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.     

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 &#55 m) is a function of the Pacinian-weighted power of the vibrations it elicits.     

Time course and action spectrum of vibrotactile adaptation

In a series of experiments designed to explore the processes underlying adaptation of the sense of flutter-vibration, vibrotactile threshold was measured on the pad of the index finger, using Békésy tracking. Unadapted thresholds were first measured, for a number of frequencies (4-90 Hz) and contactor sizes (1-8 mm diameter). As expected, these measurements indicated the presence of (1) a Pacinian system possessing spatial summation and increasing in sensitivity, as frequency was raised, at the rate of 12 dB/octave; and (2) a non-Pacinian system showing little spatial summation, and with a frequency characteristic matching that of the NP I mechanism of Bolanowski et al. (1988). These baseline data of Experiment 1 guided the selection of stimulus parameters for subsequent experiments, in which threshold for a test stimulus was measured before, during, and after periods of vibrotactile adaptation. In Experiment 2, test stimuli of 10 Hz and 50 Hz were combined factorially with 30-dB SL adapting stimuli of the same two frequencies. When the test stimulus was 10 Hz, the two adapting frequencies were equally effective in raising threshold; however, when the 50-Hz test stimulus was used, the 50-Hz adapting stimulus raised threshold by a greater amount than did the 10-Hz adapter. These results confirm on the finger the independence of adaptation in Pacinian and non-Pacinian channels, a result previously established on the thenar by other workers. For all four frequency combinations, threshold rose exponentially with a time constant of 1.5-2 min. In Experiment 3, an action spectrum was determined, showing the adapting amplitude needed at each of a series of frequencies to raise the threshold of a 10-Hz stimulus by 10 dB; this spectrum was essentially flat from 30 to 90 Hz. The results, taken in conjunction with what is known about rapidly adapting cutaneous mechanoreceptors, imply that the effectiveness of an adapting stimulus is not determined solely by the amount of activity it generates in first-order afferents.     

Time Course and Action Spectrum of Vibrotactile Adaptation

In a series of experiments designed to explore the processes underlying adaptation of the sense of flutter-vibration, vibrotactile threshold was measured on the pad of the index finger, using Békésy tracking. Unadapted thresholds were first measured, for a number of frequencies (4-90 Hz) and contactor sizes (1-8 mm diameter). As expected, these measurements indicated the presence of (1) a Pacinian system possessing spatial summation and increasing in sensitivity, as frequency was raised, at the rate of 12 dB/octave; and (2) a non-Pacinian system showing little spatial summation, and with a frequency characteristic matching that of the NP I mechanism of Bolanowski et al. (1988). These baseline data of Experiment 1 guided the selection of stimulus parameters for subsequent experiments, in which threshold for a test stimulus was measured before, during, and after periods of vibrotactile adaptation.

In Experiment 2, test stimuli of 10 H_z and 50 H_z were combined factorially with 30-dB SL adapting stimuli of the same two frequencies. When the test stimulus was 10 Hz, the two adapting frequencies were equally effective in raising threshold; however, when the 50-Hz test stimulus was used, the 50-Hz adapting stimulus raised threshold by a greater amount than did the 10-Hz adapter. These results confirm on the finger the independence of adaptation in Pacinian and non-Pacinian channels, a result previously established on the thenar by other workers. For all four frequency combinations, threshold rose exponentially with a time constant of 1.5-2 min.

In Experiment 3, an action spectrum was determined, showing the adapting amplitude needed at each of a series of frequencies to raise the threshold of a 10-Hz stimulus by 10 dB; this spectrum was essentially flat from 30 to 90 Hz. The results, taken in conjunction with what is known about rapidly adapting cutaneous mechanoreceptors, imply that the effectiveness of an adapting stimulus is not determined solely by the amount of activity it generates in first-order afferents.     

