Neural correlates of motion-induced blindness in the human brain

Cortical depth profiles in primary visual cortex for illusory and imaginary experiences

Visual illusions and mental imagery are non-physical sensory experiences that involve cortical feedback processing in the primary visual cortex. Using laminar functional magnetic resonance imaging (fMRI) in two studies, we investigate if information about these internal experiences is visible in the activation patterns of different layers of primary visual cortex (V1). We find that imagery content is decodable mainly from deep layers of V1, whereas seemingly 'real' illusory content is decodable mainly from superficial layers. Furthermore, illusory content shares information with perceptual content, whilst imagery content does not generalise to illusory or perceptual information. Together, our results suggest that illusions and imagery, which differ immensely in their subjective experiences, also involve partially distinct early visual microcircuits. However, overlapping microcircuit recruitment might emerge based on the nuanced nature of subjective conscious experience.

Conscious interpretation: A distinct aspect for the neural markers of the contents of consciousness

Progress in the science of consciousness depends on the experimental paradigms and varieties of contrastive analysis available to researchers. Here we highlight paradigms where the object is represented in consciousness as a set of its features but the interpretation of this set alternates in consciousness. We group experimental paradigms with this property under the label "conscious interpretation". We compare the paradigms studying conscious interpretation of the already consciously perceived objects with other types of experimental paradigms. We review previous and recent studies investigating this interpretative aspect of consciousness and propose future directions. We put forward the hypothesis that there are types of stimuli with a hierarchy of interpretations for which the rule applies: conscious experience is drawn towards higher-level interpretation and reverting back to the lower level of interpretation is impossible. We discuss how theories of consciousness might incorporate knowledge and constraints arising from the characteristics of conscious interpretation.

A Common Mechanism for Perceptual Reversals in Motion-Induced Blindness, the Troxler Effect, and Perceptual Filling-In

Several striking visual phenomena involve a physically present stimulus that alternates between being perceived and being "invisible." For example, motion-induced blindness, the Troxler effect, and perceptual filling-in all consist of subjective alternations where an item repeatedly changes from being seen to unseen. In the present study, we explored whether these three specific visual phenomena share any commonalities in their alternation rates and patterns to better understand the mechanisms of each. Data from 69 individuals revealed moderate to strong correlations across the three phenomena for the number of perceptual disappearances and the accumulated duration of the disappearances. Importantly, these effects were not correlated with eye movement patterns (saccades) assessed through eye tracking, differences in motion sensitivity as indexed by dot coherence and speed perception thresholds, or simple reaction time abilities. Principal component analyses revealed a single component that explained 67% of the variance for the number of perceptual reversals and 60% for the accumulated duration of the disappearances. The temporal dynamics of illusory disappearances was also compared for each phenomenon, and normalized durations of disappearances were well fit by a gamma distribution with similar shape parameters for each phenomenon, suggesting that they may be driven by a single oscillatory mechanism.

The temporal advantage for reloading vs. uploading conscious representations decays over time

In motion-induced blindness (MIB), a static target superimposed on a global moving pattern frequently disappears and reappears into consciousness. We previously reported an intriguing illusory temporal reversal whereby a new stimulus onset (e.g. a dot flash) presented during MIB triggers an early reappearance of the target, yet is systematically perceived as occurring after the target reappearance. This illusion implies that the unconscious target representation can be quickly reactivated, with a temporal advantage for its conscious reloading as compared to the conscious uploading of a newly presented visual stimulus. However, it remains unclear whether the temporal advantage for conscious representation reloading strengthens, decays, or remains constant over time after we lose the initial conscious access to the stimulus. To address this question, we examined the relation between the duration of MIB and the percentage of illusory temporal reversals, and we found a negative correlation between the two measures, both between and within observers. The results suggest that although the unconscious target representation retains a certain level of activation during MIB, the temporal advantage for reloading its preexisting representation into consciousness decays over time.

Speed and Lateral Inhibition of Stimulus Processing Contribute to Individual Differences in Stroop-Task Performance

The Stroop task is a popular neuropsychological test that measures executive control. Strong Stroop interference is commonly interpreted in neuropsychology as a diagnostic marker of impairment in executive control, possibly reflecting executive dysfunction. However, popular models of the Stroop task indicate that several other aspects of color and word processing may also account for individual differences in the Stroop task, independent of executive control. Here we use new approaches to investigate the degree to which individual differences in Stroop interference correlate with the relative processing speed of word and color stimuli, and the lateral inhibition between visual stimuli. We conducted an electrophysiological and behavioral experiment to measure (1) how quickly an individual’s brain processes words and colors presented in isolation (P3 latency), and (2) the strength of an individual’s lateral inhibition between visual representations with a visual illusion. Both measures explained at least 40% of the variance in Stroop interference across individuals. As these measures were obtained in contexts not requiring any executive control, we conclude that the Stroop effect also measures an individual’s pre-set way of processing visual features such as words and colors. This study highlights the important contributions of stimulus processing speed and lateral inhibition to individual differences in Stroop interference, and challenges the general view that the Stroop task primarily assesses executive control.

