Monoptic and dichoptic metacontrasf

Four-dot masking in monoptic and dichoptic viewing

In visual backward masking paradigms, the visibility of a target is reduced using various kinds of mask stimuli presented immediately after the target. Four-dot masking is one such kind of backward masking, caused by four surrounding dots neither spatially adjacent nor similar to the target. Four-dot masking is often considered to involve object-level interferences. However, low-level contributions such as lateral inhibition and motion detection are also possible. To elucidate the loci of the underlying mechanism within the visual hierarchy, we compared the masking effect between monoptic and dichoptic viewing conditions. A target and a four-dot mask, which also served as a spatial cue to the target location, were presented to the same eye in monoptic viewing, whereas they were presented to different eyes in dichoptic viewing. Observers were then asked to discriminate the target shape. We found a significant decline in the correct response rate compared to the baseline condition in which the four-dot mask was not presented, and the masking effect was equivalent between the monoptic and dichoptic viewings. These results demonstrate that four-dot masking stems exclusively from processing within the binocular pathway.

Differences in perceptual masking between humans and rats

INTRODUCTION

The perception of a target stimulus can be impaired by a subsequent mask stimulus, even if they do not overlap temporally or spatially. This "backward masking" is commonly used to modulate a subject's awareness of a target and to characterize the temporal dynamics of vision. Masking is most apparent with brief, low-contrast targets, making detection difficult even in the absence of a mask. Although necessary to investigate the underlying neural mechanisms, evaluating masking phenomena in animal models is particularly challenging, as the task structure and critical stimulus features to be attended must be learned incrementally through rewards and feedback. Despite the increasing popularity of rodents in vision research, it is unclear if they are susceptible to masking illusions.

METHODS

We characterized how spatially surrounding masks affected the detection of sine-wave grating targets.

RESULTS

In humans (n = 5) and rats (n = 7), target detection improved with contrast and was reduced by the presence of a mask. After controlling for biases to respond induced by the presence of the mask, a clear reduction in detectability was caused by masks. This reduction was evident when data were averaged across all animals, but was only individually significant in three animals.

CONCLUSIONS

While perceptual masking occurs in rats, it may be difficult to observe consistently in individual animals because the complexity of the requisite task pushes the limits of their behavioral capabilities. We suggest methods to ensure that masking, and similarly subtle effects, can be reliably characterized in future experiments.

Dichoptic Metacontrast Masking Functions to Infer Transmission Delay in Optic Neuritis

Optic neuritis (ON) has detrimental effects on the transmission of neuronal signals generated at the earliest stages of visual information processing. The amount, as well as the speed of transmitted visual signals is impaired. Measurements of visual evoked potentials (VEP) are often implemented in clinical routine. However, the specificity of VEPs is limited because multiple cortical areas are involved in the generation of P1 potentials, including feedback signals from higher cortical areas. Here, we show that dichoptic metacontrast masking can be used to estimate the temporal delay caused by ON. A group of 15 patients with unilateral ON, nine of which had sufficient visual acuity and volunteered to participate, and a group of healthy control subjects (N = 8) were presented with flashes of gray disks to one eye and flashes of gray annuli to the corresponding retinal location of the other eye. By asking subjects to report the subjective visibility of the target (i.e. the disk) while varying the stimulus onset asynchrony (SOA) between disk and annulus, we obtained typical U-shaped masking functions. From these functions we inferred the critical SOA_max at which the mask (i.e. the annulus) optimally suppressed the visibility of the target. ON-associated transmission delay was estimated by comparing the SOA_max between conditions in which the disk had been presented to the affected and the mask to the other eye, and vice versa. SOA_max differed on average by 28 ms, suggesting a reduction in transmission speed in the affected eye. Compared to previously reported methods assessing perceptual consequences of altered neuronal transmission speed the presented method is more accurate as it is not limited by the observers’ ability to judge subtle variations in perceived synchrony.

