Biomedical subjects

Mark Hollins

Publications and source records attributed to Mark Hollins.

11 recordsLinked to original sources

PubMed2006

Somatosensory coding of roughness: the effect of texture adaptation in direct and indirect touch.

To examine somatosensory mechanisms contributing to the perception of roughness, subjects examined surfaces with rigorously specified spatial textures under conditions of direct moving contact between the fingertip and the surface (direct touch) and contact through a rigid probe (indirect touch). Subjects were trained to scan the surfaces along a consistent path and with a speed of 2.7 cm/s. With each mode of touch, periods of prolonged inspection of a single adapting surface were followed by shorter periods in which the roughness of multiple test surfaces was reported. Adaptation caused a drop in perceived roughness under conditions of indirect touch, reflecting the reduced sensitivity of vibrotactile mechanisms that are the main recipients of textural information transmitted through the probe. During direct touch, adaptation had no significant effect on the perception of textures with spatial period >200 mum, which are spatially encoded. The results have an important implication for the physiological basis of the spatial code, which is believed to involve somatosensory cortical neurons with highly structured receptive fields: these cortical populations appear to be less susceptible to adaptation than otherwise similar neuronal populations in the visual system.

Action Potentials↗

PubMed2005

Factors contributing to the integration of textural qualities: evidence from virtual surfaces.

Two experiments involving indirect touch were carried out to explore the relationships among perceptual dimensions of haptically examined surfaces. Subjects in both experiments used a stylus to evaluate the properties of virtual surfaces created by a force-feedback device; four surface properties (resistance to normal force, coefficient of friction, texture scale, and vibration amplitude) were manipulated in various combinations. In Experiment 1, the extent to which there was a one-to-one relationship between specific stimulus properties and perceptual qualities ("perceptual separability") was evaluated. A substantial failure of separability was demonstrated, with friction tending to be more separable from the other properties than they were from one another. The pattern of results suggests that the amount of measured separability depends crucially on the way stimulus properties are defined (e.g., force versus displacement). In Experiment 2, surfaces with known perceptual properties were used to study the metric(s) of the relevant perceptual space. By specifying the perceptual, rather than the stimulus, coordinates of the surfaces, it was possible to bypass issues of perceptual separability. For surfaces of equal friction, a Euclidean metric captured the results (r(2) = 0.75) more effectively than a city-block metric did; neither metric did well when differences in friction were involved. The fact that-unlike stickiness-hardness, roughness, and perceived vibration intensity are all increasing functions of surface-normal forces may facilitate their integration into a Euclidean space, in both direct and indirect touch.

Adolescent↗

PubMed2005

Vibrotactile intensity and frequency information in the pacinian system: a psychophysical model.

The objective of the study was to characterize the Pacinian representation of stimulus waveform. Subjects were presented with pairs of high-frequency vibrotactile stimuli that varied in intensity and/or frequency content and made same-different judgments under conditions of low-frequency adaptation designed to minimize the contribution of the RA system. We wished to infer the nature of the information conveyed by the Pacinian system about the stimuli from measured sensitivity (d') to stimulus differences. We first tested the hypothesis that the Pacinian system conveys only intensive information about vibratory stimuli and found that intensive cues could not account for much of the variance in the discrimination data. We then proposed a model characterizing the Pacinian-mediated representation of an arbitrary stimulus as a pattern of activation in a set of frequency-tuned minichannels. The model was shown to predict the discriminability of the stimulus pairs presented in the psychophysical experiments. Furthermore, the model parameters, optimized to fit the discrimination data, were compatible with analogous values obtained in other experimental contexts. One of the assumptions underlying the model is that information about individual spectral components is conveyed in parallel and quasi-independently. By simulating the response of a population of Pacinian afferents to a polyharmonic stimulus, we demonstrated that such a population can simultaneously convey information about multiple frequency components, despite having a homogeneous spectral profile.

Adult↗

PubMed2005

Pacinian representations of fine surface texture.

