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Temporal integration and segregation have been investigated both in the research on the temporal mechanisms in visual perception and in the research on visual masking. Although both research lines share theoretical, methodological, and empirical similarities, there is little overlap between them and their models of temporal processing are incompatible. As a first step toward the unification of both lines of research, we investigated the electrophysiological correlates of temporal integration and segregation in a metacontrast masking paradigm. Participants reported in each trial whether they perceived the target-mask sequence as a simultaneous or temporally segregated percept while their EEG was recorded. A comparison of both temporal report categories resulted in an ERP difference after stimulus presentation (200-450 ms) that closely resembles the contour integration negativity. Moreover, we found that phase states were shifted between perceptual report categories in the alpha (450-250 ms) and beta (225-125 ms) frequency band before stimulus presentation and induced a sinusoidal periodicity in later temporal report proportions. Thus, we show that neural correlates of temporal integration and segregation can be generalized to metacontrast masking. These findings emphasize the potential role of temporal mechanisms in the emergence of the masking phenomenon. Additionally, our findings validate our phenomenological approach by demonstrating similar neural correlates of temporal integration and segregation as in performance-based tasks. Future research may profit from our phenomenological approach to disentangle the (neural) interplay between temporal and masking mechanisms.

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A brief visual display can give rise to a sensation that outlasts the duration of the physical stimulus. The duration of this visible persistence has been estimated with paradigms that require the temporal integration of two brief sequential displays (frames) separated by a blank temporal gap. Temporal integration is said to occur when the visible persistence generated by the first frame is sufficiently long to bridge the inter-frame temporal gap. The longest gap at which integration still occurs is taken as an estimate of the duration of visible persistence. In the present work, we show that the duration of visible persistence has been underestimated in at least some of the experiments involving the temporal integration of successive displays. This is because the trailing frame can act as a metacontrast mask that foreshortens the visibility of the leading frame. Specifically, we show that operations that reduce the strength of metacontrast masking yield longer estimates of visible persistence. The relationship between metacontrast masking and visible persistence had been mentioned in some individual studies, but a comprehensive examination of that relationship is currently unavailable. Finally, we show that estimates based on single displays (e.g., the Sperling paradigm) also fail to provide untainted estimates because, in single displays, visible persistence is confounded with informational persistence.

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To investigate the relation between attention and awareness, we manipulated visibility/awareness and stimulus-driven attention capture among metacontrast-masked visual stimuli. By varying the time interval between target and mask, we manipulated target visibility measured as target discrimination accuracies (ACCs; Experiments 1 and 2) and as subjective awareness ratings (Experiment 3). To modulate stimulus-driven attention capture, we presented the masked target either as a color-singleton (the target stands out by its unique color among homogeneously colored non-singletons), as a non-singleton together with a distractor singleton elsewhere (an irrelevant distractor has a unique color, whereas the target is colored like the other stimuli) or without a singleton (no stimulus stands out; only in Experiment 1). As color singletons capture attention in a stimulus-driven way, we expected target visibility/discrimination performance to be best for target singletons and worst with distractor singletons. In Experiments 1 and 2, we confirmed that the masking interval and the singleton manipulation influenced ACCs in an independent way and that attention capture by the singletons, with facilitated performance in target-singleton compared to distractor-singleton conditions, was found regardless of the interval-induced (in-)visibility of the targets. In Experiment 1, we also confirmed that attention capture was the same among participants with worse and better visibility/discrimination performance. In Experiment 2, we confirmed attention capture by color singletons with better discrimination performance for probes presented at singleton position, compared to other positions. Finally, in Experiment 3, we found that attention capture by target singletons also increased target awareness and that this capture effect on subjective awareness was independent of the effect of the masking interval, too. Together, results provide new evidence that stimulus-driven attention and awareness operate independently from one another and that stimulus-driven attention capture can precede awareness.