Masking reduces orientation selectivity in rat visual cortex

In visual masking the perception of a target stimulus is impaired by a preceding (forward) or succeeding (backward) mask stimulus. The illusion is of interest because it allows uncoupling of the physical stimulus, its neuronal representation, and its perception. To understand the neuronal correlates of masking, we examined how masks affected the neuronal responses to oriented target stimuli in the primary visual cortex (V1) of anesthetized rats (n = 37). Target stimuli were circular gratings with 12 orientations; mask stimuli were plaids created as a binarized sum of all possible target orientations. Spatially, masks were presented either overlapping or surrounding the target. Temporally, targets and masks were presented for 33 ms, but the stimulus onset asynchrony (SOA) of their relative appearance was varied. For the first time, we examine how spatially overlapping and center-surround masking affect orientation discriminability (rather than visibility) in V1. Regardless of the spatial or temporal arrangement of stimuli, the greatest reductions in firing rate and orientation selectivity occurred for the shortest SOAs. Interestingly, analyses conducted separately for transient and sustained target response components showed that changes in orientation selectivity do not always coincide with changes in firing rate. Given the near-instantaneous reductions observed in orientation selectivity even when target and mask do not spatially overlap, we suggest that monotonic visual masking is explained by a combination of neural integration and lateral inhibition.

A temporal window for estimating surface brightness in the Craik-O'Brien-Cornsweet effect

The central edge of an opposing pair of luminance gradients (COC edge) makes adjoining regions with identical luminance appear to be different. This brightness illusion, called the Craik-O'Brien-Cornsweet effect (COCe), can be explained by low-level spatial filtering mechanisms (Dakin and Bex, 2003). Also, the COCe is greatly reduced when the stimulus lacks a frame element surrounding the COC edge (Purves et al., 1999). This indicates that the COCe can be modulated by extra contextual cues that are related to ideas about lighting priors. In this study, we examined whether processing for contextual modulation could be independent of the main COCe processing mediated by the filtering mechanism. We displayed the COC edge and frame element at physically different times. Then, while varying the onset asynchrony between them and changing the luminance contrast of the frame element, we measured the size of the COCe. We found that the COCe was observed in the temporal range of around 600–800 ms centered at the 0 ms (from around −400 to 400 ms in stimulus onset asynchrony), which was much larger than the range of typical visual persistency. More importantly, this temporal range did not change significantly regardless of differences in the luminance contrast of the frame element (5–100%), in the durations of COC edge and/or the frame element (50 or 200 ms), in the display condition (interocular or binocular), and in the type of lines constituting the frame element (solid or illusory lines). Results suggest that the visual system can bind the COC edge and frame element with a temporal window of ~1 s to estimate surface brightness. Information from the basic filtering mechanism and information of contextual cue are separately processed and are linked afterwards.

Probing feedforward and feedback contributions to awareness with visual masking and transcranial magnetic stimulation

A number of influential theories posit that visual awareness relies not only on the initial, stimulus-driven (i.e., feedforward) sweep of activation but also on recurrent feedback activity within and between brain regions. These theories of awareness draw heavily on data from masking paradigms in which visibility of one stimulus is reduced due to the presence of another stimulus. More recently transcranial magnetic stimulation (TMS) has been used to study the temporal dynamics of visual awareness. TMS over occipital cortex affects performance on visual tasks at distinct time points and in a manner that is comparable to visual masking. We draw parallels between these two methods and examine evidence for the neural mechanisms by which visual masking and TMS suppress stimulus visibility. Specifically, both methods have been proposed to affect feedforward as well as feedback signals when applied at distinct time windows relative to stimulus onset and as a result modify visual awareness. Most recent empirical evidence, moreover, suggests that while visual masking and TMS impact stimulus visibility comparably, the processes these methods affect may not be as similar as previously thought. In addition to reviewing both masking and TMS studies that examine feedforward and feedback processes in vision, we raise questions to guide future studies and further probe the necessary conditions for visual awareness.