Vibrotactile thresholds at the fingertip, volar forearm, large toe, and heel

Thresholds for the perception of vibration vary with location on the body due to the organization of tactile channels in hairy and non-hairy skin, and variations in receptor density. This study determined vibration thresholds at four locations on the body with two different contactors so as to assist the identification of the tactile channel determining the threshold at each location. Vibrotactile thresholds at six frequencies from 8 to 250 Hz were measured on the distal phalanx of the index finger, the volar forearm, the large toe, and the heel with two contactors: (i) a 1-mm diameter circular probe with a 1-mm gap to a fixed circular surround (i.e., 7.1-mm(2) excitation area), and (ii) a 6-mm diameter circular probe with a 2-mm gap to a fixed circular surround (i.e., 79-mm(2) excitation area). At all frequencies and with both contactors, thresholds on the fingertip were lower than thresholds on the volar forearm, the large toe, and the heel, consistent with a greater density of mechanoreceptors at the fingertip. Thresholds with the larger contactor were lower than thresholds with the smaller contactor on the fingertip at high frequencies (63, 125, and 250 Hz), on the large toe (except at 250 Hz), on the heel (at all frequencies), and on the volar forearm at 250 Hz. It is concluded that at least two tactile channels (Pacinian from 63 to 250 Hz, and non-Pacinian from 8 to 31.5 Hz) determined vibrotactile thresholds at the fingertip, whereas non-Pacinian channels had a dominant influence on vibrotactile thresholds at the volar forearm. The role of Pacinian and non-Pacinian channels could not be confirmed at the large toe or the heel despite some evidence of spatial summation.     

Vibrotactile difference thresholds: Effects of vibration frequency,vibration magnitude,contact area,and body location

It has not been established whether the smallest perceptible change in the intensity of vibrotactile stimuli depends on the somatosensory channel mediating the sensation. This study investigated intensity difference thresholds for vibration using contact conditions (different frequencies, magnitudes, contact areas, body locations) selected so that perception would be mediated by more than one psychophysical channel. It was hypothesized that difference thresholds mediated by the non-Pacinian I (NPI) channel and the Pacinian (P) channel would differ. Using two different contactors (1-mm diameter contactor with 1-mm gap to a fixed surround; 10-mm diameter contactor with 2-mm gap to the surround) vibration was applied to the thenar eminence and the volar forearm at two frequencies (10 and 125?Hz). The up-down-transformed-response method with a three-down-one-up rule provided absolute thresholds and also difference thresholds at various levels above the absolute thresholds of 12 subjects (i.e., sensation levels, SLs) selected to activate preferentially either single channels or multiple channels. Median difference thresholds varied from 0.20 (thenar eminence with 125-Hz vibration at 10?dB SL) to 0.58 (thenar eminence with 10-Hz vibration at 20?dB SL). Median difference thresholds tended to be lower for the P channel than the NPI channel. The NPII channel may have reduced difference thresholds with the smaller contactor at 125?Hz. It is concluded that there are large and systematic variations in difference thresholds associated with the frequency, the magnitude, the area of contact, and the location of contact with vibrotactile stimuli that cannot be explained without increased understanding of the perception of supra-threshold vibrotactile stimuli.     

Perception of the tactile texture of raised-dot patterns: a multidimensional analysis

An ALSCAL multidimensional scaling analysis in Euclidean space revealed that three orthogonal perceptual dimensions can account for the judged tactile dissimilarities of raised-dot patterns. Through magnitude estimates of various perceptual attributes, it was determined that the three dimensions consist of blur, roughness, and clarity. The only effect that selective adaptation of the Pacinian (P) channel had was to change the perceptual clarity of the raised dots against their background. Adaptation of the P channel with a 20 dB SL 250 Hz stimulus enhanced clarity. As indicated by magnitude estimates, adaptation of the P channel by the 250 Hz stimulus had no effect on the perceived roughness of the dot pattern but did cause the individual dots of the textured pattern to feel smoother. When the observer was required to estimate magnitude "overall roughness" defined as a combination of dot-pattern roughness and individual-dot roughness, adaptation of the P channel affected perceived roughness by reducing it. Taken as a whole, the results are consistent with the hypothesis that the NP channels and the P channel jointly influence the perception of textured surfaces.     

Vibration influences haptic perception of surface compliance during walking

Background

The haptic perception of ground compliance is used for stable regulation of dynamic posture and the control of locomotion in diverse natural environments. Although rarely investigated in relation to walking, vibrotactile sensory channels are known to be active in the discrimination of material properties of objects and surfaces through touch. This study investigated how the perception of ground surface compliance is altered by plantar vibration feedback.

Methodology/Principal Findings

Subjects walked in shoes over a rigid floor plate that provided plantar vibration feedback, and responded indicating how compliant it felt, either in subjective magnitude or via pairwise comparisons. In one experiment, the compliance of the floor plate was also varied. Results showed that perceived compliance of the plate increased monotonically with vibration feedback intensity, and depended to a lesser extent on the temporal or frequency distribution of the feedback. When both plate stiffness (inverse compliance) and vibration amplitude were manipulated, the effect persisted, with both factors contributing to compliance perception. A significant influence of vibration was observed even for amplitudes close to psychophysical detection thresholds.

