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.     

On the relationship between stimulus intensity and duration of visible persistence

Stimulus intensity and duration of visible persistence have been found to vary directly in some studies but inversely in others. Hawkins and Shulman (1979) have proposed that this inconsistency can be resolved by separating the studies that employed a decrement-threshold measure of persistence (Type I measure) from those that measured the total duration of persistence (Type II measure). They suggested that Type I measures yield an inverse relationship, whereas Type II measures yield a direct relationship between intensity and persistence. Hawkins and Shulman's model is incomplete in ways that are easily remedied. However, the model is totally contradicted by the experimental evidence. A new resolution of the inconsistent results is proposed in terms of retinal afterimages.     

Unconscious processing of color and form in metacontrast masking

Three experiments employed a metacontrast masking procedure to examine the extent and nature of priming effects from visual stimuli not consciously perceived. The results showed effects of unconscious stimuli on subsequent target responses that (1) were more consistent, reliable, and not subject to strategic control, as compared with consciously perceived stimuli (Experiment 1); (2) produced both facilitation and interference of subsequent processing (Experiment 2); and (3) did not influence indirect response-related levels of processing (Experiment 3). These results demonstrate that color and form attributes of unconscious stimuli are sufficiently registered within the visual system to influence behavior, and that some of these unconscious effects occur at early levels of stimulus encoding, prior to higher level perceptual and response-related processes.     

The electrophysiological correlates of stimulus visibility and metacontrast masking

There are conflicting views concerning the electrophysiological correlates of visual consciousness. Whereas one view considers a relatively late positive deflection (LP) as a primary correlate of consciousness, another model links consciousness with earlier negativity (VAN). The present experiment utilized metacontrast masking in investigating the electrophysiological correlates of visual consciousness. The participants were presented with target-mask sequences in three stimulus onset asynchronies. The target stimuli were followed by either a metacontrast mask or a similar-looking, but ineffective pseudomask. The results showed that the first deflection that correlated with target visibility was VAN which was followed by LP. We argue that the VAN is the primary correlate target visibility, while the LP reflects later, postperceptual processing stages.     

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.     

Interaction of stimulus size and retinal eccentricity in metacontrast masking

Metacontrast masking occurs both at the fovea and in the retinal periphery; foveally, the smallest stimulus elicited the strongest masking, whereas peripherally the reverse was the case. An analysis of variance showed a significant size effect, eccentricity effect, and size-eccentricity interaction. As stimulus size increased, the stimulus onset asynchrony of maximum masking shifted to greater values. Both foveal metacontrast and peak shifts contradicted predictions made by the hypothesis that metacontrast is mediated by an interaction of sustained and transient channels in the visual system. The data are consistent, however, with a lateral inhibitory model of metacontrast masking and stimulus coding.     

Duration of visible persistence in relation to stimulus complexity

Visible persistence refers to the phenomenal impression that a stimulus is still present after its offset. A dispute exists whether visible persistence is due to temporal sluggishness in the visual pathway (neural hypothesis) or whether it is a byproduct of information-extraction processes under cognitive control (process hypothesis). This was investigated by manipulating stimulus complexity in five temporal integration experiments and one recognition memory experiment. According to the process hypothesis, complex stimuli should persist longer than simple stimuli because they require more information extraction. This prediction was not confirmed; in all six experiments, complexity was found to have no reliable effect on the duration of visible persistence. By contrast, and in accordance with earlier findings, complexity was shown to have a significant effect on a short-lived, nonvisible form of memory known as schematic persistence. This pattern of results supports two major conclusions: First, that the effects of complexity reported in earlier research were probably on schematic—rather than visible—persistence; and second, that visible persistence must be regarded as a residual neural trace of an extinguished stimulus, rather than as a byproduct of information-extraction processes.     

Individual differences in metacontrast masking regarding sensitivity and response bias

In metacontrast masking target visibility is modulated by the time until a masking stimulus appears. The effect of this temporal delay differs across participants in such a way that individual human observers' performance shows distinguishable types of masking functions which remain largely unchanged for months. Here we examined whether individual differences in masking functions depend on different response criteria in addition to differences in discrimination sensitivity. To this end we reanalyzed previously published data and conducted a new experiment for further data analyses. Our analyses demonstrate that a distinction of masking functions based on the type of masking stimulus is superior to a distinction based on the target-mask congruency. Individually different masking functions are based on individual differences in discrimination sensitivities and in response criteria. Results suggest that individual differences in metacontrast masking result from individually different criterion contents.     

Duration of visible persistence in relation to stimulus complexity.

Visible persistence refers to the phenomenal impression that a stimulus is still present after its offset. A dispute exists whether visible persistence is due to temporal sluggishness in the visual pathway (neural hypothesis) or whether it is a byproduct of information-extraction processes under cognitive control (process hypothesis). This was investigated by manipulating stimulus complexity in five temporal integration experiments and one recognition memory experiment. According to the process hypothesis, complex stimuli should persist longer than simple stimuli because they require more information extraction. This prediction was not confirmed; in all six experiments, complexity was found to have no reliable effect on the duration of visible persistence. By contrast, and in accordance with earlier findings, complexity was shown to have a significant effect on a short-lived, nonvisible form of memory known as schematic persistenc. This pattern of results supports two major conclusions: First, that the effects of complexity reported in earlier research were probably on schematic--rather than visible--persistence; and second, that visible persistence must be regarded as a residual neural trace of an extinguished stimulus, rather than as a byproduct of information-extraction processes.     

