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Abstract: An important unresolved question in sensory neuroscience is whether, and if so with what time course, tactile perception is enhanced by visual deprivation. In three experiments involving 158 normally sighted human participants, we assessed whether tactile spatial acuity improves with short-term visual deprivation over periods ranging from under 10 to over 110 minutes. We used an automated, precisely controlled two-interval forced-choice grating orientation task to assess each participant's ability to discern the orientation of square-wave gratings pressed against the stationary index finger pad of the dominant hand. A two-down one-up staircase (Experiment 1) or a Bayesian adaptive procedure (Experiments 2 and 3) was used to determine the groove width of the grating whose orientation each participant could reliably discriminate. The experiments consistently showed that tactile grating orientation discrimination does not improve with short-term visual deprivation. In fact, we found that tactile performance degraded slightly but significantly upon a brief period of visual deprivation (Experiment 1) and did not improve over periods of up to 110 minutes of deprivation (Experiments 2 and 3). The results additionally showed that grating orientation discrimination tends to improve upon repeated testing, and confirmed that women significantly outperform men on the grating orientation task. We conclude that, contrary to two recent reports but consistent with an earlier literature, passive tactile spatial acuity is not enhanced by short-term visual deprivation. Our findings have important theoretical and practical implications. On the theoretical side, the findings set limits on the time course over which neural mechanisms such as crossmodal plasticity may operate to drive sensory changes; on the practical side, the findings suggest that researchers who compare tactile acuity of blind and sighted participants should not blindfold the sighted participants. Wong M, Gnanakumaran V, Goldreich D (2011) Tactile spatial acuity enhancement in blindness: evidence for experience-dependent mechanisms. J Neurosci 31: 7028-7037. Abstract: Tactile spatial acuity is enhanced in blindness, according to several studies, but the cause of this enhancement has been controversial. Two competing hypotheses are the tactile experience hypothesis (reliance on the sense of touch drives tactile-acuity enhancement) and the visual deprivation hypothesis (the absence of vision itself drives tactile-acuity enhancement). Here, we performed experiments to distinguish between these two hypotheses. We used force-controlled grating orientation tasks to compare the passive (finger stationary) tactile spatial acuity of 28 profoundly blind and 55 normally sighted humans on the index, middle, and ring fingers of each hand, and on the lips. The tactile experience hypothesis predicted that blind participants would outperform the sighted on the fingers, and that Braille reading would correlate with tactile acuity. The visual deprivation hypothesis predicted that blind participants would outperform the sighted on fingers and lips. Consistent with the tactile experience hypothesis, the blind significantly outperformed the sighted on all fingers, but not on the lips. Additionally, among blind participants, proficient Braille readers on their preferred reading index finger outperformed nonreaders. Finally, proficient Braille readers performed better with their preferred reading index finger than with the opposite index finger, and their acuity on the preferred reading finger correlated with their weekly reading time. These results clearly implicate reliance on the sense of touch as the trigger for tactile spatial acuity enhancement in the blind, and suggest the action of underlying experience-dependent neural mechanisms such as somatosensory and/or cross-modal cortical plasticity. Bhattacharjee A, Ye AJ, Lisak JA, Vargas MG, Goldreich D (2010) Vibrotactile masking experiments reveal accelerated somatosensory processing in congenitally blind Braille readers. J Neurosci 30: 14288-14298. Abstract: Braille reading is a demanding task that requires the identification of rapidly varying tactile patterns. During proficient reading, neighboring characters impact the fingertip at 100 ms intervals, and adjacent raised dots within a character at 50 ms intervals. Because the brain requires time to interpret afferent sensorineural activity, among other reasons, tactile stimuli separated by such short temporal intervals pose a challenge to perception. How, then, do proficient Braille readers successfully interpret inputs arising from their fingertips at such rapid rates? We hypothesized that somatosensory perceptual consolidation occurs more rapidly in proficient Braille readers. If so, Braille readers should outperform sighted participants on masking tasks, which demand rapid perceptual processing, but would not necessarily outperform the sighted on tests of simple vibrotactile sensitivity. To investigate, we conducted two-interval forced-choice vibrotactile detection, amplitude discrimination, and masking tasks on the index fingertips of 89 sighted and 57 profoundly blind humans. Sighted and blind participants had similar unmasked detection (25 ms target tap) and amplitude discrimination (compared with 100 µm reference tap) thresholds, but congenitally blind Braille readers, the fastest readers among the blind participants, exhibited significantly less masking than the sighted (masker, 50 Hz, 50 µm; target–masker delays, ±50 and ±100 ms). Indeed, Braille reading speed correlated significantly and specifically with masking task performance, and in particular with the backward masking decay time constant. We conclude that vibrotactile sensitivity is unchanged but that perceptual processing is accelerated in congenitally blind Braille readers. Peters RM, Hackeman E, Goldreich D (2009) Diminutive digits discern delicate details: fingertip size and the sex difference in tactile spatial acuity. J Neurosci 29: 15756 –15761. Abstract: We have observed that passive tactile spatial acuity, the ability to resolve the spatial structure of surfaces pressed upon the skin, differs subtly but consistently between the sexes, with women able to perceive finer surface detail than men. Eschewing complex central explanations, we hypothesized that this sex difference in somatosensory perception might result from simple physical differences between the fingers of women and men. To investigate, we tested 50 women and 50 men on a tactile grating orientation task, and measured the surface area of the participants' index fingertips. In subsets of participants, we additionally measured finger skin compliance and optically imaged the fingerprint microstructure to count sweat pores. We