Metacontrast investigations of sustained-transient channel inhibitory interactions

Serial dependence in position occurs at the time of perception

Observers perceive objects in the world as stable over space and time, even though the visual experience of those objects is often discontinuous and distorted due to masking, occlusion, camouflage, or noise. How are we able to easily and quickly achieve stable perception in spite of this constantly changing visual input? It was previously shown that observers experience serial dependence in the perception of features and objects, an effect that extends up to 15 seconds back in time. Here, we asked whether the visual system utilizes an object's prior physical location to inform future position assignments in order to maximize location stability of an object over time. To test this, we presented subjects with small targets at random angular locations relative to central fixation in the peripheral visual field. Subjects reported the perceived location of the target on each trial by adjusting a cursor's position to match its location. Subjects made consistent errors when reporting the perceived position of the target on the current trial, mislocalizing it toward the position of the target in the preceding two trials (Experiment 1). This pull in position perception occurred even when a response was not required on the previous trial (Experiment 2). In addition, we show that serial dependence in perceived position occurs immediately after stimulus presentation, and it is a fast stabilization mechanism that does not require a delay (Experiment 3). This indicates that serial dependence occurs for position representations and facilitates the stable perception of objects in space. Taken together with previous work, our results show that serial dependence occurs at many stages of visual processing, from initial position assignment to object categorization.

The functional role of time compression

Multisensory integration provides continuous and stable perception from separate sensory inputs. Here, we investigated the functional role of temporal binding between the visual and the tactile senses. To this end we used the paradigm of compression that induces shifts in time when probe stimuli are degraded, e.g., by a visual mask (Zimmermann et al. 2014). Subjects had to estimate the duration of temporal intervals of 500 ms defined by a tactile and a visual, masked stimulus. We observed a strong (~100 ms) underestimation of the temporal interval when the stimuli from both senses appeared to occur at the same position in space. In contrast, when the positions of the visual and tactile stimuli were spatially separate, interval perception was almost veridical. Temporal compression furthermore depended on the correspondence of probe features and was absent when the orientation of the tactile and visual probes was incongruent. An additional experiment revealed that temporal compression also occurs when objects were presented outside the attentional focus. In conclusion, these data support a role for spatiotemporal binding in temporal compression, which is at least in part selective for object features.

Subjective and objective learning effects dissociate in space and in time

Perceptual learning not only improves sensitivity, but it also changes our subjective experience. However, the question of how these two learning effects relate is largely unexplored. Here we investigate how subjects learn to see initially indiscriminable metacontrast-masked shapes. We find that sensitivity and subjective awareness increase with training. However, sensitivity and subjective awareness dissociate in space: Learning effects on performance are lost when the task is performed at an untrained location in another quadrant, whereas learning effects on subjective awareness are maintained. This finding indicates that improvements in shape sensitivity involve visual areas up to V4, whereas changes in subjective awareness involve other brain regions. Furthermore, subjective awareness dissociates from sensitivity in time: In an early phase of perceptual learning, subjects perform above chance on trials that they rate as subjectively invisible. Later, this phenomenon disappears. Subjective awareness is thus neither necessary nor sufficient for achieving above-chance objective performance.

Spatial Extent and Figural Factors in Backward Masking

Spatial and figural characteristics of backward masking were studied, with two collinear arcs presented end-to-end and serving as target and mask, respectively. Stimulus onset asynchrony was 50 ms while interstimulus interval was 0 ms. Mask exposure duration required for masking was determined as a function of target length with mask length as a parameter. The exposure duration of the mask required for complete masking varied directly with target length, but inversely with mask length. The fact that masking strength increased with mask duration while all other parameters were kept constant suggests that masking depended on stimulus termination asynchrony. Maximal masking occurred for target arcs as long as 5.0 deg of visual angle, exceeding previously reported distances. Misaligned or differently shaped stimuli produced less masking, suggesting that figural factors play a role in long-range backward masking.

