Individually different weighting of multiple processes underlies effects of metacontrast masking

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.     

Metacontrast masking is processed before grapheme–color synesthesia

We investigated the physiological mechanism of grapheme–color synesthesia using metacontrast masking. A metacontrast target is rendered invisible by a mask that is delayed by about 60 ms; the target and mask do not overlap in space or time. Little masking occurs, however, if the target and mask are simultaneous. This effect must be cortical, because it can be obtained dichoptically. To compare the data for synesthetes and controls, we developed a metacontrast design in which nonsynesthete controls showed weaker dichromatic masking (i.e., the target and mask were in different colors) than monochromatic masking. We accomplished this with an equiluminant target, mask, and background for each observer. If synesthetic color affected metacontrast, synesthetes should show monochromatic masking more similar to the weak dichromatic masking among controls, because synesthetes could add their synesthetic color to the monochromatic condition. The target–mask pairs used for each synesthete were graphemes that elicited strong synesthetic colors. We found stronger monochromatic than dichromatic U-shaped metacontrast for both synesthetes and controls, with optimal masking at an asynchrony of 66 ms. The difference in performance between the monochromatic and dichromatic conditions in the synesthetes indicates that synesthesia occurs at a later processing stage than does metacontrast masking.     

Metacontrast suppression and criterion content: A discriminant function analysis

Three test and three mask energies of a metacontrast display were varied orthogonally and randomly over trials. The stimulus onset asynchrony (SOA) separating them was varied over blocks of trials from 0 to 180 msec in 30-msec steps. Both the accuracy in judging the test and the coherence (consistency) of the judgments were U-shaped functions of SOA. Thus, metacontrast suppression is in part due to inadequate information. In addition, mask energy was found to correlate negatively with judgments of the test at short SO As but positively at longer SOAs. This indicates that part of the masking effect is due to inappropriate use of information. Certain similarities were noted between these findings and those obtained with judgments of frequency in the auditory-recognition masking paradigm. In general, the results indicate that subjects respond to different features of the stimulus situation as SOA varies.     

Quantitative theories of metacontrast masking

In metacontrast masking, the effect of a visual mask stimulus on the perceptual strength of a target stimulus varies with the stimulus-onset asynchrony (SOA) between them. As SOA increases, the target percept first becomes weaker, bottoms out at an intermediate SOA, and then increases for still larger SOAs. As a result, a plot of target percept strength against SOA produces a U-shaped masking curve. Theories have proposed special mechanisms to account for this curve, but new mathematical analyses indicate that it is a robust characteristic of a large class of neurally plausible systems. The author describes 3 quantitative methods of accounting for the U-shaped masking effect and analyzes 4 previously published mathematical models of masking. The models produce the masking curve through mask blocking, whereby a strong internal representation of the target blocks the mask's effects.     

Apparent movement and metacontrast suppression: A decisional analysis

Three test and three mask energies were varied orthogonally and randomly over trials. The stimulus onset asynchrony (ISOA) separating test and mask was varied between trial blocks within each of two display conditions, apparent movement (two-object) and metacontrast (threeobject). Subjects were required to makebrightness judgments of both test and mask energies by responding “bright,” “medium,” or “dim” with respect to the apparent intensity of each stimulus. The accuracy and the coherence lconsistencyt of test judgments were U-shaped functions of SOA for both apparent movement and metacontrast situations. However, the accuracy and the coherence of mask judgments did not vary with SOA for either apparent movement or metacontrast. It was noted that substantially the same results have been reported previously when subjects were required to makecontour judgments. Hence, it is argued that apparent movement and metacontrast suppression are intimately related.     

OBJECT SUBSTITUTION:

Abstract —Can four dots that surround, but do not touch, a target shape act as a mask to reduce target discriminability? Although existing theories of metacontrast and pattern masking say "no." we report this occurs when targets appear in unpredictable locations. In three experiments, a four-dot mask was compared with a standard metacontrast mask that surrounded the target. Although accuracy was predictably different for the two masks at a central display location in Experiment I. both masks had similar strong effects on accuracy in parafoveal locations. Experiment 2 revealed that both four-dot and metacontrast masking were insensitive to contour proximity in parafoveal display locations, and Experiment 3 showed that four-dot masking could occur even at a central location if attention was distributed among several targets. We propose that targets in unattended locations are coded with low spotiotemporal resolution, leaving them vulnerable to substitution by the four dots when attention is directed to them.     