Motion-induced blindness continues outside visual awareness and without attention

Visual phenomena demonstrating striking perceptual disappearances of salient stimuli have fascinated researchers because of their utility in identifying neural processes that underlie subjective visibility and invisibility. Motion-induced blindness (MIB) is appealing for such purposes because it, like a class of ostensibly related paradigms such as binocular rivalry, features periods of unequivocal subjective disappearances despite constant physical stimulation. It remains unclear, however, exactly how the mechanisms that cause MIB are related to subjectively observed fluctuations in visual awareness. To address this question, we used continuous flash suppression (CFS) to present the MIB stimulus outside visual awareness. Results indicated that MIB occasionally reappeared from suppression with its salient yellow target absent. To quantify this observation, we measured reaction times (RTs) to detect the yellow dot target following visible or perceptually suppressed MIB and indeed found no difference in RTs between these conditions. We also provide evidence that MIB fluctuations can occur without attention. In sum, these experiments indicate that MIB fluctuations are effectively changes in stimulus strength, which under typical conditions result in unmistakable subjective disappearances, but are not inherently fluctuations in stimulus visibility. More broadly, these results challenge the assumed privileged link between bistable stimuli and visual awareness.

Fluctuations of visual awareness: combining motion-induced blindness with binocular rivalry

Binocular rivalry (BR) and motion-induced blindness (MIB) are two phenomena of visual awareness where perception alternates between multiple states despite constant retinal input. Both phenomena have been extensively studied, but the underlying processing remains unclear. It has been suggested that BR and MIB involve the same neural mechanism, but how the two phenomena compete for visual awareness in the same stimulus has not been systematically investigated. Here we introduce BR in a dichoptic stimulus display that can also elicit MIB and examine fluctuations of visual awareness over the course of each trial. Exploiting this paradigm we manipulated stimulus characteristics that are known to influence MIB and BR. In two experiments we found that effects on multistable percepts were incompatible with the idea of a common oscillator. The results suggest instead that local and global stimulus attributes can affect the dynamics of each percept differently. We conclude that the two phenomena of visual awareness share basic temporal characteristics but are most likely influenced by processing at different stages within the visual system.

Rotary motion impairs attention to color change in 4-month-old infants

Continuous color changes of an array of elements appear to stop changing if the array undergoes a coherent motion. This silencing illusion was demonstrated for adults by Suchow and Alvarez (Current Biology, 2011, vol. 21, pp. 140-143). The current forced-choice preferential looking study examined 4-month-old infants' sensitivity to the silencing illusion. Two experimental conditions were conducted. In the dynamic condition, infants were tested with two rotating rings of circular different-colored dots. In one of these rings the dots continuously changed color, whereas in the other ring the dots did not change color. In the static condition, the global rotary motion was eliminated from the targets. Infants preferred looking at the color-changing target in the static condition but not in the dynamic condition; they attended to the color changes in the static condition but failed to detect them in the dynamic condition. This differential looking pattern is consistent with the hypothesis that the silencing illusion can be established during early infancy. A control group of adults also responded to the silencing phenomenon. This substantiates that the stimuli generate a robust illusory effect.

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 measured objectively

During central fixation, a moving pattern of nontargets induces repeated temporary blindness to even salient peripheral targets: motion-induced blindness (MIB). Hitherto, behavioral measures of MIB have relied on subjective judgments. Here, we offer an objective alternative that builds on earlier findings regarding the effects of MIB on the detectability of physical target offsets. We propose a small modification of regular MIB displays: Following a variable duration (lead time), one of the targets is physically removed. Subjects are to respond immediately afterward. We hypothesize that illusory target offsets, caused by MIB, are mistaken for physical target offsets and that errors should thus increase with lead time. Indeed, for both nonsalient and salient targets, we found that detection accuracy for physical target offsets dramatically decreased with lead time. We conclude that target offset detection accuracy is a valid objective measure of MIB. With our method, effects of guessing are minimal, and the fitting of psychometric functions is straightforward. In principle, a staircase extension--for more efficient data collection--is also possible.

