Prior and Present Evidence: How Prior Experience Interacts with Present Information in a Perceptual Decision Making Task

Vibrotactile discrimination tasks have been used to examine decision making processes in the presence of perceptual uncertainty, induced by barely discernible frequency differences between paired stimuli or by the presence of embedded noise. One lesser known property of such tasks is that decisions made on a single trial may be biased by information from prior trials. An example is the time-order effect whereby the presentation order of paired stimuli may introduce differences in accuracy. Subjects perform better when the first stimulus lies between the second stimulus and the global mean of all stimuli on the judged dimension ("preferred" time-orders) compared to the alternative presentation order ("nonpreferred" time-orders). This has been conceptualised as a "drift" of the first stimulus representation towards the global mean of the stimulus-set (an internal standard). We describe the influence of prior information in relation to the more traditionally studied factors of interest in a classic discrimination task.Sixty subjects performed a vibrotactile discrimination task with different levels of uncertainty parametrically induced by increasing task difficulty, aperiodic stimulus noise, and changing the task instructions whilst maintaining identical stimulus properties (the "context").The time-order effect had a greater influence on task performance than two of the explicit factors-task difficulty and noise-but not context. The influence of prior information increased with the distance of the first stimulus from the global mean, suggesting that the "drift" velocity of the first stimulus towards the global mean representation was greater for these trials.Awareness of the time-order effect and prior information in general is essential when studying perceptual decision making tasks. Implicit mechanisms may have a greater influence than the explicit factors under study. It also affords valuable insights into basic mechanisms of information accumulation, storage, sensory weighting, and processing in neural circuits

Encoding tactile frequency and intensity information in the temporal pattern of afferent nerve impulses

Using our hands to interact with the world around us produces complex vibrations travelling across the skin. These complex waves are transduced by tactile afferent neurons whose impulse patterns convey information about the external world. A major question in this field is how important the timing of these afferent impulses is in shaping perception. We have the means to investigate this question by artificially inducing impulse patterns using brief mechanical and electrical stimuli, allowing us to study the neural coding of vibrotactile sensory information. Our lab has used this to show that when mechanical pulses evoked impulse trains grouped into periodic bursts, perceived frequency corresponded to the duration of the silent inter-burst gap interval, rather than the periodicity or the mean impulse rate. In this thesis, we induced controlled impulse trains, while measuring the perceptual responses of human subjects using psychophysical methods to assess the dimensions of frequency and intensity. As electrical stimulation has broad utility in prosthetic applications, we first verified that the same perceived frequency as predicted by the burst gap was elicited with electrical pulses in subjects within the low frequency flutter range. We then tested whether this same coding mechanism also applied outside the flutter frequency range by conducting further experiments with higher pulse rates. We found that burst gap coding correctly predicted perceived frequencies above flutter, suggesting a generalised temporal processing strategy that operates on tactile afferent inputs spanning a broad range of frequencies. Next, we investigated perceived intensity where stimulus pulse rate was varied without changes in afferent population recruitment or in perceived frequency by using bursts of pulsatile stimuli. Increasing the number of pulses within a burst caused a significant increase in perceived intensity when electrical stimulation was used. Mechanical pulses with the same burst groupings did not produce a comparable intensity increase, possibly due to minimal variations in the population firing rate. These new insights into the encoding of tactile information through temporal patterning in peripheral impulse patterns may allow the multiplexing of frequency and intensity sensations with a fixed stimulation amplitude for use in neural interfaces to deliver sensory feedback information

Behavioural and neural correlates of vibrotactile discrimination and uncertainty

Decisions pervade everyday life, from the mundane to those that induce anxiety. The act of making simple decisions may be examined experimentally with the vibrotactile discrimination task, where the manipulation of task factors challenge the way people make decisions. The objective of this thesis is to examine how the human brain maintains above-chance levels of performance despite challenging experimental conditions that alter the way people perceive presented sensory stimuli. Two behavioural investigations of perceptual decision making and two separate analyses of a functional neuroimaging experiment were conducted. In the first experiment, we examine the influence of the time-order effect whereby prior information from task stimuli biases decision making on a current trial. In the second study, different delay periods between vibration pairs were used to examine how the working memory representation of a vibrotactile stimulus drifts over time by observing changes in accuracy and sensitivity. The neural correlates of explicit factors used in the task, including stimulus noise and context judgements, were then studied through a functional neuroimaging experiment. A number of distinct prefrontal neural regions were identified, a selection of which were then used in the model-driven network based technique of dynamic causal modelling. This thesis makes the following conclusions: Even when not explicitly incorporated into experimental design, the history of previously presented stimuli can quickly establish an internal standard and exert a powerful influence on decision making. The time-order effect exhibits its influence on decision making in a nonlinear fashion across short interval delay periods between paired stimuli, in a way that depends upon prior experience with time-dependent tasks. Distinct prefrontal cortex regions including the inferior frontal gyrus pars triangularis and the superior frontal gyrus, are engaged when precision estimates of stimulus representations are required for decision making. These prefrontal regions exert their influence through nonlinear, hierarchical network connections. The findings of this thesis could be extended to elucidate cognitive disturbances in depression where deficits in decision making are a debilitating daily experience