Brightness interactions between rods and cones

Two parafoveal test targets with different spectral compositions were matched in brightness to a fixed-luminance foveal reference target under scotopic adaptation conditions. The idea of the experiment was to find a reference luminance for which one of the matching test targets stimulated only rods while the other stimulated both rods and cones. If brightness was proportional to the linear sum of rod and cone responses, then the luminance of the matching rod+cone target would be predictably closer to rod threshold than would that of the rod target. The results were complicated by evidence that rod responses to the test targets selectively enhanced weak chromatic signals. Nevertheless, it was possible to show that cone activity never reduced the matching luminance as much as predicted by the additivity hypothesis, and sometimes even increased it. These findings suggest that cone activity can suppress brightness signals from rods.     

Target-mask shape congruence impacts the type of metacontrast masking

Visual metacontrast masking may depend on the time intervals between target and mask in two qualitatively different ways: in type-A masking the smaller the mask delay from target the stronger the masking while in type-B masking maximal masking effect is obtained with a larger temporal delay of the mask. Variability in the qualitative apperance of masking functions has been explained by variability in stimuli parameters and tasks. Recent research on metacontrast masking has surprisingly shown that both of these types of functions can be found with an identical range of stimulation parameters depending on individual differences between observers. Here we show that obtaining clear-cut type-A masking depends on whether target and mask shapes are congruent or incongruent and whether observers use the cues available due to the congruence factor. Conspicuously expressed type-A masking is selectively associated with incongruent target-mask pairings. In the latter conditions target identification level significantly drops with the shortest target-to-mask delays.     

Temporal characteristics of visibility in chimpanzees (Pan troglodytes) and humans (Homo sapiens) assessed by a visual-masking paradigm

We used the visual-masking paradigm to compare temporal characteristics of chimpanzee vision with those of humans. Two types of masking experiments were conducted. One type involved masking by noise, in which the visibility of the geometric pattern target was tested with a spatially overlapping noise as the mask stimulus. The other type involved paracontrast and metacontrast masking, in which the mask stimuli flanked but did not spatially overlap the target stimuli. Temporal characteristics regarding the visibility of target stimuli, displayed as functions of temporal asynchrony between target and mask stimuli, differed with the mask type in chimpanzees as in humans. Peak deterioration in visibility occurred at the point of minimum temporal asynchrony both in forward and backward masking by noise, but was not at 0 ms temporal asynchrony when the target and mask stimuli did not spatially overlap. These results suggest that chimpanzees and humans share the underlying mechanisms in two kinds of temporal inhibition caused by spatially overlapping and non-overlapping mask stimuli.     

Visual backward masking as measured by voice reaction time

Instead of using percent correct identifications or detections as the dependent variable, latency in voicing the target stimulus was measured in a backward masking paradigm. Reaction time (RT) to target letters was reliably increased when they were simultaneously encircled by a black ring mask of a size found to produce masking using an identification or detection criterion. The masking function in terms of RT was typical in shape, a decreasing function of stimulus onset asynchrony (SOA) over an interval of 150 msec. Since the target remained “on” when the mask appeared, the results are incompatible with an erasure interpretation of masking effects. Analyses of the variances of the RTs supported an interpretation of a progressive decrease in masking effects as SOA increased.     

Summation versus suppression in metacontrast masking: On the potential pitfalls of using metacontrast masking to assess perceptual–motor dissociation

A briefly flashed target stimulus can become “invisible” when immediately followed by a mask—a phenomenon known as backward masking, which constitutes a major tool in the cognitive sciences. One form of backward masking is termed metacontrast masking. It is generally assumed that in metacontrast masking, the mask suppresses activity on which the conscious perception of the target relies. This assumption biases conclusions when masking is used as a tool—for example, to study the independence between perceptual detection and motor reaction. This is because other models can account for reduced perceptual performance without requiring suppression mechanisms. In this study, we used signal detection theory to test the suppression model against an alternative view of metacontrast masking, referred to as the summation model. This model claims that target- and mask-related activations fuse and that the difficulty in detecting the target results from the difficulty to discriminate this fused response from the response produced by the mask alone. Our data support this alternative view. This study is not a thorough investigation of metacontrast masking. Instead, we wanted to point out that when a different model is used to account for the reduced perceptual performance in metacontrast masking, there is no need to postulate a dissociation between perceptual and motor responses to account for the data. Metacontrast masking, as implemented in the Fehrer–Raab situation, therefore is not a valid method to assess perceptual–motor dissociations.     

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.     

A Comparison of Two Lateral Inhibitory Models of Metacontrast

Metacontrast, an apparent reduction in brightness of a target that is followed by a non-overlapping mask, has been modeled with simulated neural nets incorporating either recurrent lateral inhibition or forward and backward inhibition with lateral components. A one-layer lateral inhibitory model (B. Bridgeman, 1971, Psychological Review 78, 528-539) and a six-layer model (G. Francis, 1997, Psychological Review 104, 572-594) both simulate the basic metacontrast effect, showing that stimulus-dependent activity that reverberates for some time in the model after stimulus offset is essential to simulate metacontrast. The six-layer model does not simulate monotonic masking with low response criterion, an essential property of metacontrast; the lateral inhibitory model uses duration of reverberation to simulate the criterion. Each model simulates several variations of masking, such as changing the relative energy of target and mask, but neither can handle effects of practice or attention that apparently engage higher processing levels. Copyright 2001 Academic Press.     

