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Ma WJ, Kording KP, Goldreich D (2023) Bayesian Models of Perception and Action: An Introduction. MIT Press (ISBN: 9780262047593). An accessible introduction to
constructing and interpreting Bayesian models of perceptual
decision-making and action.
Many forms of perception and action can be mathematically modeled as probabilistic—or Bayesian—inference, a method used to draw conclusions from uncertain evidence. According to these models, the human mind behaves like a capable data scientist or crime scene investigator when dealing with noisy and ambiguous data. This textbook provides an approachable introduction to constructing and reasoning with probabilistic models of perceptual decision-making and action. Featuring extensive examples and illustrations, Bayesian Models of Perception and Action is the first textbook to teach this widely used computational framework to beginners. Unwalla K, Goldreich D, Shore DI (2021) Exploring Reference Frame Integration Using Response Demands in a Tactile Temporal-Order Judgement Task. Multisensory Research 34: 807–838. doi: 10.1163/22134808-bja10057. (PDF) Exploring the world through touch
requires the integration of internal (e.g., anatomical) and
external (e.g., spatial) reference frames — you only know what you
touch when you know where your hands are in space. The deficit
observed in tactile temporal-order judgements when the hands are
crossed over the midline provides one tool to explore this
integration. We used foot pedals and required participants to
focus on either the hand that was stimulated first (an anatomical
bias condition) or the location of the hand that was stimulated
first (a spatiotopic bias condition). Spatiotopic-based responses
produce a larger crossed-hands deficit, presumably by focusing
observers on the external reference frame. In contrast,
anatomical-based responses focus the observer on the internal
reference frame and produce a smaller deficit. This manipulation
thus provides evidence that observers can change the relative
weight given to each reference frame. We quantify this effect
using a probabilistic model that produces a population estimate of
the relative weight given to each reference frame. We show that a
spatiotopic bias can result in either a larger external weight
(Experiment 1) or a smaller internal weight (Experiment 2) and
provide an explanation of when each one would occur.
Bharadwaj A, Shaw SB, Goldreich D (2019) Comparing Tactile to Auditory Guidance for Blind Individuals. Front Hum Neurosci 13:443. doi: 10.3389/fnhum.2019.00443. (PDF) The ability to travel independently
is crucial to an individual's quality of life but compromised by
visual impairment. Several navigational aids have been developed
for blind people to address this limitation. These devices
typically employ auditory instructions to guide users to desired
waypoints. Unfortunately, auditory instructions may interfere with
users' awareness of environmental sounds that signal dangers or
provide cues for spatial orientation. Accordingly, there is a need
to explore the use of non-auditory modalities to convey
information for safe and independent travel. Here, we explored the
efficacy of a tactile navigational aid that provides turn signals
via vibrations on a hip-worn belt. We compared the performance of
12 blind participants as they navigated a series of paths under
the direction of the tactile belt or conventional auditory turn
commands; furthermore, we assessed the effect of repeated testing,
both in the presence and absence of simulated street sounds. A
computer-controlled system triggered each turn command, measured
participants’ time-to-path-completion, and detected major
navigational errors. When participants navigated in a silent
environment, they performed somewhat worse with the tactile belt
than the auditory device, taking longer to complete each trial and
committing more errors. When participants navigated in the
presence of simulated street noises, the difference in completion
time between auditory and tactile navigation diminished. These
results suggest that tactile navigation holds promise as an
effective method in everyday environments characterized by ambient
noise such as street sounds.
