Disruption of early face recognition processes by object substitution masking

Visual stimuli can fail to reach conscious awareness when surrounded by dots sharing a common onset but remaining visible after target removal, a phenomenon known as object substitution masking (OSM). We assessed the locus of substitution by recording the N170 component of the human event-related potential during a face/house classification task. Delayed-offset dots impaired target categorization, confirming the generality of object substitution masking by extending the effect to realistic, complex stimuli. Furthermore, object substitution masking eliminated the N170 amplitude difference between faces and houses, providing the first neural evidence that object substitution masking disrupts structural encoding stages during object recognition.     

Different effects of the two types of spatial pre-cueing: what precisely is “attention” in Di Lollo’s and Enns’ substitution masking theory?

Enns and Di Lollo [Psychological Science, 8 (2), 135-139, 1997] have introduced the object substitution theory of visual masking. Object substitution masking occurs when focusing attention on the target is delayed. However, Posner (Quarterly Journal of Experimental Psychology, 32, 3-25, 1980) has already shown that attention can be directed to a target at least in two ways: intentionally (endogenously) and automatically (exogenously). We conducted two experiments to explore the effects of endogenous and exogenous cues on substitution masking. The results showed that when attention was shifted to the target location automatically (using a local peripheral pre-cue), masking was attenuated. A decrease in target identification dependent on a delay of mask offset, typical to substitution masking, was not observed. However, strong substitution masking occurred when the target location was not pre-cued or when attention was directed to the target location intentionally (using a symbolic pre-cue displayed centrally). The hypothesis of two different mechanisms of attentional control in substitution masking was confirmed.     

When the target becomes the mask: using apparent motion to isolate the object-level component of object substitution masking

J. T. Enns and V. Di Lollo (1997) discovered a new form of visual masking that they labeled object substitution masking (OSM). OSM occurs when 4 dots, presented around a target, trail in the display after target offset. The present study showed that the physical presence of the masking dots after target offset is not necessary for OSM. Instead, the continued presence of a changing high-level representation associated with the target suffices to yield OSM. Apparent motion was used to define such representation. In these experiments, the initial display offset and was followed by a 2nd display where masks appeared at new locations. Only when the spatiotemporal properties of the stimuli on the 2nd display supported the perception of the target moving and turning into the mask was OSM observed.     

Warning: Attending to a mask may be hazardous to your perception

Object substitution is a type of backward masking that occurs when a mask appears during visual search for a target. We tested the hypothesis that object substitution is an overwriting process triggered by attentional selection of the mask. Impeding attentional selection of a mask by embedding it in an array of distractors eliminated object substitution. Similarly, object substitution did not occur when the mask appeared in advance of the target and, therefore, could not capture attention during search for the target. However, masking was reinstated when the mask was revealed from background contours at the moment of target onset and could therefore capture attention during search. These observations demonstrate that attentional selection of the mask is a necessary step in this type of masking and suggest that object substitution is active overwriting of unattended information triggered by selection of other visual information at a nearby location.     

Feature integration and spatial attention: common processes for endogenous and exogenous orienting

Briand (J Exp Psychol Hum Percept Perform 24:1243–1256, 1998) and Briand and Klein (J Exp Psychol Hum Percept Perform 13:228–241, 1987) demonstrated that spatial cueing effects are larger for detecting conjunction of features than for detecting simple features when spatial attention is oriented exogenously, and not when attention is oriented endogenously. Their results were interpreted as if only exogenous attention affects the posterior spatial attention system that performs the feature binding function attributed to spatial attention by Treisman’s feature integration theory (FIT; 1980). In a series of 6 experiments, we attempted to replicate Briand’s findings. Manipulations of distractor string size and symmetry of stimulus presentation left and right from fixation were implemented in Posner’s cueing paradigm. The data indicate that both exogenous and endogenous cueing address the same attentional mechanism needed for feature binding. The results also limit the generalisability of Briand’s proposal concerning the role of exogenous attention in feature integration. Furthermore, the importance to control the effect of unintended attentional capture in a cueing task is demonstrated.     

Breaking Surface Feature Continuity of Previewed Mask Reinstates Object Substitution Masking12

Perception of a briefly presented target is impaired when a sparse surrounding mask (e.g., four-dot) persists after target offset compared to when the target and mask offset together (i.e., object substitution masking [OSM]). Previous studies have reported a mask preview effect in which OSM is largely attenuated by prior presentation of the mask. Here, we investigated how breaking object continuity of the previewed mask affects the mask preview effect. Introducing an abrupt surface color change of the previewed mask at target onset disrupted the beneficial effect of mask preview, that is, OSM was reinstated (Experiment 1). The masking induced by mask color change exhibited the same two characteristics as conventional OSM: target location specificity and non-necessity of voluntary attention to the mask (Experiment 2). These results suggest that a sudden change in surface color breaks object continuity of the previewed mask and causes the mask to be represented as a new one, which triggers OSM anew.     

