Signal strength determines the nature of the relationship between perception and working memory

Neurophysiological and behavioral studies have shown that perception and memory share neural substrates and functional properties. But are perception and the active working memory of a stimulus one and the same? To address this question in the spatial domain, we compared the percept and the working memory of the position of a target stimulus embedded within a surround of moving dots. Motion in a particular direction after the target's offset biased the memory of target location in the same direction. However, motion simultaneous with a high-contrast, perceptually strong target biased the percept of target location in the opposite direction. Thus, perception and working memory can be modified by motion in qualitatively different ways. Manipulations to strengthen the memory trace had no effect on the direction of the memory bias, indicating that memory signal strength can never equal that of the percept of a strong stimulus. However, the percept of a weak stimulus was biased in the direction of motion. Thus, although perception and working memory are not inherently different, they can differ behaviorally depending on the strength of the perceptual signal. Understanding how a changing surround biases neural representations in general, and postsensory processes in particular, can help one understand past reports of spatial mislocalization

Dynamic Perceptual Changes in Audiovisual Simultaneity

Background: The timing at which sensory input reaches the level of conscious perception is an intriguing question still awaiting an answer. It is often assumed that both visual and auditory percepts have a modality specific processing delay and their difference determines perceptual temporal offset. Methodology/Principal Findings: Here, we show that the perception of audiovisual simultaneity can change flexibly and fluctuates over a short period of time while subjects observe a constant stimulus. We investigated the mechanisms underlying the spontaneous alternations in this audiovisual illusion and found that attention plays a crucial role. When attention was distracted from the stimulus, the perceptual transitions disappeared. When attention was directed to a visual event, the perceived timing of an auditory event was attracted towards that event. Conclusions/Significance: This multistable display illustrates how flexible perceived timing can be, and at the same time offers a paradigm to dissociate perceptual from stimulus-driven factors in crossmodal feature binding. Our findings suggest that the perception of crossmodal synchrony depends on perceptual binding of audiovisual stimuli as a common event

Perceiving a discontinuity in motion

Studies have shown that the position of a target stimulus is misperceived owing to ongoing motion. Although static forces (fixation, landmarks) affect perceived position, motion remains the overwhelming force driving estimates of position. Motion endpoint estimates biased in the direction of motion are perceptual signatures of motion's dominant role in localization. We sought conditions in which static forces exert the predominant influence over perceived position: stimulus displays for which target position is perceived backward relative to motion. We used a target that moved diagonally with constant speed, abruptly turned 90° and continued at constant speed; observers localized the discontinuity. This yielded a previously undescribed effect, “turn-point shift,” the tendency of observers to estimate the position of orthogonal direction change backward relative to subsequent motion direction. Display and mislocalization direction differ from past studies. Static forces (foveal attraction, repulsion by subsequently occupied spatial positions) were found to be responsible. Delayed turn-point estimates, reconstructed from probing the entire trajectory, shifted the horizontal coordinate forward in the direction of motion. This implies more than one percept of turn-point position. As various estimates of turn-point position arise at different times, under different task demands, the perceptual system does not necessarily resolve conflicts between them

Adapting to an aftereffect

We report a new type of orientation-contingent color aftereffect in which the color aftereffect is opposite to the classical McCollough effect, i.e., the perceived color of the aftereffect is the same as the inducer's color. Interleaved exposure to red, horizontal and achromatic (gray), horizontal gratings led to a long-lasting aftereffect in which achromatic horizontal gratings appeared reddish. The effect, termed the anti-McCollough effect, although weaker than the classical aftereffect, remained stable for a moderate duration of time (24 hours). Unlike the classical aftereffect, which is known to not transfer interocularly, the new after-aftereffect transferred 100%, suggesting that its locus in the brain was downstream of the classical effect. It is likely that neurons in a higher-order area adapted to the classical color aftereffect that was represented in a lower-order area, thus forming an aftereffect of an aftereffect, i.e., an after-aftereffect. Our finding has implications as to how neural activity in lower- and higher-level areas in the brain interacts to yield conscious visual experience