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Most studies investigating the influence of primes on the processing of subsequent targets involve a main task in which responses are made to the targets, and a task that tests prime awareness. If the participant is not aware of the prime location/identity but an influence of the prime is observed in the main task, researchers conclude that this influence can be ascribed to unconscious processing of the prime. This implies the assumption that the prime's influence is independent of task instructions: a prime consciously perceived in the prime task is consciously perceived in the main task. In the metacontrast-masking study, we compared motor- and attention-related electroencephalographic (EEG) components in three tasks with the same stimuli but different instructions and showed that early posterior contralateral negativities (PCNs) and lateralized readiness potentials (LRPs) were smaller when primes were task-relevant than when targets were task-relevant. This suggests that early components may depend on task instruction and are not purely prime-related.

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Masked priming has been employed to study the role of consciousness for different levels of visual processing. However, masking procedures differ systematically between studies. To examine these procedural differences we contrasted priming effects with metacontrast masking, which is often applied in the context of perceptual priming, and priming effects with sandwich pattern masking, frequently used in studies on semantic priming. Results indicate that the amount of masking neither affects perceptual nor semantic priming effects in a semantic categorization task when a metacontrast masking paradigm was used. However, perceptual and semantic priming effects increased with increasing prime visibility when a sandwich pattern masking paradigm was used. Findings suggest that different types of masking procedures affect the processing of the masked stimuli in substantially different ways even if the masking effect on conscious perception of these stimuli is comparable.

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A recent fMRI study by Webb et al. (Cortical networks involved in visual awareness independent of visual attention, Proc Natl Acad Sci U S A 2016;113:13923-28) proposes a new method for finding the neural correlates of awareness by matching attention across awareness conditions. The experimental design, however, seems at odds with known features of attention. We highlight logical and methodological points that are critical when trying to disentangle attention and awareness.

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In most models of vision, a stimulus is processed in a series of dedicated visual areas, leading to categorization of this stimulus, and possible decision, which subsequently may be mapped onto a motor-response. In these models, stimulus processing is thought to be independent of the response modality. However, in theories of event coding, common coding, and sensorimotor contingency, stimuli may be very specifically mapped onto certain motor-responses. Here, we compared performance in a shape localization task and used three different response modalities: manual, saccadic, and verbal. Meta-contrast masking was employed at various inter-stimulus intervals (ISI) to manipulate target visibility. Although we found major differences in reaction times for the three response modalities, accuracy remained at the same level for each response modality (and all ISIs). Our results support the view that stimulus-response (S-R) associations exist only for specific instances, such as reflexes or skills, but not for arbitrary S-R pairings.

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Visual masking and attention have been known to control the transfer of information from sensory memory to visual short-term memory. A natural question is whether these processes operate independently or interact. Recent evidence suggests that studies that reported interactions between masking and attention suffered from ceiling and/or floor effects. The objective of the present study was to investigate whether metacontrast masking and attention interact by using an experimental design in which saturation effects are avoided. We asked observers to report the orientation of a target bar randomly selected from a display containing either two or six bars. The mask was a ring that surrounded the target bar. Attentional load was controlled by set-size and masking strength by the stimulus onset asynchrony between the target bar and the mask ring. We investigated interactions between masking and attention by analyzing two different aspects of performance: (i) the mean absolute response errors and (ii) the distribution of signed response errors. Our results show that attention affects observers' performance without interacting with masking. Statistical modeling of response errors suggests that attention and metacontrast masking exert their effects by independently modulating the probability of "guessing" behavior. Implications of our findings for models of attention are discussed.

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Metacontrast masking occurs when a mask follows a target stimulus in close spatial proximity. Target visibility varies with stimulus onset asynchrony (SOA) between target and mask in individually different ways leading to different masking functions with corresponding phenomenological reports. We used individual differences to determine the processes that underlie metacontrast masking. We assessed individual masking functions in a masked target discrimination task using different masking conditions and applied factor-analytical techniques on measures of sensitivity. Results yielded two latent variables that (1) contribute to performance with short and long SOA, respectively, (2) relate to specific stimulus features, and (3) differentially correlate with specific subjective percepts. We propose that each latent variable reflects a specific process. Two additional processes may contribute to performance with short and long SOAs, respectively. Discrimination performance in metacontrast masking results from individually different weightings of two to four processes, each of which contributes to specific subjective percepts.