W-shaped and U-shaped functions obtained for monoptic and dichoptic disk-disk masking

For nonmetacontrast (disk-disk) masking, if the outer diameter of the mask equals that of an annulus which produces typical metacontrast functions, U- and W-shaped functions will be obtained if (1) the stimuli are viewed dichoptically and monoptically, respectively. and (2) the mask is no more than about 10 times the energy (luminance x duration) of the target. As this energy ratio is approached, both types of functions become monotonic. These shapes suggest that interactions at different visual-pathway loci may he mapped by visual masking functions.     

Unconscious priming by color and form: different processes and levels

Using a metacontrast masking paradigm, prior studies have shown (a) that a target's color information and form information, can be processed without awareness and (b) that unconscious color processing occurs at early, wavelength-dependent levels in the cortical information processing hierarchy. Here we used a combination of paracontrast and metacontrast masking techniques to explore unconscious color and form priming effects produced by blue, green, and neutral stimuli. We found that color priming in normal observers is significantly reduced when an additional paracontrast mask precedes the target at optimal masking SOAs. However, no reduction of form-priming effects was obtained at similar optimal paracontrast SOAs. We conclude that unconscious color priming depends on an early, wavelength- or stimulus-dependent response of color neurons located at early cortical levels whereas unconscious form priming occurs at later levels.     

Decoupling stimulus duration from brightness in metacontrast masking: data and models

A brief target that is visible when displayed alone can be rendered invisible by a trailing stimulus (metacontrast masking). It has been difficult to determine the temporal dynamics of masking to date because increments in stimulus duration have been invariably confounded with apparent brightness (Bloch's law). In the research reported here, stimulus luminance was adjusted to maintain constant brightness across all durations. Increasing target duration yielded classical U-shaped masking functions, whereas increasing mask duration yielded monotonic decreasing functions. These results are compared with predictions from 6 theoretical models, with the lateral inhibition model providing the best overall fit. It is tentatively suggested that different underlying mechanisms may mediate the U-shaped and monotonic functions obtained with increasing durations of target and mask, respectively.     

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.     

Rapid enumeration within a fraction of a single glance: The role of visible persistence in object individuation capacity

The number of items that can be individuated at a single glance is limited. Here, we investigate object individuation at a higher temporal resolution, in fractions of a single glance. In two experiments involving object individuation we manipulated the duration of visual persistence of the target items with a forward masking procedure. The number of items as well as their stimulus–onset asynchrony (SOA) to the mask was varied independently. The results showed main effects of numerosity and SOA, as well as an interaction. These effects were not caused by a generic reduction of item visibility by the mask. Instead, the SOA manipulation appeared to fractionate the time to access the sensory image. These findings suggest that the capacity limit of 3–4 items found in object individuation is, at least partially, the consequence of the temporal window of access to sensory information.     

Backward masking by pattern stimulus offset

The backward masking effects of the offset of a pattern stimulus on the apparent contrast of a target stimulus were determined to be a function of target onset-mask offset asynchrony. With spatially overlapping stimuli and binocular viewing, a monotonic function similar to that characterizing early dark adaptation was obtained; with a dichoptically presented disk onset as target and a surrounding ring offset as mask, a typical U-shaped metacontrast effect as a function of target onset-mask offset asynchrony was obtained. These mask-offset effects are related to the possible roles of (a) peripheral "off" mechanisms and (b) central metacontrast mechanisms in terminating visual response persistence in sustained channels.     

On the labile memory buffer in the attentional blink: masking the T2 representation by onset transients mediates the AB

Report of a second target (T2) is impaired when presented within 500 ms of the first (T1). This attentional blink (AB) is known to cause a delay in T2 processing during which T2 must be stored in a labile memory buffer. We explored the buffer's characteristics using different types of masks after T2. These characteristics were inferred by determining what attributes of T2 are hindered by a given form of masking. In Experiments 1-3, trailing metacontrast and four-dot masks produced ABs of equal magnitudes, implicating the onset-transient triggered by the mask as the mechanism underlying the AB and strongly suggesting a locus in early vision. In Experiment 4, metacontrast and four-dot masks were presented in a common-onset masking (COM) paradigm in which a brief, combined display of T2 and the mask was followed by a longer display of the mask alone. COM is thought to occur late in the sequence of processing events. No AB occurred with COM, confirming the critical role of the mask's onset transients and ruling out a high-level locus for the labile memory buffer.