Morrison JA, Valdizón-Rodríguez R, Goldreich D, Faure PA (2018) Tuning for rate and duration of frequency-modulated sweeps in the mammalian inferior colliculus. J Neurophysiol. 120(3):985-997. doi: 10.1152/jn.00065.2018. (PDF) Responses of auditory duration-tuned
neurons (DTNs) are selective for stimulus duration. We used
single-unit extracellular recording to investigate how the
inferior colliculus (IC) encodes frequency-modulated (FM) sweeps
in the big brown bat. It was unclear whether the responses of
so-called “FM DTNs” encode signal duration, like classic pure-tone
DTNs, or the FM sweep rate. Most FM cells had spiking responses
selective for downward FM sweeps. We presented cells with linear
FM sweeps whose center frequency (CEF) was set to the best
excitatory frequency and whose bandwidth (BW) maximized the spike
count. With these baseline parameters, we stimulated cells with
linear FM sweeps randomly varied in duration to measure the range
of excitatory FM durations and/or sweep rates. To separate FM rate
and FM duration tuning, we doubled (and halved) the BW of the
baseline FM stimulus while keeping the CEF constant and then
recollected each cell’s FM duration tuning curve. If the cell was
tuned to FM duration, then the best duration (or range of
excitatory durations) should remain constant despite changes in
signal BW; however, if the cell was tuned to the FM rate, then the
best duration should covary with the same FM rate at each BW. A
Bayesian model comparison revealed that the majority of neurons
were tuned to the FM sweep rate, although a few cells showed
tuning for FM duration. We conclude that the dominant parameter
for temporal tuning of FM neurons in the IC is FM sweep rate and
not FM duration.
Li L, Chan A, Iqbal SM, Goldreich D (2017) An adaptation-induced repulsion illusion in tactile spatial perception. Front Hum Neurosci 11: 331. doi: 10.3389/fnhum.2017.00331. (PDF) Following focal sensory adaptation,
the perceived separation between visual stimuli that straddle the
adapted region is often exaggerated. For instance, in the tilt
aftereffect illusion, adaptation to tilted lines causes
subsequently viewed lines with nearby orientations to be
perceptually repelled from the adapted orientation. Repulsion
illusions in the nonvisual senses have been less studied. Here, we
investigated whether adaptation induces a repulsion illusion in
tactile spatial perception. In a two-interval forced-choice task,
participants compared the perceived separation between two
point-stimuli applied on the forearms successively. Separation
distance was constant on one arm (the reference) and varied on the
other arm (the comparison). In Experiment 1, we took three
consecutive baseline measurements, verifying that in the absence
of manipulation, participants' distance perception was unbiased
across arms and stable across experimental blocks. In Experiment
2, we vibrated a region of skin on the reference arm, verifying
that this focally reduced tactile sensitivity, as indicated by
elevated monofilament detection thresholds. In Experiment 3, we
applied vibration between the two reference points in our distance
perception protocol and discovered that this caused an illusory
increase in the separation between the points. We conclude that
focal adaptation induces a repulsion aftereffect illusion in
tactile spatial perception. The illusion provides clues as to how
the tactile system represents spatial information. The analogous
repulsion aftereffects caused by adaptation in different stimulus
domains and sensory systems may point to fundamentally similar
strategies for dynamic sensory coding.
Tong
J, Ngo V, Goldreich D (2016) Tactile length contraction as
Bayesian inference. J Neurophysiol 116: 369-379.
(PDF)
To perceive, the brain must
interpret stimulus-evoked neural activity. This is challenging:
the stochastic nature of the neural response renders its
interpretation inherently uncertain. Perception would be optimized
if the brain used Bayesian inference to interpret inputs in light
of expectations derived from experience. Bayesian inference would
improve perception on average but cause illusions when stimuli
violate expectation. Intriguingly, tactile, auditory, and visual
perception are all prone to length contraction illusions,
characterized by the dramatic underestimation of the distance
between punctate stimuli delivered in rapid succession; the origin
of these illusions has been mysterious. We previously proposed
that length contraction illusions occur because the brain
interprets punctate stimulus sequences using Bayesian inference
with a low-velocity expectation. A novel prediction of our
Bayesian observer model is that length contraction should
intensify if stimuli are made more difficult to localize. Here, we
report a tactile psychophysical study that tested this prediction.
