Retinotopic Patterns of Correlated Fluctuations in Visual Cortex Reflect the Dynamics of Spontaneous Perceptual Suppression

While viewing certain stimuli, perception changes spontaneously in the face of constant input. For example, during "motion-induced blindness" (MIB), a small salient target spontaneously disappears and reappears when surrounded by a moving mask. Models of such bistable perceptual phenomena posit spontaneous fluctuations in neuronal activity throughout multiple stages of the visual cortical hierarchy. We used fMRI to link correlated activity fluctuations across human visual cortical areas V1 through V4 to the dynamics (rate and duration) of MIB target disappearance. We computed the correlations between the time series of fMRI activity in multiple retinotopic subregions corresponding to MIB target and mask. Linear decomposition of the matrix of temporal correlations revealed spatial patterns of activity fluctuations, regardless of whether or not these were time-locked to behavioral reports of target disappearance. The spatial pattern that dominated the activity fluctuations during MIB was spatially nonspecific, shared by all subregions, but did not reflect the dynamics of perception. By contrast, the fluctuations associated with the rate of MIB disappearance were retinotopically specific for the target subregion in V4, and the fluctuations associated with the duration of MIB disappearance states were target-specific in V1. Target-specific fluctuations in V1 have not previously been identified by averaging activity time-locked to behavioral reports of MIB disappearance. Our results suggest that different levels of the visual cortical hierarchy shape the dynamics of perception via distinct mechanisms, which are evident in distinct spatial patterns of spontaneous cortical activity fluctuations

Motion-induced blindness and Troxler fading: common and different mechanisms

Extended stabilization of gaze leads to disappearance of dim visual targets presented peripherally. This phenomenon, known as Troxler fading, is thought to result from neuronal adaptation. Intense targets also disappear intermittently when surrounded by a moving pattern (the "mask"), a phenomenon known as motion-induced blindness (MIB). The similar phenomenology and dynamics of these disappearances may suggest that also MIB is, likewise, solely due to adaptation, which may be amplified by the presence of the mask. Here we directly compared the dependence of both phenomena on target contrast. Observers reported the disappearance and reappearance of a target of varying intensity (contrast levels: 8%-80%). MIB was induced by adding a mask that moved at one of various different speeds. The results revealed a lawful effect of contrast in both MIB and Troxler fading, but with opposite trends. Increasing target contrast increased (doubled) the rate of disappearance events for MIB, but decreased the disappearance rate to half in Troxler fading. The target mean invisible period decreased equally strongly with target contrast in MIB and in Troxler fading. The results suggest that both MIB and Troxler are equally affected by contrast adaptation, but that the rate of MIB is governed by an additional mechanism, possibly involving antagonistic processes between neuronal populations processing target and mask. Our results link MIB to other bi-stable visual phenomena that involve neuronal competition (such as binocular rivalry), which exhibit an analogous dependency on the strength of the competing stimulus components

Motion-induced blindness and microsaccades: cause and effect

It has been suggested that subjective disappearance of visual stimuli results from a spontaneous reduction of microsaccade rate causing image stabilization, enhanced adaptation, and a consequent fading. In motion-induced blindness (MIB), salient visual targets disappear intermittently when surrounded by a moving pattern. We investigated whether changes in microsaccade rate can account for MIB. We first determined that the moving mask does not affect microsaccade metrics (rate, magnitude, and temporal distribution). We then compared the dynamics of microsaccades during reported illusory disappearance (MIB) and physical disappearance (Replay) of a salient peripheral target. We found large modulations of microsaccade rate following perceptual transitions, whether illusory (MIB) or real (Replay). For MIB, the rate also decreased prior to disappearance and increased prior to reappearance. Importantly, MIB persisted in the presence of microsaccades although sustained microsaccade rate was lower during invisible than visible periods. These results suggest that the microsaccade system reacts to changes in visibility, but microsaccades also modulate MIB. The latter modulation is well described by a Poisson model of the perceptual transitions assuming that the probability for reappearance and disappearance is modulated following a microsaccade. Our results show that microsaccades counteract disappearance but are neither necessary nor sufficient to account for MIB

Spatial structure affects temporal judgments: evidence for a synchrony binding code

Visual binding is the process by which the brain groups the elements belonging to one object, whilst segregating them from other scene elements. A computationally parsimonious mechanism of visual binding is the binding-by-synchrony (BBS) hypothesis. According to this hypothesis, detectors that respond to elements of a single object fire in synchrony, while detectors that respond to elements of different objects do not. Current psychophysical and physiological evidence are inconclusive about the role of BBS in the visual integration process. Here we provide psychophysical and computational evidence suggesting that the visual system implements a mechanism that synchronizes response onsets to object parts and attenuates or cancels their latency differences. In three experiments, observers had to judge the synchrony of two flickering Gabor patches embedded in a static Gabor contour, passing through fixation. We found that a smooth contour, as compared to a jagged one, impedes judgments of temporal synchrony between the targets, whilst facilitating non-temporal judgments of contrast on the same targets. We show that the results are consistent with a simple computational model that implements synchronization of responses via lateral interactions, enabling the visual system to pick up objects by synchrony among a temporally diverse background

Pupil size tracks perceptual content and surprise in a visual illusion

Changes in pupil size at constant light levels reflect the activity of neuromodulatory brainstem centers that control global brain state. These endogenously driven pupil dynamics can be synchronized with cognitive acts. For example, the pupil dilates during the spontaneous switches of perception of a constant sensory input in bistable perceptual illusions. It is unknown whether this pupil dilation only indicates the occurrence of perceptual switches, or also their content. Here, we measured pupil diameter in human subjects reporting the subjective disappearance and re-appearance of a physically constant visual target surrounded by a moving pattern (‘motion-induced blindness’ illusion). We show that the pupil dilates during the perceptual switches in the illusion and a stimulus-evoked ‘replay’ of that illusion. Critically, the switch-related pupil dilation encodes perceptual content, with larger amplitude for disappearance than re-appearance. This difference in pupil response amplitude enables prediction of the type of report (disappearance vs. re-appearance) on individual switches (receiver-operating characteristic: 61%). The amplitude difference is independent of the relative durations of target-visible and target-invisible intervals and subjects' overt behavioral report of the perceptual switches. Further, we show that pupil dilation during the replay also scales with the level of surprise about the timing of switches, but there is no evidence for an interaction between the effects of surprise and perceptual content on the pupil response. Taken together, our results suggest that pupil-linked brain systems track both the content of, and surprise about, perceptual events