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Postdiction effects are phenomena in which a stimulus influences the appearance of events taking place before it. In metacontrast masking, for instance, a masking stimulus can render a target stimulus shown before the mask invisible. This and other postdiction effects have been considered incompatible with a simple explanation according to which (i) our perceptual experiences are delayed for only the time it takes for a distal stimulus to reach our sensory receptors and for our neural mechanisms to process it, and (ii) the order in which the processing of stimuli is completed corresponds with the apparent temporal order of stimuli. As a result, the theories that account for more than a single postdiction effect reject at least one of these theses. This paper presents a new framework for the timing of experiences-the non-linear latency difference view-in which the three most discussed postdiction effects-apparent motion, the flash-lag effect, and metacontrast masking-can be accounted for while simultaneously holding theses (i) and (ii). This view is grounded in the local reentrant processes, which are known to have a crucial role in perception. Accordingly, the non-linear latency difference view is both more parsimonious and more empirically plausible than the competing theories, all of which remain largely silent about the neural implementation of the mechanisms they postulate.

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Dopaminergic system of the brain is believed to be strongly involved in normal and pathological behavioral phenotypes of attention. In metacontrast masking studies attentional effects on metacontrast are predominantly expressed when time intervals between a target stimulus and a masking stimulus are longer rather than shorter. Taken together, this predicts that variability in common genes known to be involved in dopaminergic function could interact with target/mask intervals in determining the effects of metacontrast masking. We tested this by genotyping participants of the masking experiment for the COMT Val158Met, DAT1 3'UTR 40 bp VNTR, and DRD4 exon 3 48b p VNTR variability. We found that Val homozygotes and subjects with long repeat variants of the DRD4 gene showed relatively higher level of correct target perception with a longer target/mask time interval than with a shorter time interval while DAT1 variability did not have any effects. Implications of this result for the development of psychophysical testing based methods of screening for vulnerability/resilience in relation to the pathology of the dopaminergic systems related attentional dysfunction are considered.

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It is known that 5HTR2A (rs6311) receptors have high concentration in the cortical layer-5 pyramidal neurons and that these receptors play an important role in the modulation of neurocognitive functions. For example, layer-5 pyramidal neurons mediate cellular level integrative interaction of primary sensory afferent signals and top-down signals exerting contextual modulatory influence. It is also known that genetic variability of 5HTR2A is implicated in individual differences in mental processes. Interestingly, serotonin selectively enhances the asynchronous type of glutamate release when modulating the activity of cortical layer-5 pyramidal neurons, with a post-stimulation delay of this effect at about 50 ms. There are not many behavioral tasks capable of tapping that small temporal intervals in terms of change of the values of independent variables leading to an observable change in the subjects' behavior. However, in the metacontrast masking vision task stimulus onset asynchronies between target and mask critical for the change in the expression of the masking effect correspond to this small temporal value. Thus we hypothesized that genetic variability in 5HTR2A (rs6311) is likely to be associated with different behavioral effects of metacontrast masking and more specifically, that because A allele carriers typically demonstrate greater promoter activity, target-to-mask asynchrony variations should have stronger impact on the masking effect especially with this group of subjects. We obtained support for this hypothesis, but only when target and mask shapes were mutually incongruent.

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The visibility of a target can be strongly suppressed by metacontrast masking. Still, some features of the target can be perceived within the mask. Usually, these rare cases of feature mis-localizations are assumed to reflect errors of the visual system. To the contrary, I will show that feature "mis-localizations" in metacontrast masking follow rules of motion grouping and, hence, should be viewed as part of a systematic feature attribution process.