Malleability of time through progress bars and throbbers.

Compared to a stationary pattern, a moving pattern dilates the perception of time. However, when it comes to comparing only moving stimulus, the exact dilation effects are less clear. The time dilation may be attributed to either speed of motion, temporal and spatial frequency, stimulus complexity, or the number of changes in the stimulus pattern. In the present study, we used progress bars and throbbers for inducing impressions of fast and slow "apparent" motions while the speed of motion and distance covered was actually equivalent across all conditions. The results indicate that higher number of steps produced the impression of a faster progression leading to an underestimation of time, whereas a progression in large fewer steps, produced slower apparent progression, creating the illusion of dilated time. We suggest that the perception of time depends on the nature of the stimulus rather than the speed of motion or the distance covered by the stimulus.

Diffusion modeling of interference and decay in auditory short-term memory.

Decay and interference are two leading proposals for the cause of forgetting from working and/or short-term memory, and mathematical models of both processes exist. In the present study, we apply a computational model to data from a simple short-term memory task and demonstrate that decay and interference can co-occur in the same experimental paradigm, and that neither decay nor interference alone can account for all cases of forgetting.

Overwriting and intrusion in short-term memory.

Studies of interference in working and short-term memory suggest that irrelevant information may overwrite the contents of memory or intrude into memory. While some previous studies have reported greater interference when irrelevant information is similar to the contents of memory than when it is dissimilar, other studies have reported greater interference for dissimilar distractors than for similar distractors. In the present study, we find the latter effect in a paradigm that uses auditory tones as stimuli. We suggest that the effects of distractor similarity to memory contents are mediated by the type of information held in memory, particularly the complexity or simplicity of information.

A shared short-term memory system for stimulus duration and stimulus frequency.

Recent research has suggested the existence of a modality-independent memory system that is responsible for storing representations of simple, scalar stimulus attributes, such as the frequency of an auditory pure tone or the duration of a stimulus. In the present study, we modify an existing computational model of short-term memory (STM) for stimulus frequency to allow it to perform STM tasks for both stimulus frequency and stimulus duration, supporting the notion of a common scalar STM system. We further demonstrate the utility of the model by showing that it can reproduce the subjective shortening effect, a classic finding in the psychophysical literature.

TMS-induced neural noise in sensory cortex interferes with short-term memory storage in prefrontal cortex.

In a previous study, Harris et al. (2002) found disruption of vibrotactile short-term memory after applying single-pulse transcranial magnetic stimulation (TMS) to primary somatosensory cortex (SI) early in the maintenance period, and suggested that this demonstrated a role for SI in vibrotactile memory storage. While such a role is compatible with recent suggestions that sensory cortex is the storage substrate for working memory, it stands in contrast to a relatively large body of evidence from human EEG and single-cell recording in primates that instead points to prefrontal cortex as the storage substrate for vibrotactile memory. In the present study, we use computational methods to demonstrate how Harris et al.'s results can be reproduced by TMS-induced activity in sensory cortex and subsequent feedforward interference with memory traces stored in prefrontal cortex, thereby reconciling discordant findings in the tactile memory literature.

Does stimulus complexity determine whether working memory storage relies on prefrontal or sensory cortex?

Traditionally, working and short-term memory (WM/STM) have been believed to rely on storage systems located in prefrontal cortex (PFC). However, recent experimental and theoretical efforts have suggested that, in many cases, sensory or other task-relevant cortex is the actual storage substrate for WM/STM. What factors determine whether a given WM/STM task relies on PFC or sensory cortex? In the present article, we outline recent experimental findings and suggest that the dimensionality or complexity of the to-be-remembered property or properties of a stimulus can be a determining factor.

Irrelevant sensory stimuli interfere with working memory storage: evidence from a computational model of prefrontal neurons.

The encoding of irrelevant stimuli into the memory store has previously been suggested as a mechanism of interference in working memory (e.g., Lange & Oberauer, Memory, 13, 333-339, 2005; Nairne, Memory & Cognition, 18, 251-269, 1990). Recently, Bancroft and Servos (Experimental Brain Research, 208, 529-532, 2011) used a tactile working memory task to provide experimental evidence that irrelevant stimuli were, in fact, encoded into working memory. In the present study, we replicated Bancroft and Servos's experimental findings using a biologically based computational model of prefrontal neurons, providing a neurocomputational model of overwriting in working memory. Furthermore, our modeling results show that inhibition acts to protect the contents of working memory, and they suggest a need for further experimental research into the capacity of vibrotactile working memory.

Probing the neural basis of perceptual phenomenology with the touch-induced visual illusion.

Using the touch-induced visual illusion we examine whether the brain regions involved in coding sensory information are dissociable from those that contain decision information. Activity in the intraparietal sulcus, as measured by functional magnetic resonance imaging, was associated with the illusion suggesting a sensory coding role whereas activity in the middle occipital gyrus differentially modulated activity according to the decisions made by subjects consistent with their reported perceptual phenomenology.

Can vibrotactile working memory store multiple items?

