Prestimulus oscillatory brain activity interacts with evoked recurrent processing to facilitate conscious visual perception

We investigated whether prestimulus alpha-band oscillatory activity and stimulus-elicited recurrent processing interact to facilitate conscious visual perception. Participants tried to perceive a visual stimulus that was perceptually masked through object substitution masking (OSM). We showed that attenuated prestimulus alpha power was associated with greater negative-polarity stimulus-evoked ERP activity that resembled the visual awareness negativity (VAN), previously argued to reflect recurrent processing related to conscious perception. This effect, however, was not associated with better perception. Instead, when prestimulus alpha power was elevated, a preferred prestimulus alpha phase was associated with a greater VAN-like negativity, which was then associated with better cue perception. Cue perception was worse when prestimulus alpha power was elevated but the stimulus occurred at a nonoptimal prestimulus alpha phase and the VAN-like negativity was low. Our findings suggest that prestimulus alpha activity at a specific phase enables temporally selective recurrent processing that facilitates conscious perception in OSM.

Revealing a competitive dynamic in rapid categorization with object substitution masking

Categorization at different levels of abstraction have distinct time courses, but the different levels are often considered separately. Superordinate-level categorization is typically faster than basic-level categorization at ultra-rapid exposure durations (< 33 ms) while basic-level categorization is faster than superordinate-level categorization at longer exposure durations. This difference may be due to a competitive dynamic between levels of categorization. By leveraging object substitution masking, we found a distinct time course of masking effects for each level of categorization. Superordinate-level categorization showed a masking effect earlier than basic-level categorization. However, when basic-level categorization first showed a masking effects, superordinate-level categorization was spared despite its earlier masking effect. This unique pattern suggests a trade-off between the two levels of categorization over time. Such an effect supports an account of categorization that depends on the interaction of perceptual encoding, selective attention, and competition between levels of category representation.

Chinese Character Processing in Visual Masking

It has not been clarified if attention influences perception of targets in visual masking. Three forms of common masks (random pattern, para-/metacontrast, and four dots) were thus chosen in the present study and presented with character targets in three temporal sequences (forward, backward, and sandwiched mask or forward-backward mask combination). In order to pinpoint the level of processing where masking arises, character targets were varied in depth of processing from random arrangements of strokes up to real Chinese characters. The attentional influence was examined under perceptual discrimination and lexical decision tasks, respectively. The results revealed significant interactions among four factors (mask form, temporal sequence, depth of processing, and task). Identification of character targets in each form of mask sequence varied with task demand, with greater suppression in the perceptual discrimination task. These findings suggested that attentional demand can bias processing in favor of task-related information in visual masking. Variations in masking effects may be contributed by both attentional demand and spatio-temporal interaction.

Standardizing measurement in psychological studies: On why one second has different value in a sprint versus a marathon

Here we highlight the importance of considering relative performance and the standardization of measurement in psychological research. In particular, we highlight three key analytic issues. The first is the fact that the popular method of calculating difference scores can be misleading because current approaches rely on absolute differences, neglecting what proportion of baseline performance this change reflects. We propose a simple solution of dividing absolute differences by mean levels of performance to calculate a relative measure, much like a Weber fraction from psychophysics. The second issue we raise is that there is an increasing need to compare the variability of effects across studies. The standard deviation score (SD) represents the average amount by which scores differ from their mean, but is sensitive to units, and to where a distribution lies along a measure even when the units are common. We propose two simple solutions to calculate a truly standardized SD (SSD), one for when the range of possible scores is known (e.g., scales, accuracy), and one for when it is unknown (e.g., reaction time). The third and final issue we address is the importance of considering relative performance in applying exclusion criteria to screen overly slow reaction time scores from distributions.

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.

What was that object? On the role of identity information in the formation of object files and conscious object perception

Object files are a psychological representation that allows the human brain to keep track of objects, as they move and change across time. The question regarding what information is used to individuate versus update object files has been the focus of considerable scientific debate. Historically, the role of an object's spatiotemporal history was emphasised, whereas more recent work has demonstrated a key contribution from surface features, such as colour. The purpose of the present study was to investigate the role of identity-level information in the formation and individuation of object files, and how it compares to the contribution of featural information. Using a modified spatial repetition-blindness paradigm, across four experiments, there was convergent evidence that surface features contribute to the formation of object files, whereas the role of identity information was at best much smaller and less reliable than the clear contribution from surface features, and the most parsimonious explanation is that it was not present at all.