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.     

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.     

Object substitution masking and its relationship with visual crowding

Object substitution masking (OSM) occurs when the perceptibility of a brief target is reduced by a trailing surround mask typically composed of four dots. Camp et al. (Journal of Experimental Psychology: Human Perception and Performance, 41, 940–957, 2015) found that crowding a target by adding adjacent flankers, in addition to OSM, had a more deleterious effect on performance than expected based on the combined individual effects of crowding and masking alone. The current experiments test why OSM and crowding interact in this way. In three experiments, target-flanker distance is manipulated whilst also varying mask duration in a digit identification task. The OSM effect—as indexed by the performance difference between unmasked and masked conditions—had a quadratic function with respect to target-flanker distance. Results suggest it is OSM affecting crowding rather than the converse: Masking seems to amplify crowding at intermediate target-distractor distances at the edge of the crowding interference zone. These results indicate that OSM and crowding share common mechanisms. The effect of OSM is possibly a consequence of changes to the types of feature detectors which are pooled together for target identification when that target must compete for processing with a trailing mask in addition to competition from adjacent flankers.     

Delayed offset of distracters masks a local target

Object substitution masking (OSM) is observed when a brief target surrounded with a mask is presented among distracter stimuli and cannot be identified when it and the distracters disappear but the mask remains in view. We probed whether OSM also occurs without a local mask object when the distracters remain after target offset. We also varied the congruence between the local target and the global search display and the grouping properties of the delayed offset distracters. A target was briefly presented in a global object configuration of distracters that had delayed offset. Results showed that OSM could be observed with delayed offset of distracters grouped into a global mask shape. Congruence of the shapes of the global and local objects did not affect OSM, suggesting that a generalized abstract visual pattern representation of the global object may not be involved in OSM nor did the grouping properties of the delayed offset distracters influence OSM.     

Size (mostly) doesn’t matter: the role of set size in object substitution masking

Conscious detection and discrimination of a visual target stimulus can be prevented by the presentation a spatially nonoverlapping, but temporally trailing, visual masking stimulus. This phenomenon, known as object substitution masking (OSM), has long been associated with spatial attention, with diffuse attention seemingly being key for the effect to be observed. Recently, this hypothesis has been questioned. We sought to provide a definitive test of the involvement of spatial attention in OSM by using an eight-alternative forced choice task under a range of mask durations, set sizes, and target/distractor spatial configurations. The results provide very little evidence that set size, and thus the distribution of spatial attention, interacts with masking magnitude. These findings have implications for understanding the mechanisms underlying OSM and the relationship between consciousness and attention.     

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.     

The role of executive attention in object substitution masking

It was long thought that a key characteristic of object substitution masking (OSM) was the requirement for spatial attention to be dispersed for the mask to impact visual sensitivity. However, recent studies have provided evidence questioning whether spatial attention interacts with OSM magnitude, suggesting that the previous reports reflect the impact of performance being at ceiling for the low attention load conditions. Another technique that has been employed to modulate attention in OSM paradigms involves presenting the target stimulus foveally, but with another demanding task shown immediately prior, and thus taxing executive/temporal attention. Under such conditions, when the two tasks occur in close temporal proximity relatively to greater temporal separation, masking is increased. However this effect could also be influenced by performance being at ceiling in some conditions. Here, we manipulated executive attention for a foveated target using a dual-task paradigm. Critically, ceiling performance was avoided by thresholding the target stimulus prior to it being presented under OSM conditions. We found no evidence for an interaction between executive attention load and masking. Collectively, along with the previous findings, our results provide compelling evidence that OSM as a phenomenon occurs independently of attention.     

Colored photisms prevent object-substitution masking in digit-color synesthesia

For C, a digit-color synesthete, viewing a digit elicits a photism (an experience of a highly specific color), which is perceived as externally projected onto the digit. We used an object substitution paradigm to demonstrate the influence of C's photisms on her perception of digits. Object substitution refers to a form of masking that depends upon two critical factors: attention must be taxed (e.g., distributed among numerous distracters), and the mask must remain on-screen for a sufficient duration after the target has been removed. Results showed that for C, even under conditions that produced maximal object substitution in control participants, her endogenous coloring of target digits prevented object substitution. Thus, the present study clearly showed that C's photisms influence her ability to detect digits.     