Twenty humans compared two distances on the forearm: a fixed
reference distance defined by two taps with 1-s temporal
separation, and an adjustable comparison distance defined by two
taps with temporal separation t ≤ 1 s. We observed significant
length contraction: as t was decreased, participants perceived the
two distances as equal only when the comparison distance was made
progressively greater than the reference distance. Furthermore,
the use of weaker taps significantly enhanced participants' length
contraction. These findings confirm the model's predictions,
supporting the view that the spatiotemporal percept is a best
estimate resulting from a Bayesian inference process. Keywords:
cutaneous rabbit illusion, Bayesian inference, sensory saltation,
somatosensory psychophysics, spatial illusion, uncertainty.
Peters RM, Staibano P, Goldreich D (2015) Tactile orientation perception: an ideal observer analysis of human psychophysical performance in relation to macaque area 3b receptive fields. J Neurophysiol 114: 3076-3096. (PDF) The ability to resolve the
orientation of edges is crucial to daily tactile and sensorimotor
function, yet the means by which edge perception occurs is not
well understood. Primate cortical area 3b neurons have diverse
receptive field (RF) spatial structures that may participate in
edge orientation perception. We evaluated five candidate RF models
for macaque area 3b neurons previously recorded while an oriented
bar contacted the monkey's fingertip. We used a Bayesian
classifier to assign each neuron a best-fit RF structure. We
generated predictions for human performance by implementing an
ideal observer that optimally decoded stimulus-evoked spike counts
in the model neurons. The ideal observer predicted a saturating
reduction in bar orientation discrimination threshold with
increasing bar length. We tested 24 humans on an automated,
precision-controlled bar orientation discrimination task and
observed performance consistent with that predicted. We next
queried the ideal observer to discover the RF structure and number
of cortical neurons that best matched each participant's
performance. Human perception was matched with a median of 24
model neurons firing throughout a 1 s period. The ten
lowest-performing participants were fit with RFs lacking
inhibitory sidebands, whereas 12 of the 14 higher-performing
participants were fit with RFs containing inhibitory sidebands.
Participants whose discrimination improved as bar length increased
to 10 mm were fit with longer RFs; those who performed well on the
2 mm bar, with narrower RFs. These results suggest plausible RF
features and computational strategies underlying tactile spatial
perception and may have implications for perceptual learning.
Keywords: Bayesian inference, cortex, linear filter,
somatosensory, spatial acuity.
Peters RM, Goldreich D (2013) Tactile spatial acuity in childhood: effects of age and fingertip size. PLOS ONE 8(12): e84650. (PDF) Tactile acuity is known to decline
with age in adults, possibly as the result of receptor loss, but
less is understood about how tactile acuity changes during
childhood. Previous research from our laboratory has shown that
fingertip size influences tactile spatial acuity in young adults:
those with larger fingers tend to have poorer acuity, possibly
because mechanoreceptors are more sparsely distributed in larger
fingers. We hypothesized that a similar relationship would hold
among children. If so, children's tactile spatial acuity might be
expected to worsen as their fingertips grow. However, concomitant
CNS maturation might result in more efficient perceptual
processing, counteracting the effect of fingertip growth on
tactile acuity. To investigate, we conducted a cross-sectional
study, testing 116 participants ranging in age from 6 to 16 years
on a precision-controlled tactile grating orientation task. We
measured each participant's grating orientation threshold on the
dominant index finger, along with physical properties of the
fingertip: surface area, volume, sweat-pore spacing, and
temperature. We found that, as in adults, children with larger
fingertips (at a given age) had significantly poorer acuity, yet
paradoxically acuity did not worsen significantly with age. We
propose that finger growth during development results in a gradual
decline in innervation density as receptive fields reposition to
cover an expanding skin surface. At the same time, central
maturation presumably enhances perceptual processing.