Waves of visibility: probing the depth of inter-ocular suppression with transient and sustained targets

In order to study non-conscious visual processing, researchers render otherwise consciously perceived images into invisible stimuli. Through the years, several psychophysical techniques have been developed for this purpose. Yet the comparison of experimental results across techniques remains a difficult task as the depth of suppression depends on the interactions between the type of stimuli and the suppression methods employed. This poses a limit to the inferences that researchers make about the extent of non-conscious processes. We investigated the mechanisms underlying inter-ocular suppression during continuous flash suppression (CFS) and dichoptic visual masking using a transient onset target stimulus and a variety of stimulus/mask temporal manipulations. We show that target duration, timing of target onset, and mask frequency are key aspects of inter-ocular suppression during CFS with transient targets. The differences between our results and sustained target CFS studies suggest that two distinct mechanisms are involved in the detection of transient and prolonged target stimuli during CFS. Our results provide insight into the dynamics of CFS together with evidence for similarities between transient target CFS and dichoptic visual masking.

Visual masking: past accomplishments, present status, future developments

Visual masking, throughout its history, has been used as an investigative tool in exploring the temporal dynamics of visual perception, beginning with retinal processes and ending in cortical processes concerned with the conscious registration of stimuli. However, visual masking also has been a phenomenon deemed worthy of study in its own right. Most of the recent uses of visual masking have focused on the study of central processes, particularly those involved in feature, object and scene representations, in attentional control mechanisms, and in phenomenal awareness. In recent years our understanding of the phenomenon and cortical mechanisms of visual masking also has benefited from several brain imaging techniques and from a number of sophisticated and neurophysiologically plausible neural network models. Key issues and problems are discussed with the aim of guiding future empirical and theoretical research.

Functional hierarchies of nonconscious visual processing

A number of psychophysical techniques can be used to eliminate the registration of stimuli in visual awareness and to study the dynamics of conscious and nonconscious information processing in the visual system. However, little is known about how these techniques relate to each other. We chose to compare binocular rivalry, induced by orthogonal gratings presented separately to the two eyes, and metacontrast suppression, produced when a target stimulus is followed by a spatially surrounding mask stimulus, to investigate relative levels and correlates of nonconscious processing. Combined with prior results, our findings indicate that binocular rivalry expresses its suppressive effects prior to the level at which the mechanism of metacontrast does. Implications for theories of masking and interpretations of the loss or perceptual effects when stimulus visibility is suppressed by different psychophysical methods are discussed.

Central factors contributing to para-contrast modulation of contour and brightness perception

Following up on a prior study of contour and brightness processing in visual masking (Breitmeyer et al., 2006), we investigated the effects of a binocular and dichoptic para-contrast masking on the visibility of the contour and brightness of a target presented to the other eye. Combined, the results support the contributions of several cortical processes to para-contrast: (1) two central sources of inhibition, one long-latency and prolonged and the other short-latency and brief; (2) binocular rivalry suppression; and (3) a facilitatory effect peaking at different SOAs for the contour and the brightness tasks, reflecting; (4) known properties of two separate cortical systems, one a fast contour-processing pathway and the other a slower brightness-processing pathway.

Visual attention and the mechanism of metacontrast

The U-shaped metacontrast function may result from the superimposition of two monotonic components which reflect the effects of mechanisms similar to the peripheral and central processes suggested for backward pattern masking by Turvey (Psychol Rev 80:1-52, 1973). In an experiment using the disc-ring paradigm, it was demonstrated that the decreasing and increasing branches of the metacontrast function are differently affected by the exposure duration of the mask and a task-irrelevant stimulus (distractor) appearing in the contralateral visual hemifield. The phenomenal representation of masking is different for the two parts of the curve. It is suggested that masking in the second part of the masking function, but not in the first, is related to the control of visual attention.

Generalized flash suppression of salient visual targets

A pattern of light striking the retina of an alert observer is normally readily perceived. While a handful of conditions exist in which even salient visual stimuli can be rendered invisible, the mechanisms underlying such suppression remain poorly understood. Here, we describe experiments using a novel stimulation sequence that gives rise to the sudden and reliable subjective disappearance of a wide range of visual patterns. We found that a parafoveal target immediately vanished from perception following the abrupt onset of a surrounding texture. The probability of disappearance was influenced by the ocular configuration of the target and surround, as well as their spatial separation. In addition, suppression was critically dependent upon several hundred milliseconds of stimulus-specific adaptation. These findings demonstrate that the all-or-none disappearance of a salient visual target, which is reminiscent of a high-level selection process, is inextricably linked to topographic stimulus representations, presumably in the early visual cortex.

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.