Target recovery in metacontrast: the effect of contrast

The visibility of a target stimulus (T) can be reduced by an aftercoming and spatially non-overlapping mask stimulus (M1), a phenomenon known as metacontrast masking. Interestingly, the visibility of the masked target can be recovered when a secondary mask (M2) is added to the T-M1 sequence. We analyzed a computational model of retino-cortical dynamics (RECOD) and derived the prediction that contrast dependence of metacontrast and target recovery should parallel the contrast dependence of afferent magnocellular and parvocellular pathways, respectively. In a psychophysical experiment, we tested this prediction by systematically varying (a) M2's contrast and (b) the M1-M2 onset asynchrony (SOA). At the optimal M1-M2 SOA, target recovery effect increased with M2's contrast without saturating, but at the optimal M1-M2 metacontrast SOA, reduction of M1's visibility saturated very rapidly as M2's contrast increased. Quantitative comparisons of psychophysical results with model simulations provide support for our prediction. We conclude that metacontrast masking is driven by signals originating from the magnocellular pathway and target recovery in metacontrast is driven by signals originating from the parvocellular pathway.

Dynamics of shape interaction in human vision

Spatial context can alter perceived shape, and temporal context can influence the perception of a stimulus. We sought to determine the time course of shape interactions by using a paradigm in which closed shape contours are laterally displaced over space and time. Target and masks are separated by various stimulus onset asynchrony (SOA) values, yielding forward, backward, and simultaneous masking conditions. Results indicate that spatial lateral interactions of shape are amplified by temporal asynchrony, reaching a peak at SOAs of 80-110 ms. Mask amplitude scales all effects and masking is shape specific. When a single mask follows the target, both spatial configuration and mask onset transient are critical in determining depth of masking. When the target is followed by two sequential masks, the possibility of apparent motion determines whether one or both masks drive masking. These findings suggest that temporal interactions of shape are dependent on an interactive combination of shape specificity and transients, that apparent motion plays a modulatory role, and that target shape is determined after a temporal window, not at its onset.

A comparison of masking by visual and transcranial magnetic stimulation: implications for the study of conscious and unconscious visual processing

Visual stimuli as well as transcranial magnetic stimulation (TMS) can be used: (1) to suppress the visibility of a target and (2) to recover the visibility of a target that has been suppressed by another mask. Both types of stimulation thus provide useful methods for studying the microgenesis of object perception. We first review evidence of similarities between the processes by which a TMS mask and a visual mask can either suppress the visibility of targets or recover such suppressed visibility. However, we then also point out a significant difference that has important implications for the study of the time course of unconscious and conscious visual information processing and for theoretical accounts of the processes involved. We present evidence and arguments showing: (a) that visual masking techniques, by revealing more detailed aspects of target masking and target recovery, support a theoretical approach to visual masking and visual perception that must take into account activities in two separate neural channels or processing streams and, as a corollary, (b) that at the current stage of methodological sophistication visual masks, by acting in more highly specifiable ways on these pathways, provide information about the microgenesis of form perception not available with TMS masks.

Recent models and findings in visual backward masking: a comparison, review, and update

Visual backward masking not only is an empirically rich and theoretically interesting phenomenon but also has found increasing application as a powerful methodological tool in studies of visual information processing and as a useful instrument for investigating visual function in a variety of specific subject populations. Since the dual-channel, sustained-transient approach to visual masking was introduced about two decades ago, several new models of backward masking and metacontrast have been proposed as alternative approaches to visual masking. In this article, we outline, review, and evaluate three such approaches: an extension of the dual-channel approach as realized in the neural network model of retino-cortical dynamics (Ogmen, 1993), the perceptual retouch theory (Bachmann, 1984, 1994), and the boundary contour system (Francis, 1997; Grossberg & Mingolla, 1985b). Recent psychophysical and electrophysiological findings relevant to backward masking are reviewed and, whenever possible, are related to the aforementioned models. Besides noting the positive aspects of these models, we also list their problems and suggest changes that may improve them and experiments that can empirically test them.