Implicit semantic perception in object substitution masking

Decades of research on visual perception has uncovered many phenomena, such as binocular rivalry, backward masking, and the attentional blink, that reflect ‘failures of consciousness’. Although stimuli do not reach awareness in these paradigms, there is evidence that they nevertheless undergo semantic processing. Object substitution masking (OSM), however, appears to be the exception to this rule. In OSM, a temporally-trailing four-dot mask interferes with target perception, even though it has different contours from and does not spatially overlap with the target. Previous research suggests that OSM has an early locus, blocking the extraction of semantic information. Here, we refute this claim, showing implicit semantic perception in OSM using a target-mask priming paradigm. We conclude that semantic information suppressed via OSM can nevertheless guide behavior.     

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.     

Semantic priming occurs for word but not location pronunciation in the postcue task

Semantic priming refers to the finding that a word response is facilitated if it is preceded by a related word compared to when it is preceded by an unrelated word. Dallas and Merikle (Can J Psychol 30: 15–21 1976a; Bull Psychon Soc 8: 441–444 1976b) demonstrated that semantic priming occurred under conditions in which a pair of simultaneously displayed words was previewed for over a second prior to the onset of a cue indicating which of the words should be pronounced aloud (postcue task). In contrast, semantic interference effects have been reported for postcue picture-naming tasks (Dean et al. in J Exp Psychol Learn Mem Cogn 27: 733–743, 2001; Humphreys et al. in J Exp Psychol Learn Mem Cogn 21: 961–980, 1995). According to Dean et al., the semantic interference effects in postcue picture naming occur because the integration of the object and the cued attribute in memory is more difficult for categorically related pictures than for unrelated pictures. The aim of this experiment was to determine whether this idea was true for postcue word pronunciation tasks. Participants completed two postcue tasks, one requiring pronunciation of the target word indicated by a locational cue and another requiring pronunciation of the location of a centrally presented word. Results indicated a semantic priming effect only for the locational cue condition suggesting that the integration of the cue and identity information was unaffected by word context. These data suggest that priming in a postcue word pronunciation task may be due to feedback from residual activation within the semantic system facilitating access to the target word’s phonology.     

What's new in visual masking?

A brief display that is clearly visible when shown alone can be rendered invisible by the subsequent presentation of a second visual stimulus. Several recently described backward masking effects are not predicted by current theories of visual masking, including masking by four small dots that surround (but do not touch) a target object and masking by a surrounding object that remains on display after the target object has been turned off. A crucial factor in both of these effects is attention: almost no masking occurs if attention can be rapidly focused on the target, whereas powerful masking ensues if attention directed at the target is delayed. A new theory of visual masking, inspired by developments in neuroscience, can account for these effects, as well as more traditional masking effects. In addition, the new theory sheds light on related research, such as the attentional blink, inattentional blindness and change blindness.     

Exogenous spatial precuing reliably modulates object processing but not object substitution masking

Object substitution masking (OSM) is used in behavioral and imaging studies to investigate processes associated with the formation of a conscious percept. Reportedly, OSM occurs only when visual attention is diffusely spread over a search display or focused away from the target location. Indeed, the presumed role of spatial attention is central to theoretical accounts of OSM and of visual processing more generally (Di Lollo, Enns, & Rensink, Journal of Experimental Psychology: General 129:481–507, 2000). We report a series of five experiments in which valid spatial precuing is shown to enhance the ability of participants to accurately report a target but, in most cases, without affecting OSM. In only one experiment (Experiment 5) was a significant effect of precuing observed on masking. This is in contrast to the reliable effect shown across all five experiments in which precuing improved overall performance. The results are convergent with recent findings from Argyropoulos, Gellatly, and Pilling (Journal of Experimental Psychology: Human Perception and Performance 39:646–661, 2013), which show that OSM is independent of the number of distractor items in a display. Our results demonstrate that OSM can operate independently of focal attention. Previous claims of the strong interrelationship between OSM and spatial attention are likely to have arisen from ceiling or floor artifacts that restricted measurable performance.     

Masking by object substitution: dissociation of masking and cuing effects

In a newly discovered form of visual masking, a target stimulus is masked by 4 flanking dots if their offset is delayed relative to the target (V. Di Lollo, J. T. Enns, & R. A. Rensink, 2000). In Di Lollo et al. (2000), the dot pattern also cued the relevant target and therefore required deliberate attention. In the present Experiments 2-6, a central arrow cued 1 of 2 letters for an E/F discrimination, with dots flanking both letters. Masking was reduced compared with the mask-cue procedure but was still robust. Delayed-offset dots flanking the nontarget also impaired performance, indicating competition for attention. Masking was unaffected by brightness of the dots relative to the target. Masking was attenuated not only by precuing attention to the target location but also by preview of an uninformative dot mask. Theories of masking by object substitution must therefore accommodate the prior context into which the target stimulus is introduced.