolution. An in vivo human fingerpad is investigated in three dimensions with an image size of 480 points (vertical) x 300 points (horizontal) x 1024 points (depth), which corresponds to 2.1 x 1.4 x 1.3 mm. The acquisition time for a single cross section is 1 ms and that for a single volume is 10 s. The system sensitivity is 75.6 dB at a probe beam power of 1.1 mW.

Dynamic lumped element response of the human fingerpad.

The dynamic response of the fingerpad plays an important role in the tactile sensory response and precision manipulation, as well as in ergonomic design. This paper investigates the dynamic lumped element response of the human fingerpad in vivo to a compressive load. A flat probe indented the fingerpad at a constant velocity, then held a constant position. The resulting force (0-2 N) increased rapidly with indentation then relaxed during the hold phase. A quasilinear viscoelastic model successfully explained the experimental data. The instantaneous elastic response increased exponentially with position, and the reduced relaxation function included three decaying exponentials (with time constants of approximately 4 ms, 70 ms, and 1.4 s) plus a constant. The model was confirmed with data from sinusoidal displacement trajectories.

Tactual discrimination of softness.

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)

Responses of human mechanoreceptive afferents to embossed dot arrays scanned across fingerpad skin

The spatial resolving capacities of the four classes of mechanoreceptive afferents innervating human fingerpad skin were investigated to determine which class sets the limit of tactile spatial resolution for scanning stimuli. The stimulus consisted of an array of embossed dots (0.7 mm diameter, 0.5 mm high) arranged in a tetragonal pattern with dot spacing decreasing linearly from 6.4 mm at one end of the array to 0.87 mm at the other. The pattern was wrapped around a drum and repeatedly scanned across the receptive field of single afferents by continuously rotating the drum. Responses to many closely spaced scans were obtained by imposing a lateral shift of the pattern between each revolution. Impulses were recorded microneurographically. Responses were plotted in raster form to produce a neural image of the pattern. Responses of rapidly and slowly adapting type I (FAI and SAI) afferents resolved dots down to a spacing of about 1.5 mm. Responses of type II (FAII and SAII) afferents resolved dots down to a spacing of about 3.5 mm. Variation in scanning speed (range, 20-90 mm/sec) and contact force (range, 0.4-1.0 N) had minimal effects on spatial resolution of all afferents. The response clusters associated with individual widely spaced dots were used to investigate receptive field structure. FAI and SAI fields (mean areas, 6.1 and 4.8 mm2, respectively) each contained several zones of maximal sensitivity. FAI fields had five to eight such zones, whereas SAI fields had three to five such zones. As dot spacing decreased, neighboring dots interacted to affect the responses associated with the individual zones within a field. Initially, one or more zones were deactivated, effectively reducing receptive field size and allowing representation of finer spatial detail than would be predicted from the overall area of the receptive field. At very close dot spacings responses were only obtained when more than one sensitive zone within a field were simultaneously activated by different dots.

Specific [125I]Bolton-Hunter substance P binding sites in human and rat skin

[125I]Bolton-Hunter substance P [( 125I]BH-SP) binding sites in rat and human skin were investigated, using quantitative receptor autoradiographic and emulsion autoradiographic methods. [125I]BH-SP binding sites were discretely localized in skin areas anatomically corresponding to dermal papillae, sweat glands, and hair follicles. The highest density of the binding sites was in the dermal papilla of the finger, followed by the sweat gland. [125I]BH-SP binding to the dermal papillae of the human finger pad skin and rat paw pad skin was displaced by unlabeled SP, with a high affinity, and Kd values were calculated to be 744 pM and 297 pM, respectively. The existence of [125I]BH-SP binding sites supports the idea of the neurotransmitter role of substance P in skin dermal papilla.

Calcitonin gene-related peptide-like immunoreactivity in nerve fibers in the human skin

Calcitonin gene-related peptide-like immunoreactivity was demonstrated in in sensory nerve fibers in the epidermis and dermis as free nerve endings and around blood vessels and hair follicles of the human finger pad and arm skin. The vast majority of the calcitonin gene-related immunoreactive fibers was shown to display also substance P-like immunoreactivity and a few fibers in the dermis were somatostatin positive. No fibers displaying both substance P and somatostatin-like immunoreactivity were found but a few substance P immunoreactive fibers in the dermis-epidermis region were found to contain also vasointestinal polypeptide-like immunoreactivity. In the sweat glands, abundant calcitonin gene-related peptide positive, but substance P negative, fibers were observed with a similar distribution pattern as the vasoactive intestinal polypeptide immunoreactive fibers and these fibers were suggested to be of sympathetic origin.

Tactile discrimination of shape: responses of slowly adapting mechanoreceptor afferents to a step stroked across the monkey fingerpad

The representation of shape in the responses of slowly adapting mechanoreceptive afferent fibers (SAs) in monkeys was investigated. A series of flat plates was used, each having an increase in thickness (a step) 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 of a sinusoid. The height of the step was fixed at 0.5 mm, while its width (half-cycle wavelength) was varied from 0 to 3.13 mm, resulting in step shapes that varied in steepness and curvature. The steps fell into 2 categories, characterized as “steep” and “gradual.” A servocontrolled mechanical stimulator stroked each step across the distal fingerpad from the high to the low side of the step and back, while maintaining the contact force at 20 gm wt. Evoked action potentials in single SAs innervating the fingerpads of anesthetized monkeys were recorded. Each SA's response to a step provided a spatial response profile (discharge rate as a function of step position) that reflected the distribution of curvature across the step shape. All the major features of the SA response could be consistently explained as being due to the sensitivity of the SA to the amount and rate of change in skin curvature. The response profile was altered by changes in stroke direction, step shape, and stroke velocity. Differences in stroke direction (back and forth) were indicated by differences in pattern of response: a “burst-pause-burst” for strokes from high to low, and a “pause-burst-pause” for strokes from low to high; a greater discharge rate in response to the step for low to high strokes, and for some SAs, the reduction or absence of basal discharge in one of the directions. The discharge rate during the burst for either direction of stroking was greater for steep than for gradual steps, and increased, for a given step shape, with increases in stroke velocity. Regardless of differences in stroke velocity, steep steps were distinguished from gradual steps by having narrower burst widths for low-to-high strokes and narrower pause widths for high-to-low strokes. The same stimuli were delivered to the human fingerpad, and the capacities of humans to discriminate between the steps were measured. It was concluded that the spatial features of SA responses, representing the widths of regions of active and inactive SA populations, as well as the intensive feature of discharge rate, accounted for the gross sensory discriminations of shape.(ABSTRACT TRUNCATED AT 400 WORDS)

Tactile discrimination of shape: responses of slowly and rapidly adapting mechanoreceptive afferents to a step indented into the monkey fingerpad

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)

The role of specialized nerve terminals in cutaneous sensibility.

Correlation of sensory testing and histological examination does not support the belief that the morphology of the nerve terminals in the human lip and finger pad determines a sensory I specificity. No ‘organized endings’ have been found in hairy skin from the abdomen, the dorsum of the finger or the lip, in spite of the use of techniques capable of satisfactorily demonstrating them elsewhere in the same histological section. It is suggested that there may be a simple, consistent and non-specific plan of construction underlying the apparently diverse morphology of both the ‘organized endings' and the terminals around the hairs.