Vibrotactile working memory is increasing in popularity as a model system to test theories of working memory. Notably, however, we know little about vibrotactile working memory capacity. While most other domains of working memory are able to store multiple items (for example, the seven-plus-or-minus-two capacity of verbal memory [17]), previous examinations of vibrotactile working memory suggest that stored items may suffer from high levels of interference in the form of overwriting or representation-based interference [2,4], potentially limiting capacity and also limiting our ability to draw comparisons between vibrotactile working memory and other forms of working memory. In the present study, we use a two-item delayed match-to-sample paradigm to demonstrate that subjects are able to store multiple items in vibrotactile working memory, suggesting that interference does not catastrophically limit capacity, and strengthening our ability to compare vibrotactile working memory to other working memory tasks.

Diffusion modeling of interference in vibrotactile working memory.

The nature of interference in working memory has been a subject of discussion for decades. It has previously been argued that irrelevant stimuli can interfere with working memory by being encoded into memory. Previous findings have suggested that irrelevant sensory activity can interfere with the storage of information in tactile working memory. More recently, it has been suggested that this type of interference may operate through the overwriting of stored information by interfering sensory stimuli, even when participants are instructed to ignore such stimuli. Such a mechanism of interference is consistent with previous theoretical proposals. In the present study, we use a computational diffusion model to demonstrate that previous empirical findings are best explained by the encoding of irrelevant sensory information and subsequent interference.

The postcentral gyrus shows sustained fMRI activation during the tactile motion aftereffect.

The tactile motion aftereffect (tMAE) is a perceptual illusion in which a stationary stimulus feels as though it is moving when presented following adaptation to a unidirectionally moving tactile stimulus. Using functional magnetic resonance imaging (fMRI), we localized the brain areas responsive to tactile motion and then investigated whether these areas underlie the tMAE. Tactile stimulation was delivered to the glabrous surface of the right hand by means of a plastic cylinder with a square-wave patterned surface. In the tactile motion localizer, we contrasted periods in which the cylinder rotated at 15 rpm with periods of rest (stationary contact). Activation was observed in the contralateral (left) thalamus, postcentral gyrus, and parietal operculum. In the tMAE experiment, the cylinder rotated at 15 or 60 rpm for 2 min. The 60-rpm speed induced reliable tMAEs, whereas the 15-rpm speed did not. Of the areas activated by the tactile motion localizer, only the postcentral gyrus showed a sustained fMRI response following the offset of 60-rpm (but not 15-rpm) stimulation, presumably reflecting the illusory perception of motion.

Mechanisms of interference in vibrotactile working memory.

In previous studies of interference in vibrotactile working memory, subjects were presented with an interfering distractor stimulus during the delay period between the target and probe stimuli in a delayed match-to-sample task. The accuracy of same/different decisions indicated feature overwriting was the mechanism of interference. However, the distractor was presented late in the delay period, and the distractor may have interfered with the decision-making process, rather than the maintenance of stored information. The present study varies the timing of distractor onset, (either early, in the middle, or late in the delay period), and demonstrates both overwriting and non-overwriting forms of interference.

Distractor frequency influences performance in vibrotactile working memory.

We use a vibrotactile-delayed match-to-sample paradigm to evaluate the effects of interference on working memory. One of the suggested mechanisms through which interference affects performance in working memory is feature overwriting: Short-term representations are maintained in a finite set of feature units (such as prefrontal neurons), and distractor stimuli co-opt some or all of those units, degrading the stored representation of an earlier stimulus. Subjects were presented with two vibrotactile stimuli and were instructed to determine whether they were of the same or different frequencies. A distractor stimulus was presented between the target and probe stimuli, the frequency of which was a function of the target stimulus. Performance on the task was affected by the frequency of the distractor, with subjects making more erroneous same judgments on different trials when the distractor frequency was closer to the probe than to the target, than when the distractor was further from the probe than the target. The results suggest that the frequency of the distractor partially overwrites the stored frequency information of the probe stimulus, providing support for the feature-overwriting explanation of working memory interference.

The effect of adapting speed, duration, and distance on the tactile motion aftereffect.

We investigated the effect of adapting speed, duration, and distance on the frequency of occurrence, duration, and vividness of the tactile motion aftereffect (tMAE). Using a cylindrical drum with a patterned surface we adapted the glabrous surface of the right hand at two speeds (14 and 28 cm/s) and three durations (60, 120, and 240 s). Distance was explored in the interaction of adapting speed and duration. The results showed that the frequency of occurrence, duration, and vividness of the tMAE increased with adapting speed. There was also a positive relationship between adapting duration and the frequency of occurrence, but not the duration or vividness, of the illusion. Distance was only a factor when it came to the duration of the tMAE. Taken together, these results show the importance of adapting parameters, particularly speed, on the tMAE.

Site of stimulation effects on the prevalence of the tactile motion aftereffect.