Perceptual Errors Support the Notion of Masking by Object Substitution

Two experiments examined the effect of object substitution masking (OSM) on the perceptual errors in reporting the orientation of a target. In Experiment 1, a four-dot trailing mask was compared with a simultaneous-noise mask. In Experiment 2, the four-dot and noise masks were factorially varied. Responses were modelled using a mixture regression model and Bayesian inference to deduce whether the relative impacts of OSM on guessing and precision were the same as those of a noise mask, and thus whether the mechanism underpinning OSM is based on increasing noise rather than a substitution process. Across both experiments, OSM was associated with an increased guessing rate when the mask trailed target offset and a reduction in the precision of the target representation (although the latter was less reliable across the two experiments). Importantly, the noise mask also influenced both guessing and precision, but in a different manner, suggesting that OSM is not simply caused by increasing noise. In Experiment 2, the effects of OSM and simultaneous-noise interacted, suggesting the two manipulations involve common mechanisms. Overall results suggest that OSM is often a consequence of a substitution process, but there is evidence that the mask increases noise levels on trials where substitution does not occur.

Object-substitution masking weakens but does not eliminate shape interactions

At any moment, some objects in the environment are seen clearly, whereas others go unnoticed. Whether or not these gaps in awareness are actually problematic may depend on the extent that information about unseen objects is lost. Determining when and how visual awareness and visual processing become linked is thus of great importance. Previous research using object-substitution masking (OSM) demonstrated that relatively simple visual features, such as size or orientation, are still processed even when they are not visible. Yet this does not appear to be the case for more complex features like faces. This suggests that, during OSM, disruptions of visual processing and awareness may tend to co-occur beginning at some intermediate stage along the ventral pathway. We tested this hypothesis by evaluating the extent to which OSM disrupted the perception and processing of two-dimensional objects. Specifically, we evaluated whether an unseen shape's aspect ratio would influence the appearance of another shape that was briefly visible nearby. As expected, the aspect ratios of two shapes appeared to be more similar to each other when both were visible. This averaging effect was weakened, but not eliminated, when one ellipse in each pair received OSM. These shape interactions persisted even when one ellipse from each pair was invisible. When combined with previous work, these results suggest that during object-substitution masking, disruptions of visual processing tend to strengthen with increases in stimulus complexity, becoming more tightly bound to the mechanisms of visual awareness at intermediate stages of visual analysis.

Dissociations Among Attention, Perception, and Awareness During Object-Substitution Masking

When a visual target object is surrounded by four dots that onset at the same time as the target but remain visible after the target terminates, the four dots dramatically impair target discrimination performance. This phenomenon is called object-substitution masking, reflecting the hypothesis that both the target and the four dots are identified, but the representation of the four dots replaces the representation of the target object before the target can be reported. The present study used the event-related potential technique to demonstrate that a target masked in this manner is identified by the visual system and triggers a shift of attention. However, by the time attention is shifted to the target, only the mask remains visible, leading to impaired behavioral detection performance. These findings support the object-substitution hypothesis and provide new evidence that perception, attention, and awareness can be dissociated.

Object-substitution masking degrades the quality of conscious object representations

Object-substitution masking (OSM) is a unique paradigm for the examination of object updating processes. However, existing models of OSM are underspecified with respect to the impact of object updating on the quality of target representations. Using two paradigms of OSM combined with a mixture model analysis we examine the impact of post-perceptual processes on a target's representational quality within conscious awareness. We conclude that object updating processes responsible for OSM cause degradation in the precision of object representations. These findings contribute to a growing body of research advocating for the application of mixture model analysis to the study of how cognitive processes impact the quality (i.e., precision) of object representations.