The temporal pattern of impulses in primary afferents analogously encodes touch and hearing information

An open question in neuroscience is the contribution of temporal relations between individual impulses in primary afferents in conveying sensory information. We investigated this question in touch and hearing, while looking for any shared coding scheme. In both systems, we artificially induced temporally diverse afferent impulse trains and probed the evoked perceptions in human subjects using psychophysical techniques. First, we investigated whether the temporal structure of a fixed number of impulses conveys information about the magnitude of tactile intensity. We found that clustering the impulses into periodic bursts elicited graded increases of intensity as a function of burst impulse count, even though fewer afferents were recruited throughout the longer bursts. The interval between successive bursts of peripheral neural activity (the burst-gap) has been demonstrated in our lab to be the most prominent temporal feature for coding skin vibration frequency, as opposed to either spike rate or periodicity. Given the similarities between tactile and auditory systems, second, we explored the auditory system for an equivalent neural coding strategy. By using brief acoustic pulses, we showed that the burst-gap is a shared temporal code for pitch perception between the modalities. Following this evidence of parallels in temporal frequency processing, we next assessed the perceptual frequency equivalence between the two modalities using auditory and tactile pulse stimuli of simple and complex temporal features in cross-sensory frequency discrimination experiments. Identical temporal stimulation patterns in tactile and auditory afferents produced equivalent perceived frequencies, suggesting an analogous temporal frequency computation mechanism. The new insights into encoding tactile intensity through clustering of fixed charge electric pulses into bursts suggest a novel approach to convey varying contact forces to neural interface users, requiring no modulation of either stimulation current or base pulse frequency. Increasing control of the temporal patterning of pulses in cochlear implant users might improve pitch perception and speech comprehension. The perceptual correspondence between touch and hearing not only suggests the possibility of establishing cross-modal comparison standards for robust psychophysical investigations, but also supports the plausibility of cross-sensory substitution devices

Challenges and Opportunities for Designing Tactile Codecs from Audio Codecs

Haptic communications allows physical interaction over long distances and greatly complements conventional means of communications, such as audio and video. However, whilst standardized codecs for video and audio are well established, there is a lack of standardized codecs for haptics. This causes vendor lock-in and thereby greatly limits scalability, increases cost and prevents advanced usage scenarios with multi-sensors/actuators and multi-users. The aim of this paper is to introduce a new approach for understanding and encoding tactile signals, i.e. the sense of touch, among haptic interactions. Inspired by various audio codecs, we develop a similar methodology for tactile codecs. Notably, we demonstrate that tactile and audio signals are similar in both time and frequency domains, thereby allowing audio coding techniques to be adapted to tactile codecs with appropriate adjustments. We also present the differences between audio and tactile signals that should be considered in future designs. Moreover, in order to evaluate the performance of a tactile codec, we propose a potential direction of designing an objective quality metric which complements haptic mean opinion scores (h-MOS). This, we hope, will open the door for designing and assessing tactile codecs

Articulatory feature encoding and sensorimotor training for tactually supplemented speech reception by the hearing-impaired