Transfer of predictive signals across saccades

Predicting visual information facilitates efficient processing of visual signals. Higher visual areas can support the processing of incoming visual information by generating predictive models that are fed back to lower visual areas. Functional brain imaging has previously shown that predictions interact with visual input already at the level of the primary visual cortex (V1; Harrison et al., 2007; Alink et al., 2010). Given that fixation changes up to four times a second in natural viewing conditions, cortical predictions are effective in V1 only if they are fed back in time for the processing of the next stimulus and at the corresponding new retinotopic position. Here, we tested whether spatio-temporal predictions are updated before, during, or shortly after an inter-hemifield saccade is executed, and thus, whether the predictive signal is transferred swiftly across hemifields. Using an apparent motion illusion, we induced an internal motion model that is known to produce a spatio-temporal prediction signal along the apparent motion trace in V1 (Muckli et al., 2005; Alink et al., 2010). We presented participants with both visually predictable and unpredictable targets on the apparent motion trace. During the task, participants saccaded across the illusion whilst detecting the target. As found previously, predictable stimuli were detected more frequently than unpredictable stimuli. Furthermore, we found that the detection advantage of predictable targets is detectable as early as 50-100 ms after saccade offset. This result demonstrates the rapid nature of the transfer of a spatio-temporally precise predictive signal across hemifields, in a paradigm previously shown to modulate V1.

Oscillatory modulations in human fusiform cortex during motion-induced blindness: intracranial recording

OBJECTIVE

Motion-induced blindness (MIB) is an illusory phenomenon, in which a static target surrounded by moving distracters is perceived to disappear. We determined the electrocorticographic (ECoG) correlates of MIB.

METHODS

While undergoing intracranial ECoG recording, a patient with focal epilepsy was instructed to report the transitions of a visual target, which was designed to illusorily or physically disappear and reappear. We then determined the neural modulations associated with illusory and physical transitions of the target. We also tested whether the phase of local delta activity could predict exclusively illusory transitions.

RESULTS

High-gamma activity at 80-150 Hz was attenuated in the fusiform region prior to the reports of illusory and real visual target disappearance. Conversely, such high-gamma activity was augmented prior to the report of real target reappearance. Exclusively around illusory disappearance but not around real one, the delta phases in the fusiform region showed a highly skewed distribution with preference of the negative peak.

CONCLUSIONS

Neuronal modulations in the fusiform region may be involved in visual awareness, while spontaneous fluctuations of neural states entrained on delta rhythm may be involved in generation of MIB.

SIGNIFICANCE

Our study increases our understanding of the mechanisms of visual awareness.

The influence of spontaneous activity on stimulus processing in primary visual cortex

Spontaneous activity in the resting human brain has been studied extensively; however, how such activity affects the local processing of a sensory stimulus is relatively unknown. Here, we examined the impact of spontaneous activity in primary visual cortex on neuronal and behavioural responses to a simple visual stimulus, using functional MRI. Stimulus-evoked responses remained essentially unchanged by spontaneous fluctuations, combining with them in a largely linear fashion (i.e., with little evidence for an interaction). However, interactions between spontaneous fluctuations and stimulus-evoked responses were evident behaviourally; high levels of spontaneous activity tended to be associated with increased stimulus detection at perceptual threshold. Our results extend those found in studies of spontaneous fluctuations in motor cortex and higher order visual areas, and suggest a fundamental role for spontaneous activity in stimulus processing.

Role of prefrontal cortex in conscious visual perception

Early visual areas are required for conscious visual perception, but recent evidence suggests that parts of the frontal lobe might also play a key role. However, it remains unclear whether frontal brain areas are involved in visual perception or merely use information from visual regions to drive behavior. One such frontal cortical area, the frontal-eye field (FEF), has been shown to have fast visual responses, thought to reflect mostly low-level visual processing, and delayed responses that correlate with perceptual reports. The latter observation is consistent with the idea that FEF uses visual information from (slower) visual regions to guide behavior. Here we ask whether fast visual responses in FEF also carry information related to the perceptual state of animals. We recorded single-cell activity in two monkeys trained to report the presence or absence of a visual target under conditions that evoke the illusory disappearance of the target (motion-induced blindness). We found that fast responses in FEF strongly correlated with the perceptual report of the animal. It is unlikely that short-latency perceptually correlated activity is inherited from early visual areas, since response latencies in FEF are shorter than those of visual areas with perceptually correlated activity. These results suggest that frontal brain areas are involved in generating the contents of visual perception.

A new taxonomy for perceptual filling-in

Perceptual filling-in occurs when structures of the visual system interpolate information across regions of visual space where that information is physically absent. It is a ubiquitous and heterogeneous phenomenon, which takes place in different forms almost every time we view the world around us, such as when objects are occluded by other objects or when they fall behind the blind spot. Yet, to date, there is no clear framework for relating these various forms of perceptual filling-in. Similarly, whether these and other forms of filling-in share common mechanisms is not yet known. Here we present a new taxonomy to categorize the different forms of perceptual filling-in. We then examine experimental evidence for the processes involved in each type of perceptual filling-in. Finally, we use established theories of general surface perception to show how contextualizing filling-in using this framework broadens our understanding of the possible shared mechanisms underlying perceptual filling-in. In particular, we consider the importance of the presence of boundaries in determining the phenomenal experience of perceptual filling-in.