Odours reduce the magnitude of object substitution masking for matching visual targets in females

Recent evidence suggests that olfactory stimuli can influence early stages of visual processing, but there has been little focus on whether such olfactory-visual interactions convey an advantage in visual object identification. Moreover, despite evidence that some aspects of olfactory perception are superior in females than males, no study to date has examined whether olfactory influences on vision are gender-dependent. We asked whether inhalation of familiar odorants can modulate participants' ability to identify briefly flashed images of matching visual objects under conditions of object substitution masking (OSM). Across two experiments, we had male and female participants (N = 36 in each group) identify masked visual images of odour-related objects (e.g., orange, rose, mint) amongst nonodour-related distracters (e.g., box, watch). In each trial, participants inhaled a single odour that either matched or mismatched the masked, odour-related target. Target detection performance was analysed using a signal detection (d') approach. In females, but not males, matching odours significantly reduced OSM relative to mismatching odours, suggesting that familiar odours can enhance the salience of briefly presented visual objects. We conclude that olfactory cues exert a subtle influence on visual processes by transiently enhancing the salience of matching object representations. The results add to a growing body of literature that points towards consistent gender differences in olfactory perception.

A closer look at four-dot masking of a foveated target

Four-dot masking with a common onset mask was recently demonstrated in a fully attended and foveated target (Filmer, Mattingley & Dux, 2015). Here, we replicate and extend this finding by directly comparing a four-dot mask with an annulus mask while probing masking as a function of mask duration, and target-mask separation. Our results suggest that while an annulus mask operates via spatially local contour interactions, a four-dot mask operates through spatially global mechanisms. We also measure how the visual system’s representation of an oriented bar is impacted by a four-dot mask, and find that masking here does not degrade the precision of perceived targets, but instead appears to be driven exclusively by rendering the target completely invisible.

Invisibility and interpretation

Invisibility is often thought to occur because of the low-level limitations of the visual system. For example, it is often assumed that backward masking renders a target invisible because the visual system is simply too slow to resolve the target and the mask separately. Here, we propose an alternative explanation in which invisibility is a goal rather than a limitation and occurs naturally when making sense out of the plethora of incoming information. For example, we present evidence that (in)visibility of an element can strongly depend on how it groups with other elements. Changing grouping changes visibility. In addition, we will show that features often just appear to be invisible but are in fact visible in a way the experimenter is not aware of.

Reentrant processing mediates object substitution masking: comment on Põder (2013)

Object-substitution masking (OSM) occurs when a target stimulus and a surrounding mask are displayed briefly together, and the display then continues with the mask alone. Target identification is accurate when the stimuli co-terminate but is progressively impaired as the duration of the trailing mask is increased. In reentrant accounts, OSM is said to arise from iterative exchanges between brain regions connected by two-way pathways. In an alternative account, OSM is explained on the basis of exclusively feed-forward processes, without recourse to reentry. Here I show that the feed-forward account runs afoul of the extant phenomenological, behavioral, brain-imaging, and electrophysiological evidence. Further, the feed-forward assumption that masking occurs when attention finds a degraded target is shown to be entirely ad hoc. In contrast, the evidence is uniformly consistent with a reentrant-processing account of OSM.

Substituting objects from consciousness: a review of object substitution masking

Object substitution masking (OSM) occurs when a sparse (e.g., four-dot), temporally trailing mask obscures the visibility of a briefly presented target. Here, we review theories of OSM: those that propose that OSM reflects the interplay between feedforward and feedback/reentrant neural processes, those that predict that feedforward processing alone gives rise to the phenomenon, and theories that focus on cognitive explanations, such as object updating. We discuss how each of these theories accommodates key findings from the OSM literature. In addition, we examine the relationship between OSM and other visual-cognitive phenomena, including object correspondence through occlusion, change blindness, metacontrast masking, backward masking, and visual short-term memory. Finally, we examine the level of processing at which OSM impairs target perception. Collectively, OSM appears to reflect the conditions under which the brain confuses two visual events for one when they are encoded with low spatiotemporal resolution, due to processing resources being otherwise occupied.