Tong J, Mao O, Goldreich D (2013) Two-point orientation discrimination versus the traditional two-point test for tactile spatial acuity assessment. Front Hum Neurosci 7: 579. doi: 10.3389/fnhum.2013.00579. (PDF) Two-point discrimination is widely
used to measure tactile spatial acuity. The validity of the
two-point threshold as a spatial acuity measure rests on the
assumption that two points can be distinguished from one only when
the two points are sufficiently separated to evoke spatially
distinguishable foci of neural activity. However, some previous
research has challenged this view, suggesting instead that
two-point task performance benefits from an unintended non-spatial
cue, allowing spuriously good performance at small tip
separations. We compared the traditional two-point task to an
equally convenient alternative task in which participants attempt
to discern the orientation (vertical or horizontal) of two points
of contact. We used precision digital readout calipers to
administer two-interval forced-choice versions of both tasks to 24
neurologically healthy adults, on the fingertip, finger base,
palm, and forearm. We used Bayesian adaptive testing to estimate
the participants' psychometric functions on the two tasks.
Traditional two-point performance remained significantly above
chance levels even at zero point separation. In contrast,
two-point orientation discrimination approached chance as point
separation approached zero, as expected for a valid measure of
tactile spatial acuity. Traditional two-point performance was so
inflated at small point separations that 75%-correct thresholds
could be determined on all tested sites for fewer than half of
participants. The 95%-correct thresholds on the two tasks were
similar, and correlated with receptive field spacing. In keeping
with previous critiques, we conclude that the traditional
two-point task provides an unintended non-spatial cue, resulting
in spuriously good performance at small spatial separations.
Unlike two-point discrimination, two-point orientation
discrimination rigorously measures tactile spatial acuity. We
recommend the use of two-point orientation discrimination for
neurological assessment. Keywords: tactile perception,
somatosensory discrimination, reliability and validity,
neurological examination, psychophysics, sensory testing, spatial
acuity.
Wong M, Peters RM, Goldreich D (2013) A physical constraint on perceptual learning: Tactile spatial acuity improves with training to a limit set by finger size. J Neurosci 33(22): 9345-9352. (PDF) In touch as in vision, perceptual
acuity improves with training to an extent that differs greatly
across people: even individuals with similar initial acuity may
undergo markedly different improvement with training. What
accounts for this variability in perceptual learning? We
hypothesized that a simple physical characteristic -- fingertip
surface area -- might constrain tactile learning, because previous
research suggests that larger fingers have more widely spaced
mechanoreceptors. To test our hypothesis, we trained 10 human
participants intensively on a tactile spatial acuity task. During
four days, participants completed 1900 training trials (38
50-trial blocks) in which they discriminated the orientation of
square-wave gratings pressed onto the stationary index or ring
finger, with auditory feedback provided to signal correct and
incorrect responses. We progressively increased task difficulty by
shifting to thinner groove widths whenever participants achieved ≥
90%-correct block performance. We took optical scans to measure
surface area from the distal inter-phalangeal crease to the tip of
the finger. Participants' acuity improved markedly on the trained
finger, and to a lesser extent on the untrained finger. Crucially,
we found that participants' tactile spatial acuity improved
towards a theoretical optimum set by their finger size:
participants with worse initial performance, relative to their
finger size, improved more with training, and post-training
performance correlated better than did pre-training performance
with finger size. These results strongly support the hypothesis
that tactile perceptual learning is limited by finger size. We
suspect that analogous physical constraints on perceptual learning
will be found in other sensory modalities.