Conclusions/Significance

These findings reveal that vibrotactile sensory channels are highly salient to the perception of surface compliance, and suggest that correlations between vibrotactile sensory information and motor activity may be of broader significance for the control of human locomotion than has been previously acknowledged.     

Roughness perception in tactile channels: Evidence for an opponent process in the sense of touch

Abstract

Magnitude estimates of the tactile roughness of raised-dot surfaces revealed that perceived overall roughness, defined as the combination of the perceived roughness of the dot pattern and the perceived roughness of the individual dots in the pattern, is an inverted U-shaped function of dot spacing, reaching a maximum at approximately 3.0?mm of dot separation. The hypothesis that Pacinian corpuscles are involved in roughness perception has been supported by the finding that selective adaptation of the Pacinian corpuscle (PC) channel with a 250-Hz stimulus at 20-dB SL results in a decrease in the perceived overall roughness of the raised-dot surface at the fingertip. The effect of PC channel adaptation on perceived overall roughness was attributable entirely to a reduction in the perceived roughness of the individual raised dots; PC adaptation had no effect on the perceived roughness of the raised-dot pattern. Selective adaptation of the slowly adapting type I (SA I) channel with a 5-Hz stimulus at 20-dB SL had the opposite effect of PC channel adaptation and resulted in an increase in the perceived roughness of the individual raised dots, and consequently the perceived overall roughness of the raised-dot surface. As was the case with PC channel adaptation, SA I channel adaptation had no effect on the perceived roughness of the pattern. Adaptation with a compound adapting stimulus containing 5- and 250-Hz components at 20-dB SL had no effect on perceived overall roughness, which suggests that the PC and SA I channels operate antagonistically in an opponent-process fashion in the perception of the microstructure of a textured surface. Neither PC adaptation nor SA I adaptation affected perceived pattern roughness, which suggests that pattern roughness is coded by relative rather than by absolute spatial variation in firing rate.     

Multiple parietal operculum subdivisions in humans: tactile activation maps

We focused the present analysis on blood-oxygen-level-dependent responses evoked in four architectonic subdivisions of human posterior parietal operculum (PO) during two groups of tasks involving either vibrotactile stimulation or rubbing different surfaces against the right index finger pad. Activity localized in previously defined parietal opercular subdivisions, OP 1-4, was co-registered to a standard cortical surface-based atlas. Four vibrotactile stimulation tasks involved attention to the parameters of paired vibrations: (1) detect rare target trials when vibration frequencies matched; (2) select the presentation order of the vibration with a higher frequency or (3) longer duration; and (4) divide attention between frequency and duration before selecting stimulus order. Surface stimulation tasks involved various discriminations of different surfaces: (1) smooth surfaces required no discrimination; (2) paired horizontal gratings required determination of the direction of roughness change; (3) paired shapes entailed identifying matched and unmatched shapes; (4) raised letters involved letter recognition. The results showed activity in multiple somatosensory subdivisions bilaterally in human PO that are plausibly homologues of somatosensory areas previously described in animals. All tasks activated OP 1, but in vibrotactile tasks foci were more restricted compared to moving surface tasks. Greater spatial extents of activity especially in OP 1 and 4 when surfaces rubbed the finger pad did not support previously reported somatotopy of the second finger representation in "S2". The varied activity distributions across OP subdivisions may reflect low-level perceptual and/or cognitive processing differences between tasks.     

Higher height, higher ability: judgment confidence as a function of spatial height perception

Based on grounded cognition theories, the current study showed that judgments about ability were regulated by the subjects' perceptions of their spatial height. In Experiment 1, we found that after seeing the ground from a higher rather than lower floor, people had higher expectations about their performance on a knowledge test and assigned themselves higher rank positions in a peer comparison evaluation. In Experiment 2, we examined the boundary conditions of the spatial height effects and showed that it could still occur even if we employed photos rather than actual building floors to manipulate the perceptions of spatial heights. In addition, Experiment 2 excluded processing style as an explanation for these observations. In Experiment 3, we investigated a potential mechanism for the spatial height effect by manipulating the scale direction in the questionnaire. Consequently, consistent with our representational dependence account, the effect of spatial heights on ability judgments was eliminated when the mental representation of ability was disturbed by a reverse physical representation. These results suggest that people's judgments about their ability are correlated with their spatial perception.     