A novel paradigm reveals the role of reentrant visual processes in object substitution masking

Object substitution masking (OSM) occurs when an initial display of a target and mask continues with the mask alone, creating a mismatch between the reentrant hypothesis, triggered by the initial display, and the ongoing low-level activity. We tested the proposition that the critical factor in OSM is not whether the mask remains in view after target offset, but whether the representation of the mask is sufficiently stronger than that of the target when the reentrant signal arrives. In Experiment 1, a variable interstimulus interval (ISI) was inserted between the initial display and the mask alone. The trailing mask was presumed to selectively boost the strength of the mask representation relative to that of the target. As predicted, OSM occurred at intermediate ISIs, at which the mask was presented before the arrival of the reentrant signal, creating a mismatch, but not at long ISIs, at which a comparison between the reentrant signal and the low-level activity had already been made. Experiment 2, conducted in dark-adapted viewing, ruled out the possibility that low-level inhibitory contour interactions (metacontrast masking) had played a significant role in Experiment 1. Metacontrast masking was further ruled out in Experiment 3, in which the masking contours were reduced to four small dots. We concluded that OSM does not depend on extended presentation of the mask alone, but on a mismatch between the reentrant signals and the ongoing activity at the lower level. The present results place constraints on estimates of the timing of reentrant signals involved in OSM.

Reduced Temporal Fusion in Near-Hand Space

Object-substitution masking (OSM) is thought to reflect a failure of object individuation. That is, a briefly presented target surrounded by four dots is perceptually fused with the four-dot mask when the mask is visible after the target has disappeared, thereby obscuring the visibility of the target. If OSM depends on the inability to temporally segregate objects, then increasing the temporal precision of the visual system should reduce OSM. In the study reported here, we manipulated temporal precision by varying the proximity of participants’ hands to visual stimuli, because stimuli in near-hand space have been found to enjoy enhanced attentional processing, and attention is known to speed visual processing. Hand placement was indeed found to affect OSM: Placing participants’ hands near the visual stimuli reduced the magnitude of the masking. This finding demonstrates that object individuation can be facilitated by increasing the temporal resolution of vision via increasing the proximity of visual stimuli to the hands.

Nonretinotopic processing is related to postdictive size modulation in apparent motion

The present study briefly examined how the perceived size of a leading flash would be modulated by trailing motion signals. Observers were presented with two vertical green bars that were followed by white bars with different lengths, which were presented at different locations from the green bars. The task of observers was to discriminate which of the green bars was shorter than the other (Experiment 1) or whether the lengths of the green bars were equal or not (Experiments 2 and 3). One green bar producing apparent motion with the following shorter white bar was reported to be shorter than the other green bar producing apparent motion with the following longer white bar, not only when motion correspondence was determined on the basis of retinal proximity (Experiments 1 and 2),but also when motion correspondence was determined on the basis of nonretinotopic information-that is, a relative location within each perceptual group of bars (Experiment 3). These results indicate that motion processing involving object updating or motion deblurring in the nonretinotopic frame of reference is related to postdictive size modulation.

The effect of spatial competition between object-level representations of target and mask on object substitution masking

One of the processes determining object substitution masking (OSM) is thought to be the spatial competition between independent object file representations of the target and mask (e.g., Kahan & Lichtman, 2006). In a series of experiments, we further examined how OSM is influenced by this spatial competition by manipulating the overlap between the surfaces created by the modal completion of the target (an outline square with a gap in one of its sides) and the mask (a four-dot mask). The results of these experiments demonstrate that increasing the spatial overlap between the surfaces of the target and mask increases OSM. Importantly, this effect is not caused by the mask interfering with the processing of the target features it overlaps. Overall, the data indicate, consistent with Kahan and Lichtman, that OSM can arise through competition between independent target and mask representations.

Lateral masking in cycling displays: the relative importance of separation, flanker duration, and interstimulus interval for object-mediated updating

A central bar repeatedly presented in alternation with two flanking bars can lead to the disappearance of the central bar. Recently it has been suggested that this masking effect could be explained by object-mediated updating: the information from the central bar is integrated into the representation of the flankers, leading not only to the disappearance of the central bar as a separate object, but also to the perception of the flankers in apparent motion between their real position and the position of the central bar. This account suggests that the visibility of the central bar should depend on the same factors as those that influence the construction and maintenance of object representations. Therefore separation between central bar and flankers should not influence visibility as long as the time interval between them is adequate to make an interpretation of the scene in terms of one object moving from one location to the other possible location. We found that if the time interval between the central bar and the flankers is neither too short nor too long, the central bar becomes invisible even at large separations. These findings are inconsistent with traditional accounts of the cycling lateral masking displays in terms of local inhibitory mechanisms.