Practice Makes Imperfect: Restorative Effects of Sleep on Motor Learning

Emerging evidence suggests that sleep plays a key role in procedural learning, particularly in the continued development of motor skill learning following initial acquisition. We argue that a detailed examination of the time course of performance across sleep on the finger-tapping task, established as the paradigm for studying the effect of sleep on motor learning, will help distinguish a restorative role of sleep in motor skill learning from a proactive one. Healthy subjects rehearsed for 12 trials and, following a night of sleep, were tested. Early training rapidly improved speed as well as accuracy on pre-sleep training. Additional rehearsal caused a marked slow-down in further improvement or partial reversal in performance to observed levels below theoretical upper limits derived on the basis of early pre-sleep rehearsal. This decrement in learning efficacy does not occur always, but if and only if it does, overnight sleep has an effect in fully or partly restoring the efficacy and actual performance to the optimal theoretically achieveable level. Our findings re-interpret the sleep-dependent memory enhancement in motor learning reported in the literature as a restoration of fatigued circuitry specialized for the skill. In providing restitution to the fatigued brain, sleep eliminates the rehearsal-induced synaptic fatigue of the circuitry specialized for the task and restores the benefit of early pre-sleep rehearsal. The present findings lend support to the notion that latent sleep-dependent enhancement of performance is a behavioral expression of the brain's restitution in sleep

Posterior Beta and Anterior Gamma Oscillations Predict Cognitive Insight

Pioneering neuroimaging studies on insight have revealed neural correlates of the emotional “Aha!” component of the insight process, but neural substrates of the cognitive component, such as problem restructuring (a key to transformative reasoning), remain a mystery. Here, multivariate electroencephalogram signals were recorded from human participants while they solved verbal puzzles that could create a small-scale experience of cognitive insight. Individuals responded as soon as they reached a solution and provided a rating of subjective insight. For unsolved puzzles, hints were provided after 60 to 90 sec. Spatio-temporal signatures of brain oscillations were analyzed using Morlet wavelet transform followed by exploratory parallel-factor analysis. A consistent reduction in beta power (15–25 Hz) was found over the parieto-occipital and centro-temporal electrode regions on all four conditions—(a) correct (vs. incorrect) solutions, (b) solutions without (vs. with) external hint, (c) successful (vs. unsuccessful) utilization of the external hint, and d) self-reported high (vs. low) insight. Gamma band (30–70 Hz) power was increased in right fronto-central and frontal electrode regions for conditions (a) and (c). The effects occurred several (up to 8) seconds before the behavioral response. Our findings indicate that insight is represented by distinct spectral, spatial, and temporal patterns of neural activity related to presolution cognitive processes that are intrinsic to the problem itself but not exclusively to one's subjective assessment of insight

Does Sleep Really Influence Face Recognition Memory?

Mounting evidence implicates sleep in the consolidation of various kinds of memories. We investigated the effect of sleep on memory for face identity, a declarative form of memory that is indispensable for nearly all social interaction. In the acquisition phase, observers viewed faces that they were required to remember over a variable retention period (0–36 hours). In the test phase, observers viewed intermixed old and new faces and judged seeing each before. Participants were classified according to acquisition and test times into seven groups. Memory strength (d′) and response bias (c) were evaluated. Substantial time spent awake (12 hours or more) during the retention period impaired face recognition memory evaluated at test, whereas sleep per se during the retention period did little to enhance the memory. Wakefulness during retention also led to a tightening of the decision criterion. Our findings suggest that sleep passively and transiently shelters face recognition memory from waking interference (exposure) but does not actively aid in its long-term consolidation

Cortical representations of a class of subjective contours

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, 1996.Includes bibliographical references.by Bhavin R. Sheth.Ph.D