Perception of the tactile texture of raised-dot patterns: A multidimensional analysis

An ALSCAL multidimensional scaling analysis in Euclidean space revealed that three orthogonal perceptual dimensions can account for the judged tactile dissimilarities of raised-dot patterns. Through magnitude estimates of various perceptual attributes, it was determined that the three dimensions consist of blur, roughness, and clarity. The only effect that selective adaptation of the Pacinian (P) channel had was to change the perceptual clarity of the raised dots against their background. Adaptation of the P channel with a 20?dB SL 250?Hz stimulus enhanced clarity. As indicated by magnitude estimates, adaptation of the P channel by the 250?Hz stimulus had no effect on the perceived roughness of the dot pattern but did cause the individual dots of the textured pattern to feel smoother. When the observer was required to estimate magnitude “overall roughness” defined as a combination of dot-pattern roughness and individual-dot roughness, adaptation of the P channel affected perceived roughness by reducing it. Taken as a whole, the results are consistent with the hypothesis that the NP channels and the P channel jointly influence the perception of textured surfaces.     

Multiple parietal operculum subdivisions in humans: Tactile activation maps

We focused the present analysis on blood-oxygen-level-dependent responses evoked in four architectonic subdivisions of human posterior parietal operculum (PO) during two groups of tasks involving either vibrotactile stimulation or rubbing different surfaces against the right index finger pad. Activity localized in previously defined parietal opercular subdivisions, OP 1–4, was co-registered to a standard cortical surface-based atlas. Four vibrotactile stimulation tasks involved attention to the parameters of paired vibrations: (1) detect rare target trials when vibration frequencies matched; (2) select the presentation order of the vibration with a higher frequency or (3) longer duration; and (4) divide attention between frequency and duration before selecting stimulus order. Surface stimulation tasks involved various discriminations of different surfaces: (1) smooth surfaces required no discrimination; (2) paired horizontal gratings required determination of the direction of roughness change; (3) paired shapes entailed identifying matched and unmatched shapes; (4) raised letters involved letter recognition. The results showed activity in multiple somatosensory subdivisions bilaterally in human PO that are plausibly homologues of somatosensory areas previously described in animals. All tasks activated OP 1, but in vibrotactile tasks foci were more restricted compared to moving surface tasks. Greater spatial extents of activity especially in OP 1 and 4 when surfaces rubbed the finger pad did not support previously reported somatotopy of the second finger representation in “S2”. The varied activity distributions across OP subdivisions may reflect low-level perceptual and/or cognitive processing differences between tasks.     

Visual textures, machine vision and animal camouflage

The psychophysics of visual texture perception and texture discrimination have been investigated extensively during the past 30 years. Humans have been the main study subjects, but some research on texture perception has involved other species, and there is good reason to think that the most general results from humans apply to other vertebrates as well. Psychophysicists have suggested that some of their findings on human vision reflect adaptive 'tricks' for countering prey camouflage, but this possibility has not been widely communicated to evolutionary biologists. We review the psychophysicists' main conclusions on texture discrimination, and list additional questions that their results raise when animal coats are considered as visual textures. We also suggest ways in which advances in computer vision can be combined with psychophysics to provide new perspectives on the function of animal coat patterns.     

Flutter discrimination: neural codes,perception, memory and decision making

Recent studies combining psychophysical and neurophysiological experiments in behaving monkeys have provided new insights into how several cortical areas integrate efforts to solve a vibrotactile discrimination task. In particular, these studies have addressed how neural codes are related to perception, working memory and decision making in this model. The primary somatosensory cortex drives higher cortical areas where past and current sensory information are combined, such that a comparison of the two evolves into a behavioural decision. These and other observations in visual tasks indicate that decisions emerge from highly-distributed processes in which the details of a scheduled motor plan are gradually specified by sensory information.     

Testing meter, rhythm, and tempo discriminations in pigeons

Rhythmic grouping and discrimination is fundamental to music. When compared to the perception of pitch, rhythmic abilities in animals have received scant attention until recently. In this experiment, four pigeons were tested with three types of auditory rhythmic discriminations to investigate their processing of this aspect of sound and music. Two experiments examined a meter discrimination in which successively presented idiophonic sounds were repeated in meters of different lengths in a go/no-go discrimination task. With difficulty, the birds eventually learned to discriminate between 8/4 and 3/4 meters constructed from cymbal and tom drum sounds at 180 beats per minute. This discrimination subsequently transferred to faster tempos, but not to different drum sounds or their combination. Experiment 3 tested rhythmic and arrhythmic patterns of sounds. After 40 sessions of training, these same pigeons showed no discrimination. Experiment 4 tested repetitions of a piano sound at fast and slow tempos. This discrimination was readily learned and showed transfer to novel tempos. The pattern of results suggests that pigeons can time periodic auditory events, but their capacity to understand generalized rhythmic groupings appears limited.     