Misspecifications of stimulus presentation durations in experimental psychology: a systematic review of the psychophysics literature

BACKGROUND

In visual psychophysics, precise display timing, particularly for brief stimulus presentations, is often required. The aim of this study was to systematically review the commonly applied methods for the computation of stimulus durations in psychophysical experiments and to contrast them with the true luminance signals of stimuli on computer displays.

METHODOLOGY/PRINCIPAL FINDINGS

In a first step, we systematically scanned the citation index Web of Science for studies with experiments with stimulus presentations for brief durations. Articles which appeared between 2003 and 2009 in three different journals were taken into account if they contained experiments with stimuli presented for less than 50 milliseconds. The 79 articles that matched these criteria were reviewed for their method of calculating stimulus durations. For those 75 studies where the method was either given or could be inferred, stimulus durations were calculated by the sum of frames (SOF) method. In a second step, we describe the luminance signal properties of the two monitor technologies which were used in the reviewed studies, namely cathode ray tube (CRT) and liquid crystal display (LCD) monitors. We show that SOF is inappropriate for brief stimulus presentations on both of these technologies. In extreme cases, SOF specifications and true stimulus durations are even unrelated. Furthermore, the luminance signals of the two monitor technologies are so fundamentally different that the duration of briefly presented stimuli cannot be calculated by a single method for both technologies. Statistics over stimulus durations given in the reviewed studies are discussed with respect to different duration calculation methods.

CONCLUSIONS/SIGNIFICANCE

The SOF method for duration specification which was clearly dominating in the reviewed studies leads to serious misspecifications particularly for brief stimulus presentations. We strongly discourage its use for brief stimulus presentations on CRT and LCD monitors.

Object-substitution masking modulates spatial attention deployment and the encoding of information in visual short-term memory: insights from occipito-parietal ERP components

If object-substitution masking (OSM) arises from mask representations replacing target representations, OSM should impede the formation of representations in visual short-term memory (VSTM). We utilized event-related potentials to examine the effect of OSM on target processing. An N2pc was observed on trials with delayed-offset masks, indicating that focused attention was directed to the target. The sustained posterior contralateral negativity (SPCN), an index of VSTM storage, was observed in delayed-offset trials only on trials with correct responses. This supports the hypothesis that inaccurate performance on delayed-offset trials arises from a failure to encode the target in VSTM. On co-termination trials, accuracy was high and neither the N2pc nor SPCN was observed. This indicates that, in the absence of masking, the task was accomplished by maintaining a diffuse attentional state that enabled the joint encoding of the potential target items.

Unmasking the standing wave of invisibility: an account in terms of object-mediated representational updating

A central bar presented in counterphase with two flanking bars creates the perception of only two bars, instead of three, flickering (standing wave of invisibility illusion). Current explanations of this illusion highlight the importance of local interactions between the central bar and the flankers as a reason for the invisibility of the central bar. In three experiments, we show that the reduction in visibility of the central bar occurs even when the flankers are spatially separated from the central bar. Thus local mechanisms-low-level lateral inhibition or border-ownership competition-do not suffice to account for the decreased visibility. Furthermore, the reduced visibility of the central bar is accompanied by the perception of the flankers in apparent motion at all separations. We suggest an account of the standing wave phenomenon in terms of object updating: The representation of the central bar is updated with the representation of the flankers leaving the perception of just the flankers moving across space. The stimuli for the key conditions from this study, in QuickTime format, may be down loaded from http://app.psychonomic-journals.org/content/supplemental.

Implicit perception and level of processing in object-substitution masking

Object-substitution masking (OSM) refers to reduced target discrimination when the target is surrounded by a sparse mask that does not overlap with the target in space but trails it in time. In four experiments, we used a novel paradigm to investigate the extent of processing of a masked target in OSM. We measured response-compatibility effects between target and mask, both when the offsets were simultaneous and when the offset of the mask was delayed. Evidence for both OSM and a dissociation between perception and awareness was found when detecting the match between the target and the mask required feature but not categorical analyses. Our results suggest that the locus of disruption in OSM is likely to be beyond feature analysis of the unreported target.

Target Visibility in the Standing Wave Illusion: Is Mask—Target Shape Similarity Important?