Apparent motion can survive binocular rivalry suppression

For short-range motion, observers dichoptically viewed a random-dot cinematogram and a rival target. Upon keypress, the first frame of the cinematogram was replaced by the second frame. Observers judged the direction of motion, which was governed by the initial position of the central region. Performance was well above chance during both dominance and suppression. For long-range motion, observers rated the motion produced by sequentially flashing two small spots, with the first spot contained within a rivalrous region. Suppression reduced but did not prevent perception of this motion. Presenting the second motion frame to both eyes weakened both forms of motion.

The effects of dichoptic and binocular viewing on bistable motion percepts

Two competitive percepts are produced from a bistable stroboscopic motion display. In this display two frames, each containing three horizontally arrayed elements are presented alternately for several cycles. At short interstimulus intervals (ISIs) element or end-to-end motion responses are obtained when the two inner, spatially overlapping elements are seen as stationary and the third element moves back and forth from one end to the other end. Group motion responses are obtained at longer ISIs when the three elements are seen to move back and forth as a group. The dominance of these two percepts across ISIs was controlled by the manipulation of (1) element size, (2) frame duration, and (3) viewing conditions. Under both binocular and dichoptic viewing, element motion responses increase as element size and frame duration decrease. By maximizing pattern persistence substantial element motion responses were obtained dichoptically as well as binocularly. Instead of supporting the existence of two separate, low-level and high-level, motion systems, our data suggest that there is a single, high-level mechanism for motion whose output can be modulated by pattern persistence.

Electrophysiological correlates of visual masking

The phenomenon of paracontrast documented by means of psychophysical techniques has not been readily demonstrable using electrophysiological responses as an index of masking. We have analyzed averaged cortical potentials during presentation of visual stimuli that produce psychophysical masking. Differences in the masking profiles obtained from the two experimental conditions are apparent. However, both electrophysiological and psychophysical data do indicate an interaction between responses to an annulus and a disc which varies with the time interval between the stimuli. Both sets of data also reflect differences in masking between right and left eyes. We conclude that the cortical potentials evoked by temporally disparate stimuli do interact, but that the masking profile is not identical to the profile obtained by psychophysical methods.

Visual masking: a unified approach

The suppression effect observed in masking is assumed to be due to neural inhibition between two stimuli that interact spatially or within a narrow span of time in the fashion demonstrated in simultaneous brightness-contrast experiments. Three principles are brought together in a comprehensive mathematical model of visual masking. The three principles are: lateral inhibition, the integrated visual response function (VRF), and stimulus decay after its offset over a limited period of time (up to about 100 ms). Block's law is extended to apply to the integration of the VRF, including decay. The three principles combined are sufficient to explain well-known masking phenomena such as metacontrast, and forward and backward masking. The mathematical model presented demonstrates clearly the common underlying basis of all masking. The validity of the lesser documented decay after stimulus offset, a necessary assumption to explain masking effects with stimuli that are delayed relative to each other, is demonstrated with experimental evidence.

Temporal response characteristics of cells in monkey striate cortex measured with metacontrast masking and brightness discrimination

After measuring metacontrast masking psychophysically in two monkeys, recordings from parafoveal striate cortex of the monkeys were made while they performed a simultaneous brightness discrimination and while they judged the apparent brightness of a stimulus masked by metacontrast. The size and orientation of the stimuli were held constant regardless of receptive field parameters. In both tasks, the single-cell activity immediately following the presentation of flashed discrimination stimuli reflected only stimulus parameters, and was independent of the monkey's behavioral choice. Later activity (up to 400 msec post-stimulus) was significantly greater if the monkey was about to press the correct panel in the discrimination, or if he pressed the unmasked side (with greater apparent brightness but identical intensity) in the masking paradigm. One quarter of the cells showed a change in firing rate during the 250 msec preceding the behavioral response, though the difference in overall firing level between correct and incorrect brightness discrimination trials was diminished in this epoch, and the corresponding firing difference in metacontrast trials was not significant. The temporal pattern of firing also differed between correct and incorrect trials in the pre-response interval. The results suggest an iterative or recurrent coding of visual information, where the same cells participate in early, late, preresponse coding in different ways.