Subjects were presented with pairs of finely textured stimuli and were instructed to rate their dissimilarity, using free magnitude estimation. The subjects also rated the stimuli along each of four textural continua: roughness, hardness, stickiness, and warmth. In subsequent experimental sessions, we used a Hall effect transducer to measure the vibrations produced in the subjects' fingertip skin as the stimuli were scanned across it. We wished to assess the extent to which the perceptual dissimilarity of the textures could be explained in terms of the perceptual dissimilarity of the vibrations they elicited in the skin. To that end, we invoked a model characterizing the Pacinian representation of a vibratory stimulus. From the model, we computed the difference in the vibratory representations of the two stimuli in each pair. We found that the bulk of the variance in perceived dissimilarity of the textures was accounted for by differences in the Pacinian representations of the vibrations they produced. Our results further suggested that the textural information conveyed by the Pacinian system concerns surface roughness and, possibly, stickiness.

Adult↗

PubMed2004

Haptic perception of virtual surfaces: scaling subjective qualities and interstimulus differences.

We examined, in two experiments, the perceptual scaling of the properties of haptically examined virtual surfaces, and the way in which these properties subjectively combine. Participants used a consistent movement pattern to explore, with a stylus, virtual surfaces generated by a force-feedback device. In experiment 1, four surface properties (bump size, friction, resistance to normal force, and vibration amplitude) were varied individually, in separate blocks of trials. Free magnitude estimates of the subjective dimensions corresponding to these properties showed that all four dimensions conformed closely to the power law, except at very low stimulus values. Exponents for bump size (0.80) and stiffness (1.01) were consistent with values established in earlier work with direct touch of real surfaces. Surprisingly, the exponent for stickiness, not previously measured, was much higher than those for other dimensions (1.49). In experiment 2, dimensional combinations were analyzed by asking subjects to give magnitude estimates of the subjective difference between pairs of surfaces differing in one or two properties. Magnitude estimates of a given one-dimensional difference were generally larger when the subject was pressing down firmly on the surfaces, than when only gentle downward pressure was required; this result suggests that forces generated when a surface is haptically examined are interpreted as invariant indicators of the magnitudes of the surface properties themselves. Estimates of one-dimensional differences were also used to make predictions of two-dimensional differences, under assumptions of dimensional integrality and separability. The results fell between these two sets of predictions, indicating only modest integration of surface properties examined with indirect touch.

Computer Simulation↗

PubMed2003

Reduction of TMD pain by high-frequency vibration: a spatial and temporal analysis.

Under some conditions, vibration delivered to the skin can reduce pain (vibratory analgesia). Previous studies of this phenomenon in a clinical context have been somewhat variable in terms of stimulus control, and have not examined the way in which the spatial distribution of pain is affected. In the present study, we used rigorously controlled conditions to examine vibratory analgesia in participants (N=17) with painful temporomandibular disorders (TMD). Results of 20- and 100-Hz vibration were compared with data from a no-vibration control condition. The results document for the first time that vibratory analgesia occurs in TMD chronic pain conditions. We measured its time course using continuous visual analog scale (VAS) recording, and its spatial aspects by asking subjects to indicate painful regions on standardized drawings. VAS ratings and drawings both showed that pain is reduced by 100-Hz, but not by 20-Hz, vibration. The effectiveness of the high-frequency vibration cannot be attributed to a mechanism involving Pacinian corpuscles, since these receptors are lacking in the skin of the orofacial region. Spatial analyses revealed that ipsilateral and contralateral effects of vibration were statistically equivalent, suggesting that vibratory analgesia relies at least in part on central nervous system processes rather than local mechanisms.

Adolescent↗

PubMed2003

Vibratory antinociception: effects of vibration amplitude and frequency.

The ability of cutaneous vibration to compromise detection of a nociceptive stimulus was examined in 2 sets of psychophysical experiments. The noxious stimulus was a 10-millisecond burst of radiant heat from a CO(2) laser; at the near-threshold levels used it generally yielded a mild pricking sensation. In both experiments, the detectability (d(e)') of the laser was measured in the presence of different vibratory stimuli and in the absence of vibration. Periods of vibration lasted 10 seconds, bracketing the time of occurrence of the laser. Vibratory and laser stimuli were presented 2.3 cm apart on the dorsal surface of the forearm. Confidence rating procedures yielded receiver operating characteristic curves from which detectability of the laser was calculated. In an amplitude study, vibrations ranging from 10 to 45 dB above threshold were used; results indicated that nociceptive sensitivity gradually declined as vibration amplitude increased. In a frequency study, vibrations ranging from 20 to 230 Hz were used; all interfered with nociception. Combining the results of the 2 studies permitted the conclusion that signals in multiple vibrotactile channels are able to modulate nociception. No one mechanoreceptive channel appears to have a privileged role.