Goldreich D, Tong J (2013) Prediction, postdiction, and perceptual length contraction: a Bayesian low-speed prior captures the cutaneous rabbit and related illusions. Front Psychol 4: 221 doi: 10.3389/fpsyg.2013.00221. (PDF) Illusions provide a window into the
brain's perceptual strategies. In certain illusions, an ostensibly
task-irrelevant variable influences perception. For example, in
touch as in audition and vision, the perceived distance between
successive punctate stimuli reflects not only the actual distance
but curiously the inter-stimulus time. Stimuli presented at
different positions in rapid succession are drawn perceptually
towards one another. This effect manifests in several illusions,
among them the startling cutaneous rabbit, in which taps delivered
to as few as two skin positions appear to hop progressively from
one position to the next, landing in the process on intervening
areas that were never stimulated. Here we provide an accessible
step-by-step exposition of a Bayesian perceptual model that
replicates the rabbit and related illusions. The Bayesian observer
optimally joins uncertain estimates of spatial location with the
expectation that stimuli tend to move slowly. We speculate that
this expectation -- a Bayesian prior -- represents the statistics
of naturally occurring stimuli, learned by humans through sensory
experience. In its simplest form, the model contains a single free
parameter, tau: a time constant for space perception. We show that
the Bayesian observer incorporates both pre- and post-dictive
inference. Directed spatial attention affects the
prediction-postdiction balance, shifting the model's percept
towards the attended location, as observed experimentally in
humans. Applying the model to the perception of multi-tap
sequences, we show that the low-speed prior fits perception better
than an alternative, low-acceleration prior. We discuss the
applicability of our model to related tactile, visual, and
auditory illusions. To facilitate future model-driven experimental
studies, we present a convenient
freeware
computer program that implements the Bayesian observer; we
invite investigators to use this program to create their own
testable predictions. Keywords: probabilistic inference, sensory
saltation, motion illusions, tactile spatial attention, optimal
percepts, Kalman smoothing, somatosensory spatiotemporal
perception, sensory uncertainty.
Goldreich
D, Peterson MA (2012) A Bayesian observer replicates convexity
context effects in figure-ground perception. Seeing
and Perceiving (now Multisensory Research)
25(3-4): 365-395.
(PDF)
Peterson and Salvagio (2008)
demonstrated convexity context effects in figure-ground
perception: subjects shown displays consisting of unfamiliar
alternating convex and concave regions identified the convex
regions as foreground objects progressively more frequently as the
number of regions increased; this occurred only when the concave
regions were homogeneously colored. The origins of these effects
have been unclear. Here, we present a two-free-parameter Bayesian
observer that replicates convexity context effects. The Bayesian
observer incorporates two plausible expectations regarding
three-dimensional scenes: 1) objects tend to be convex rather than
concave, and 2) backgrounds tend (more than foreground objects) to
be homogeneously colored. The Bayesian observer estimates the
probability that a depicted scene is three-dimensional, and that
the convex regions are figures. It responds stochastically by
sampling from its posterior distributions. Like human observers,
the Bayesian observer shows convexity context effects only for
images with homogeneously colored concave regions. With optimal
parameter settings, it performs similarly to the average human
subject on the four display types tested. We propose that object
convexity and background color homogeneity are environmental
regularities exploited by human visual perception; vision achieves
figure-ground perception by interpreting ambiguous images in light
of these and other expected regularities in natural scenes.
Keywords: figure-ground, scene segregation, object convexity,
configural cue, Gestalt principles, natural scene statistics,
Bayesian inference, computational model.
Wong M, Hackeman E, Hurd C, Goldreich D (2011) Short-term visual deprivation does not enhance passive tactile spatial acuity. PLOS ONE 6(9): e25277. (PDF) 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(19): 7028-7037. (PDF) 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(43): 14288-14298. (PDF) 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. Supplemental
Information
(PDF).
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(50): 15756-15761. (PDF) 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. Supplemental
information
(PDF)
Goldreich D, Wong M, Peters RM, Kanics IM (2009) A tactile automated passive-finger stimulator (TAPS). J Vis Exp 28: e1374. doi: 10.3791/1374. (video article) 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 (2007) A Bayesian perceptual model replicates the cutaneous rabbit and other tactile spatiotemporal illusions. PLOS ONE 2(3): e333. (PDF) 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. 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. 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(8): 1363-1371. (PDF)
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(8): 3439-3445. (PDF) 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. Keywords:
tactile acuity, crossmodal plasticity, blind, Braille, grating
orientation, somatosensory psychophysics, sensory compensation.
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