A microgenetic study of the Müller-Lyer illusion

In two experiments a decomposed Müller-Lyer pattern was used to measure the time course of the illusion. A partial report procedure was used to prevent the subjects from focusing only on parts of the pattern and to maximize visual processing. The Müller-Lyer figure was decomposed into two parts, its angles and its line. A configuration of three pairs of angles, each corresponding to a row in the usual partial report arrangement, was used. A line that did or did not fit the gap was shown with a variable delay (interstimulus interval, ISI). By this procedure the relevant row (line gap) was cued. The subject had to decide whether the line fitted or was too short/long. Two exposure times for the angles were used, 50 or 200 ms in one experiment and 50 or 500 ms in the other. The result of the first experiment, with outward-pointing fins, was a stable illusion for all values of ISI (25-400 ms) and all exposure times, with one significant exception: exposure for 50 ms with an ISI of 50 ms yielded an illusion peak. It was shown that this was not caused by a reduction in length-discrimination performance. In the second experiment, with inward-pointing fins, no such peak occurred. There was only a tendency for the illusion to vanish with zero ISI. The results are discussed with respect to 'global to local' theories of visual processing.

Flicker masking and developmental dyslexia

Recent studies have provided evidence that dyslexic children tend to show longer visual persistence than control children when presented with low-spatial-frequency grating stimuli. The possibility that this phenomenon might reflect an impairment of inhibitory Y-cell activity in the visual system of dyslexics has been investigated. A flicker masking technique was used to mask Y-cell activity selectively in a group of dyslexic boys and a group of age-matched controls. There were no overall differences in reaction times to the offsets of grating patterns of various spatial frequencies between the groups, and no differences between subgroups defined by age, degree of reading impairment, or any other criterion. The results show no evidence of abnormal Y-cell function in developmental dyslexia.

Pathways in type-B (U-shaped) metacontrast

Rod and cone targets were crossed, in every combination, with rod and cone masks in flanking-bars metacontrast. Strong type-B (U-shaped) metacontrast was obtained in each condition, contrary to the claim that rod and cone masking are independent. In each condition, visibility declined steadily with stimulus-onset asynchrony (SOA) in trials in which target and mask appeared to be simultaneous, and increased with SOA in trials in which they appeared to be successive. The 'U' results from collapsing across these different types of trials, which may reflect distinct monotonic processes in masking. Under the light adaptation conditions used the time, Tmax, at which metacontrast was at a maximum was delayed by about 25 ms if rods, rather than cones, detected the target. Whether rods or cones detected the mask hardly altered Tmax.

Stationary patterns suppress the perception of stroboscopic motion

Two spatially separated vertical bar stimuli briefly flashed in temporal sequence produced strong sensations of stroboscopic apparent motion; particularly at intermediate onset asynchronies. The sustained presence of two additional stationary vertical bars flanking the two movement-inducing bars during their presentation significantly decreased the rated magnitude of the sensation of stroboscopic motion. Control experiments rule out contrast reduction of the movement-inducing bars by the stationary flanking bars as a source of the decrease of the rated magnitude of stroboscopic motion. These results are related to similar effects observed in metacontrast and suggest that sustained channels responding to stationary patterns inhibit transient channels responding to brief or rapid image displacements giving rise to perception of stroboscopic motion.

Metacontrast U-shaped functions derive from two monotonic processes

Different underlying processes account for the descending and ascending portions of the metacontrast U-shaped function obtained in the flanking-masks paradigm. One or another process is dominant on each trial. Each process is monotonic with stimulus onset asynchrony in the region in which it can be measured. The two processes may be isolated by asking the subject to report on each trial not only target visibility but also whether target and mask appear simultaneous or not. Standard U-shaped functions could be obtained only as an artifact of averaging across these different types of trials.