Adolescent↗

PubMed2003

The vibrations of texture.

The Pacinian channel has been implicated in the perception of fine textures (Hollins et al., Somatosens Mot Res 18: 253-262, 2001a). In the present study, we investigate candidate codes for Pacinian-mediated roughness perception. We use a Hall effect transducer to record the vibrations elicited in the skin when a set of textured surfaces is passively presented to the index finger. The peak frequency of the vibrations is found to decrease systematically as spatial period increases. The power of the vibrations--weighted according to the spectral sensitivity of the Pacinian system--increases with spatial period for all but the coarsest surfaces. By varying the scanning velocity, we manipulate the temporal and intensive characteristics of the texture-induced vibrations and assess the effect of the manipulation on perceived roughness. We find that doubling the scanning velocity does not result in the substantial decrease in roughness predicted by a frequency theory of vibrotactile roughness perception. On the other hand, the effects of speed on roughness match those of speed on power. We propose that the roughness of a fine surface (spatial period<200 microm) is a function of the Pacinian-weighted power of the vibrations it elicits.

Adult↗

PubMed1998

Vibrotactile amplitude and frequency discrimination in temporomandibular disorders.

The purpose of this study was to determine whether the elevation in vibrotactile detection threshold, found in many individuals with temporomandibular disorders (TMD), is paralleled by suprathreshold impairments. Participants with TMD were compared with pain-free control subjects in their ability to discriminate on the basis of differences in amplitude and frequency between vibratory stimuli delivered to the face. The TMD group was significantly impaired with respect to frequency discrimination, but not amplitude discrimination. This dissociation suggests that the cortical processing of vibrotactile signals may be affected in TMD patients. TMD participants' estimates of the intensity of their spontaneous and palpation-evoked pain did not significantly correlate with performance on either discrimination task; this finding makes it unlikely that impaired vibrotaction in TMD is primarily the result of a pain-dependent gating of tactile signals.

Adult↗

PubMed1998

Generalized vibrotactile allodynia in a patient with temporomandibular disorder.

This report presents the findings from a psychophysical study of vibrotactile responses in a patient diagnosed with temporomandibular disorder (TMD). This patient unexpectedly reported pain due to innocuous vibrotactile stimulation, and this allodynia appeared to have a component of temporal summation. The pain response occurred not only in the region of the clinical pain (the face), but also on the volar forearm, where the patient reported no clinical pain. Administration of the N-methyl-D-aspartate (NMDA) receptor antagonist dextromethorphan (DM), but not vehicle, attenuated the vibration-induced pain at both sites.

Adult↗

PubMed1996

Vibrotactile threshold is elevated in temporomandibular disorders.

Experimental pain can elevate vibrotactile threshold, a phenomenon attributed in the literature to the operation of a 'touch gate.' It is not known, however, whether clinical pain produces similar effects. To explore this possibility, we measured vibrotactile threshold in patients with temporomandibular disorders (TMD) whose pain had a prominent myalgic component. Two-interval forced-choice tracking was used to determine threshold for a 25-Hz vibratory stimulus presented on the cheek. Threshold was found to be significantly elevated in the TMD group, compared to an age- and gender-matched control group of pain-free individuals. Within the TMD group, those with a supra-median level of muscle tenderness (corrected for background levels of spontaneous pain) had significantly higher threshold than those with lower levels of palpation pain. These findings are consistent with the idea of a touch gate, and suggest the usefulness of further research in this area with clinical pain populations. The effects of an adapting stimulus (25 Hz, 20 dB SL) were also studied, and found to produce parallel elevations in vibrotactile threshold in the TMD and pain-free groups. This result indicates that at least some adaptation occurs at a higher (subsequent) level of somatosensory information processing than does the touch gating implied by the unadapted thresholds.

Adaptation, Physiological↗