The perceptibility of a flickering central bar can be dramatically reduced by the presence of two flanking bars presented in counterphase. This phenomenon, known as the ‘standing wave illusion’, has been suggested to involve local edge interactions (Macknik et al, 2000 Proceedings of the National Academy of Sciences of the USA97 7556–7560). High-level re-entrant mechanisms have also been implicated. Enns (2002, Psychonomic Bulletin & Review9 489–496) reports an association between the reported viability of the centre bar and its similarity in shape with the flanking bars. We find that this relationship between shape similarity and reported visibility seems to be contingent on the degree of experienced apparent motion. When target duration is shortened, so reducing apparent motion, reports of visibility reflect the amount of abutting contour. In a further experiment we find that luminance discriminations of the centre bar are related to the amount of abutting contour not to shape similarity. This is despite experiment 3 being conducted at stimulus durations for which experiment 2 visibility ratings indicated that shape similarity is important and contour is not. We suggest that this perceived motion may be the factor mediating shape ‘effects’ in the reported visibility task. We propose that the absence of such shape effects in the discrimination task may be because the task provides an objective measure of visibility that is immune to bias from perceived motion. We also speculate that while target luminance information may be immune to masking resulting from perceived motion, it may be subject to masking due to lateral inhibition.

The translation of cognitive paradigms for patient research

Many cognitive tasks have been developed by basic scientists to isolate and measure specific cognitive processes in healthy young adults, and these tasks have the potential to provide important information about cognitive dysfunction in psychiatric disorders, both in psychopathology research and in clinical trials. However, several practical and conceptual challenges arise in translating these tasks for patient research. Here we outline a paradigm development strategy--which involves iteratively testing modifications of the tasks in college students, in older healthy adults, and in patients--that we have used to successfully translate a large number of cognitive tasks for use in schizophrenia patients. This strategy makes it possible to make the tasks patient friendly while maintaining their cognitive precision. We also outline several measurement issues that arise in these tasks, including differences in baseline performance levels and speed-accuracy trade-offs, and we provide suggestions for addressing these issues. Finally, we present examples of 2 experiments, one of which exemplifies our recommendations regarding measurement issues and was a success and one of which was a painful but informative failure.

No representation without awareness in the lateral occipital cortex

Recent research has shown that four small dots presented in the vicinity of, but not adjacent to, a target stimulus can banish that stimulus from conscious awareness. It is thought that the mental representation of the masked stimulus is "erased" by the trailing quartet of dots. Using functional magnetic resonance adaptation, we show that there is no persisting neural representation of the successfully masked stimulus in lateral occipital cortex, a region that has been implicated in the processing of object structure. This finding rules out the alternative interpretation that a lingering neural representation is merely rendered inaccessible to consciousness, as is the fate, for example, of monocular information under conditions of binocular rivalry.

Testing models of object substitution with backward masking

A briefly presented visual target stimulus can be difficult to identify if shown in the context of a mask stimulus. There are several quantitative models that have been used to explain many different masking effects. Here, we look at modeling what has been called object substitution. We analyze four models and show that three of the models work with a common principle, whereas the fourth behaves quite differently. The three similar models have previously been used to explain many other aspects of visual masking, whereas the fourth model was designed exclusively to deal with object substitution masking effects. We identify an experimental test on the basis of which to choose between the models. The experimental results support the behavior of the fourth model.

Attention attenuates metacontrast masking

The influence of attention on perceptual awareness was examined using metacontrast masking. Attention was manipulated with endogenous cues to assess the effects on the temporal and spatial parameters of target visibility. Experiment 1 examined the time course of effective masking when the target and mask set were presented at an attended vs. an unattended location. The valid allocation of attention decreased the magnitude of the masking effect (i.e. increased visibility) for approximately 80 ms. Furthermore, even with spatial displacements of the target and mask and center-to-center separations of 1.5 degrees or 2.7 degrees of visual angle (Experiment 2), target visibility was increased when attention was validly allocated. These results indicate that attention influences low-level visual processes to enhance visual awareness.