Texture preferences of ascidian tadpole larvae during settlement

Ascidian tadpole larvae settle on hard surfaces and undergo metamorphosis into sessile adults. To test whether tadpoles evaluate the texture of surfaces they settle upon, we presented tadpoles with surfaces that were divided into halves; each half had one of four different textures: smooth, fine sandpaper, coarse sandpaper, and sandblasted. In all cases, twice as many individuals settled on one side over the other, but this was not consistently the smooth side or the rough side. More tadpoles settled on a smooth surface than one scoured by sandpaper, but more tadpoles settled on a sandblasted surface than smooth one. This indicates tadpoles are capable of finer tactile discrimination than merely detecting a hard surface, and supports the hypothesis that ascidian tadpoles have mechanoreceptive sensory neurons.     

The Effects of Aging on Information-Processing Channels in the Sense of Touch: I. Absolute Sensitivity

Thresholds for detecting vibrotactile signals of variable frequency applied to the thenar eminence of the hand by small and large contactors were measured in subjects ranging in age from 10 to 89 years. Thresholds were found to increase as a function of age, but the rate of increase was greater after than before the age of 65 years. The rate of loss of vibrotactile sensitivity was substantially greater in the P channel (mediated by Pacinian corpuscles) than in the NP I channel (mediated by rapidly adapting fibers), the NP II channel (mediated by slowly adapting type II fibers), or the NP HI channel (mediated by slowly adapting type I fibers). Women were frequently found to have greater sensitivity than men.     

Exposure to Organic Solvents Used in Dry Cleaning Reduces Low and High Level Visual Function

PurposeTo investigate whether exposure to occupational levels of organic solvents in the dry cleaning industry is associated with neurotoxic symptoms and visual deficits in the perception of basic visual features such as luminance contrast and colour, higher level processing of global motion and form (Experiment 1), and cognitive function as measured in a visual search task (Experiment 2).MethodsThe Q16 neurotoxic questionnaire, a commonly used measure of neurotoxicity (by the World Health Organization), was administered to assess the neurotoxic status of a group of 33 dry cleaners exposed to occupational levels of organic solvents (OS) and 35 age-matched non dry-cleaners who had never worked in the dry cleaning industry. In Experiment 1, to assess visual function, contrast sensitivity, colour/hue discrimination (Munsell Hue 100 test), global motion and form thresholds were assessed using computerised psychophysical tests. Sensitivity to global motion or form structure was quantified by varying the pattern coherence of global dot motion (GDM) and Glass pattern (oriented dot pairs) respectively (i.e., the percentage of dots/dot pairs that contribute to the perception of global structure). In Experiment 2, a letter visual-search task was used to measure reaction times (as a function of the number of elements: 4, 8, 16, 32, 64 and 100) in both parallel and serial search conditions.ResultsDry cleaners exposed to organic solvents had significantly higher scores on the Q16 compared to non dry-cleaners indicating that dry cleaners experienced more neurotoxic symptoms on average. The contrast sensitivity function for dry cleaners was significantly lower at all spatial frequencies relative to non dry-cleaners, which is consistent with previous studies. Poorer colour discrimination performance was also noted in dry cleaners than non dry-cleaners, particularly along the blue/yellow axis. In a new finding, we report that global form and motion thresholds for dry cleaners were also significantly higher and almost double than that obtained from non dry-cleaners. However, reaction time performance on both parallel and serial visual search was not different between dry cleaners and non dry-cleaners.ConclusionsExposure to occupational levels of organic solvents is associated with neurotoxicity which is in turn associated with both low level deficits (such as the perception of contrast and discrimination of colour) and high level visual deficits such as the perception of global form and motion, but not visual search performance. The latter finding indicates that the deficits in visual function are unlikely to be due to changes in general cognitive performance.     

Feeling Voices

Two experiments investigated deaf individuals'' ability to discriminate between same-sex talkers based on vibrotactile stimulation alone. Nineteen participants made same/different judgments on pairs of utterances presented to the lower back through voice coils embedded in a conforming chair. Discrimination of stimuli matched for F0, duration, and perceived magnitude was successful for pairs of spoken sentences in Experiment 1 (median percent correct = 83%) and pairs of vowel utterances in Experiment 2 (median percent correct = 75%). Greater difference in spectral tilt between “different” pairs strongly predicted their discriminability in both experiments. The current findings support the hypothesis that discrimination of complex vibrotactile stimuli involves the cortical integration of spectral information filtered through frequency-tuned skin receptors.