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 150-159).This thesis builds on previous efforts to develop tactile speech-reception aids for the hearing-impaired. Whereas conventional hearing aids mainly amplify acoustic signals, tactile speech aids convert acoustic information into a form perceptible via the sense of touch. By facilitating visual speechreading and providing sensory feedback for vocal control, tactile speech aids may substantially enhance speech communication abilities in the absence of useful hearing. Research for this thesis consisted of several lines of work. First, tactual detection and temporal order discrimination by congenitally deaf adults were examined, in order to assess the practicability of encoding acoustic speech information as temporal relationships among tactual stimuli. Temporal resolution among most congenitally deaf subjects was deemed adequate for reception of tactually-encoded speech cues. Tactual offset-order discrimination thresholds substantially exceeded those measured for onset-order, underscoring fundamental differences between stimulus masking dynamics in the somatosensory and auditory systems. Next, a tactual speech transduction scheme was designed with the aim of extending the amount of articulatory information conveyed by an earlier vocoder-type tactile speech display strategy. The novel transduction scheme derives relative amplitude cues from three frequency-filtered speech bands, preserving the cross-channel timing information required for consonant voicing discriminations, while retaining low-frequency modulations that distinguish voiced and aperiodic signal components. Additionally, a sensorimotor training approach ("directed babbling") was developed with the goal of facilitating tactile speech acquisition through frequent vocal imitation of visuo-tactile speech stimuli and attention to tactual feedback from one's own vocalizations. A final study evaluated the utility of the tactile speech display in resolving ambiguities among visually presented consonants, following either standard or enhanced sensorimotor training. Profoundly deaf and normal-hearing participants trained to exploit tactually-presented acoustic information in conjunction with visual speechreading to facilitate consonant identification in the absence of semantic context. Results indicate that the present transduction scheme can enhance reception of consonant manner and voicing information and facilitate identification of syllableinitial and syllable-final consonants. The sensorimotor training strategy proved selectively advantageous for subjects demonstrating more gradual tactual speech acquisition. Simple, low-cost tactile devices may prove suitable for widespread distribution in developing countries, where hearing aids and cochlear implants remain unaffordable for most severely and profoundly deaf individuals. They have the potential to enhance verbal communication with minimal need for clinical intervention.by Theodore M. Moallem.Ph.D

The burst gap is a peripheral temporal code for pitch perception that is shared across audition and touch

When tactile afferents were manipulated to fire in periodic bursts of spikes, we discovered that the perceived pitch corresponded to the inter-burst interval (burst gap) in a spike train, rather than the spike rate or burst periodicity as previously thought. Given that tactile frequency mechanisms have many analogies to audition, and indications that temporal frequency channels are linked across the two modalities, we investigated whether there is burst gap temporal encoding in the auditory system. To link this putative neural code to perception, human subjects (n = 13, 6 females) assessed pitch elicited by trains of temporally-structured acoustic pulses in psychophysical experiments. Each pulse was designed to excite a fixed population of cochlear neurons, precluding place of excitation cues, and to elicit desired temporal spike trains in activated afferents. We tested periodicities up to 150 Hz using a variety of burst patterns and found striking deviations from periodicity-predicted pitch. Like the tactile system, the duration of the silent gap between successive bursts of neural activity best predicted perceived pitch, emphasising the role of peripheral temporal coding in shaping pitch. This suggests that temporal patterning of stimulus pulses in cochlear implant users might improve pitch perception

Hierarchical and Nonlinear Dynamics in Prefrontal Cortex Regulate the Precision of Perceptual Beliefs

Actions are shaped not only by the content of our percepts but also by our confidence in them. To study the cortical representation of perceptual precision in decision making, we acquired functional imaging data whilst participants performed two vibrotactile forced-choice discrimination tasks: a fast-slow judgment, and a same-different judgment. The first task requires a comparison of the perceived vibrotactile frequencies to decide which one is faster. However, the second task requires that the estimated difference between those frequencies is weighed against the precision of each percept—if both stimuli are very precisely perceived, then any slight difference is more likely to be identified than if the percepts are uncertain. We additionally presented either pure sinusoidal or temporally degraded “noisy” stimuli, whose frequency/period differed slightly from cycle to cycle. In this way, we were able to manipulate the perceptual precision. We report a constellation of cortical regions in the rostral prefrontal cortex (PFC), dorsolateral PFC (DLPFC) and superior frontal gyrus (SFG) associated with the perception of stimulus difference, the presence of stimulus noise and the interaction between these factors. Dynamic causal modeling (DCM) of these data suggested a nonlinear, hierarchical model, whereby activity in the rostral PFC (evoked by the presence of stimulus noise) mutually interacts with activity in the DLPFC (evoked by stimulus differences). This model of effective connectivity outperformed competing models with serial and parallel interactions, hence providing a unique insight into the hierarchical architecture underlying the representation and appraisal of perceptual belief and precision in the PFC

Engineering data compendium. Human perception and performance. User's guide

The concept underlying the Engineering Data Compendium was the product of a research and development program (Integrated Perceptual Information for Designers project) aimed at facilitating the application of basic research findings in human performance to the design and military crew systems. The principal objective was to develop a workable strategy for: (1) identifying and distilling information of potential value to system design from the existing research literature, and (2) presenting this technical information in a way that would aid its accessibility, interpretability, and applicability by systems designers. The present four volumes of the Engineering Data Compendium represent the first implementation of this strategy. This is the first volume, the User's Guide, containing a description of the program and instructions for its use