Object substitution masking interferes with semantic processing: evidence from event-related potentials

Object substitution masking (OSM) refers to impaired target identification caused by common onset, but delayed offset, of a surrounding dot mask. This effect has been hypothesized to reflect reentrant processes that result in the mask replacing the target representation. However, little is known about the depth of processing associated with masked targets in this paradigm. We investigated this issue by examining the effect of OSM on the N400 component of the event-related potential, which reflects the degree of semantic mismatch between a target and its context. Participants read a context word followed by a semantically related or unrelated target word surrounded by dots. As expected, delayed dot offset significantly reduced accuracy in identifying the target. The N400 amplitude was also diminished by OSM. These findings offer the first evidence that substitution interferes with target processing prior to semantic analysis, demonstrating an important difference between OSM and other visual phenomena, such as the attentional blink, in which semantic processing is independent of awareness.

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.

Looking at object-substitution masking in depth and motion: toward a two-object theory of object substitution

When attention is divided, a briefly presented target surrounded by four small dots is difficult to identify when the dots persist beyond target offset, but not when these dots terminate with the target. This object-substitution masking effect likely reflects processes at both the image level and the object level. At the image level, visual contours of the mask make feature extraction difficult. Recent data (Lleras & Moore, 2003) suggest that, at the object level, an object file is created for the target-plus-mask, and this single-object token later morphs into a single-object token containing the mask alone. In the present experiments, we used stimuli presented in 3-D space and apparent motion; the results indicate that object-substitution masking also arises when the mask and the target are represented in two separate object tokens and the mask token interferes with the target token.

On the role of object representations in substitution masking

Three experiments were conducted to investigate the role of object representations in object substitution masking (OSM). OSM occurs when a very sparse mask is presented simultaneously with a target stimulus and the target offsets first, leaving the mask to linger in the display for some time. Results confirm earlier claims that there is an isolatable object-level component to OSM and indicate that a target can be protected from OSM if, prior to offsetting, it can be represented as a distinct object from the mask. When the mask was presented as sliding past the target (Experiment 1), as jiggling independently of the target (Experiment 2), or in a different color from the target (Experiment 3), OSM was reduced or eliminated. This suggests that OSM reflects basic updating processes that allow the perception of continuity of object identity over change.

Rapid Resumption of Interrupted Visual Search: New Insights on the Interaction Between Vision and Memory

A modified visual search task demonstrates that humans are very good at resuming a search after it has been momentarily interrupted. This is shown by exceptionally rapid response time to a display that reappears after a brief interruption, even when an entirely different visual display is seen during the interruption and two different visual searches are performed simultaneously. This rapid resumption depends on the stability of the visual scene and is not due to display or response anticipations. These results are consistent with the existence of an iterative hypothesis-testing mechanism that compares information stored in short-term memory (the perceptual hypothesis) with information about the display (the sensory pattern). In this view, rapid resumption occurs because a hypothesis based on a previous glance of the scene can be tested very rapidly in a subsequent glance, given that the initial hypothesis-generation step has already been performed.

Involvement of V5/MT+ in object substitution masking: evidence from repetitive transcranial magnetic stimulation

The visibility of a briefly presented target can be reduced by a subsequent weak mask that does not touch it, when the target is encoded in low spatiotemporal resolution. This phenomenon, called object substitution masking, has recently been proposed to reflect information updating in object-level representation, with perception of the target and the mask belonging to a single object through apparent motion. We investigated this issue by applying repetitive transcranial magnetic stimulation over V5/MT+, specialized in visual motion processing. The transient functional disruption of V5/MT+ produced by repetitive transcranial magnetic stimulation attenuated object substitution masking, while sham stimulation did not. Our results suggest that object substitution masking is mediated by normal functioning of V5/MT+. We conclude that repetitive transcranial magnetic stimulation of V5/MT+ impaired perceived object continuity and reduced object substitution masking accordingly.

Negative compatibility or object updating? A cautionary tale of mask-dependent priming

The negative compatibility effect (NCE) is the surprising result that visual targets that follow a brief prime stimulus and a mask can be identified more rapidly when they are opposite rather than identical to the prime. In a recent article in this journal, S. T. Klapp and L. B. Hinkley (2002) proposed that this reflected a competition between inhibitory unconscious processes and excitatory conscious processes. The authors of the current article report 7 experiments with results countering this theory and propose an alternative account within the framework of object substitution masking. In this account, the NCE reflects the updating of perceptual objects, including their links to responses closely associated with those objects.