show here that tactile perception improves with decreasing finger size, and that this correlation fully explains the better perception of women, who on average have smaller fingers than men. Indeed, when sex and finger size are both considered in statistical analyses, only finger size predicts tactile acuity. Thus, a man and a woman with fingers of equal size will, on average, enjoy equal tactile acuity. We further show that sweat pores, and presumably the Merkel receptors beneath them, are packed more densely in smaller fingers. Goldreich D, Wong M, Peters RM, Kanics IM (2009) A tactile automated passive-finger stimulator (TAPS). J Vis Exp 28. doi: 10.3791/1374. Abstract: Although tactile spatial acuity tests are used in both neuroscience research and clinical assessment, few automated devices exist for delivering controlled spatially structured stimuli to the skin. Consequently, investigators often apply tactile stimuli manually. Manual stimulus application is time consuming, requires great care and concentration on the part of the investigator, and leaves many stimulus parameters uncontrolled. We describe here a computer-controlled tactile stimulus system, the Tactile Automated Passive-finger Stimulator (TAPS), that applies spatially structured stimuli to the skin, controlling for onset velocity, contact force, and contact duration. TAPS is a versatile, programmable system, capable of efficiently conducting a variety of psychophysical procedures. We describe the components of TAPS, and show how TAPS is used to administer a two-interval forced-choice tactile grating orientation test. Goldreich D (2009) An ideal observer for passive tactile spatial perception. Frontiers in Systems Neuroscience. Conference Abstract: Computational and systems neuroscience. doi: 10.3389/conf.neuro.06.2009.03.058 Abstract: How does the sense of touch resolve the spatial details of stimuli pressed against the skin? Two popular tests of passive (finger stationary) tactile spatial acuity are the classic two-point discrimination task, in which the subject reports the number of caliper points (one or two) in contact with the skin, and the newer grating orientation task, in which the subject reports the orientation (horizontal or vertical) of an impressed square wave grating. Experiments suggest that the firing rates of slowly adapting type-1 (SA1) mechanoreceptive afferents encode the structure of these and other static tactile stimuli, but how the brain decodes these inputs is unknown. We developed an ideal Bayesian observer that decodes simulated somatosensory neuronal firing patterns evoked by spatially structured stimuli. The generative model incorporates the known receptive field structures and density of SA1 afferents, and firing rate noise that is either low (Fano factor < 1), characteristic of SA1 afferents, or high (Fano factor = 1, Poisson), characteristic of primary somatosensory cortical neurons. The Bayesian observer uses model selection to perform two-alternative forced-choice tasks (two-point discrimination, grating orientation), and parameter estimation for graded perceptual judgments (point stimulus localization, skin indentation estimation).... Goldreich D (2007) A Bayesian perceptual model replicates the cutaneous rabbit and other tactile spatiotemporal illusions. PLoS ONE 2(3): e333. Abstract: Background: When brief stimuli contact the skin in rapid succession at two or more locations, perception strikingly shrinks the intervening distance, and expands the elapsed time, between consecutive events. The origins of these perceptual space-time distortions are unknown. Methodology/Principal Findings: Here I show that these illusory effects, which I term perceptual length contraction and time dilation, are emergent properties of a Bayesian observer model that incorporates prior expectation for speed. Rapidly moving stimuli violate expectation, provoking perceptual length contraction and time dilation. The Bayesian observer replicates the cutaneous rabbit illusion, the tau effect, the kappa effect, and other spatiotemporal illusions. Additionally, it shows realistic tactile temporal order judgment and spatial attention effects. Conclusions/Significance: The remarkable explanatory power of this simple model supports the hypothesis, first proposed by Helmholtz, that the brain biases perception in favor of expectation. Specifically, the results suggest that the brain automatically incorporates prior expectation for speed in order to overcome spatial and temporal imprecision inherent in the sensorineural signal. Goldreich D & Kanics IM (2006) Performance of blind and sighted humans on a tactile grating detection task. Percept Psychophys 68: 1363-1371. Abstract: We compared the abilities of blind and sighted humans to distinguish grooved from smooth surfaces pressed against the stationary index fingertip. Ranging in age from 20 to 72 years, 37 blind and 47 sighted subjects participated in an automated two-alternative forced-choice tactile grating detection task. The tactile acuity of blind and sighted subjects declined with age at equivalent rates (0.011-mm threshold increase per year), but the blind subjects were able to perceive significantly thinner grooves than were their sighted peers (the average difference between blind and sighted subjects of the same age and gender was 0.267 mm). The blind Braille readers performed no better than the blind nonreaders, and the congenitally blind subjects performed equivalently to those with adult-onset blindness. The superior tactile acuity of blind persons may result from the involvement of normally visually responsive cerebrocortical areas in tactile processing, as shown by functional imaging studies. Goldreich D & Kanics IM (2003) Tactile acuity is enhanced in blindness. J Neurosci 23: 3439-3445. Abstract: Functional imaging studies in blind subjects have shown tactile activation of cortical areas that normally subserve vision, but whether blind people have enhanced tactile acuity has long been controversial. We compared the passive tactile acuity of blind and sighted subjects on a fully automated grating orientation task and used multivariate Bayesian data analysis to determine predictors of acuity. Acuity was significantly superior in blind subjects, independently of the degree of childhood vision, light perception level, or Braille reading. Acuity was strongly dependent on the force of contact between the stimulus surface and the skin, declined with subject age, and was better in women than in men. Despite large intragroup variability, the difference between blind and sighted subjects was highly significant: the average blind subject had the acuity of an average sighted subject of the same gender but 23 years younger. The results suggest that crossmodal plasticity may underlie tactile acuity enhancement in blindness. |