The motion aftereffect (MAE) refers to the apparent motion of a stationary stimulus following adaptation to a continuously moving stimulus. There is a growing consensus that the fast adapting (FA) rather than the slowly adapting (SA) afferent units mediate the tactile version of the MAE. The present study investigated which FA units underlie the tactile MAE by measuring its prevalence, duration, and vividness on different skin areas that vary in their composition of FA units. Specifically, the right cheek, volar surface of the forearm, and volar surface of the hand were adapted using a ridged cylindrical drum, which rotated at 60 rpm for 120 s. Although there was no difference in duration or vividness between the skin surfaces tested, the tactile MAE was reported twice as often on the hand compared to the cheek and forearm, which did not differ significantly from one another. This suggests that the FA I units in the glabrous skin and the hair follicle and/or the FA I and field units in the hairy skin contribute to the tactile MAE.

Somatosensory temporal discrimination learning generalizes to motor interval production.

The present study investigated whether a common timing mechanism underlies the ability to analyze incoming sensory information and control outgoing motor commands. Participants were presented with two pairs of air puffs on the ventral surface of the right forearm. One pair (the standard interval) was separated by 500 ms on every trial for half of the participants ("500 group") and 800 ms on every trial for the other half ("800 group"). The duration of the comparison interval was always longer but varied adaptively to determine discrimination thresholds. Participants indicated which of the two intervals was longer. Both groups performed two motor interval production tasks (pressing a button twice in succession with the right thumb) before and again after somatosensory training. The target inter-press interval was 500 ms in one task and 800 ms in the other. A critical feature of the design was that only one of the motor tasks for each of the groups shared temporal properties with the somatosensory discrimination task. The results showed that somatosensory discrimination learning generalizes to motor interval production when the two tasks share temporal properties. Specifically, the 500 group showed a greater reduction in motor timing variability on the 500 ms task than the 800 ms task, whereas the 800 group showed a greater reduction in motor timing variability on the 800 ms task than the 500 ms task. The possible neural basis of temporal learning generalization is discussed.

The tactile motion aftereffect revisited.

In two experiments, we measured the direction, duration, frequency, and vividness of the tactile motion aftereffect (MAE) induced by a rotating drum with a ridged surface. In Experiment 1, we adapted the: (1) fingers and palm, including the thumb, (2) fingers and palm, excluding the thumb, and (3) fingers only, excluding the thumb. In each condition the drum rotated at 60 rpm for 120 s. There was no difference in duration, frequency, or vividness between the skin surfaces tested. In Experiment 2, we tested several adapting speeds: 15, 30, 45, 60, and 75 rpm. At each speed the fingers and palm, excluding the thumb, were adapted for 120 s. The duration, frequency, and vividness of the tactile MAE increased linearly with adapting speed. Overall, the tactile MAE was reported on approximately half of the trials, suggesting that it is not as robust as its visual counterpart.

Symmetric sensorimotor somatotopy.

BACKGROUND: Functional imaging has recently been used to investigate detailed somatosensory organization in human cortex. Such studies frequently assume that human cortical areas are only identifiable insofar as they resemble those measured invasively in monkeys. This is true despite the electrophysiological basis of the latter recordings, which are typically extracellular recordings of action potentials from a restricted sample of cells. METHODOLOGY/PRINCIPAL FINDINGS: Using high-resolution functional magnetic resonance imaging in human subjects, we found a widely distributed cortical response in both primary somatosensory and motor cortex upon pneumatic stimulation of the hairless surface of the thumb, index and ring fingers. Though not organized in a discrete somatotopic fashion, the population activity in response to thumb and index finger stimulation indicated a disproportionate response to fingertip stimulation, and one that was modulated by stimulation direction. Furthermore, the activation was structured with a line of symmetry through the central sulcus reflecting inputs both to primary somatosensory cortex and, precentrally, to primary motor cortex. CONCLUSIONS/SIGNIFICANCE: In considering functional activation that is not somatotopically or anatomically restricted as in monkey electrophysiology studies, our methodology reveals finger-related activation that is not organized in a simple somatotopic manner but is nevertheless as structured as it is widespread. Our findings suggest a striking functional mirroring in cortical areas conventionally ascribed either an input or an output somatotopic function.

Preservation of Emmert's law in a visual form agnosic.

Size constancy was investigated in DF, a patient with visual form agnosia, using a technique based on Emmert's law of visual after-images. DF was first given a task in which she was asked to indicate the distance of a vertical surface and a task where she had to estimate the width of a series of squares (widths ranging from 5 cm to 35 cm) placed at varying distances and having a constant visual angle. In the distance estimation task, DF greatly overestimated the distance of the vertical surface placed in front of her. DF also had great difficulty performing the size estimation task. DF then performed a task in which she stared at a bright 5 cm square for a brief period of time at a distance of 30 cm followed by the presentation of a vertical surface which varied in distance and was asked to indicate the width of the after-image either verbally or manually. DF's after-images conformed to the size-distance relationship predicted by Emmert's law--as the distance of the vertical surface increased her perception of the size of the after-images also increased. These data demonstrate that although DF is rather impaired in tasks that require explicit estimates of size and distance, at some level, DF must have relatively intact size constancy mechanisms given that her estimates of the width of the after-image conform to Emmert's law. Thus, the processes underlying explicit judgements of size and distance appear to differ from those underlying the size and distance scaling of after-images.