Duration of visible persistence in relation to range of spatial frequencies

Five experiments examined the relationship between range of spatial frequencies contained in a visual display and duration of visible persistence. The high spatial frequency contents of the display were reduced by defocusing the image at the retina. Duration of visible persistence was measured by a task that required perceptual integration of a pattern whose parts were displayed sequentially in time. Experiments 1 and 2 showed that progressive reductions in the high-frequency contents of the display produced corresponding decrements in the duration of visible persistence. Experiments 3, 4, 5 showed that decrements in the duration of visible persistence could not be attributed to changes in the apparent size or in the brightness of the display brought about by defocusing. The results were interpreted in terms of the temporal response properties of perceptual mechanisms attuned to separate ranges of spatial frequencies. Strong parallels were drawn with the multichannel theory of visual information processing proposed by Breitmeyer and Ganz.     

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.     

Suppression of visible persistence

Five experiments were conducted to examine duration of visible persistence in sequences of stimuli. The basic display consisted of a point that stepped around a circular path on the face of an oscilloscope. Observers estimated the number of points seen simultaneously. Results were compared with a control condition in which the points were plotted in random order rather than sequentially. It was found that visible persistence of a point is suppressed if other points are shown nearby and after an appropriate delay. The degree of suppression depended on the spatial proximity of successive points. It was also found that both duration of visible persistence and degree of suppression increase with eccentricity in the visual field. The results are discussed in terms of two independent processes, persistence and suppression, that operate in a hierarchically antithetical relation.     

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.     

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.     

Iconic memory and visible persistence

There are three senses in which a visual stimulus may be said to persist psychologically for some time after its physical offset. First, neural activity in the visual system evoked by the stimulus may continue after stimulus offset (“neural persistence”). Second, the stimulus may continue to be visible for some time after its offset (“visible persistence”). Finally, information about visual properties of the stimulus may continue to be available to an observer for some time after stimulus offset (“informational persistence”). These three forms of visual persistence are widely assumed to reflect a single underlying process: a decaying visual trace that (1) consists of afteractivity in the visual system, (2) is visible, and (3) is the source of visual information in experiments on decaying visual memory. It is argued here that this assumption is incorrect. Studies of visible persistence are reviewed; seven different techniques that have been used for investigating visible persistence are identified, and it is pointed out that numerous studies using a variety of techniques have demonstrated two fundamental properties of visible persistence: theinverse duration effect (the longer a stimulus lasts, the shorter is its persistence after stimulus offset) and theinverse intensity effect (the more intense the stimulus, the briefer its persistence). Only when stimuli are so intense as to produce afterimages do these two effects fail to occur. Work on neural persistences is briefly reviewed; such persistences exist at the photoreceptor level and at various stages in the visual pathways. It is proposed that visible persistence depends upon both of these types of neural persistence; furthermore, there must be an additional neural locus, since a purely stereoscopic (and hence cortical) form of visible persistence exists. It is argued that informational persistence is defined by the use of the partial report methods introduced by Averbach and Coriell (1961) and Sperling (1960), and the term “iconic memory” is used to describe this form of persistence. Several studies of the effects of stimulus duration and stimulus intensity upon the duration of iconic memory have been carried out. Their results demonstrate that the duration of iconic memory is not inversely related to stimulus duration or stimulus intensity. It follows that informational persistence or iconic memory cannot be identified with visible persistence, since they have fundamentally different properties. One implication of this claim that one cannot investigate iconic memory by tasks that require the subject to make phenomenological judgments about the duration of a visual display. In other words, the so-called “direct methods” for studying iconic memory do not provide information about iconic memory. Another implication is that iconic memory is not intimately tied to processes going on in the visual system (as visible persistence is); provided a stimulus is adequately legible, its physical parameters have little influence upon its iconic memory. The paper concludes by pointing out that there exists an alternative to the usual view of iconic memory as a precategorical sensory buffer. According to this alternative, iconic memory is post-categorical, occurring subsequent to stimulus identification. Here, stimulus identification is considered to be a rapid automatic process which does not require buffer storage, but which provides no information about episodic properties of a visual stimulus. Information about these physical stimulus properties must, in some way, be temporarily attached to a representation of the stimulus in semantic memory; and it is this temporarily attached physical information which constitutes iconic memory.     

On the relations between crowding and visual masking

To study the question of which processes contribute to crowding and whether these are comparable to those of visual temporal masking, we varied the stimulus onset asynchrony (SOA) between target and flankers in a crowding setting. Monotonically increasing Type A masking functions observedfor small spacings and large eccentricities indicate that the integration of information from target and flankers underlies crowding. Decreasing masking functions obtained for large spacings and small eccentricities relate processes of crowding to those contributing to Type B masking. In addition, Type B masking was more frequent with letter-like nonletter flankers than with letter flankers, suggesting that Type B masking, just like crowding over large areas, is due to higher level interactions. The rapid decrease of the effects of interletter spacing and eccentricity with increasing SOA indicates that positional information is transient.     

The influence of metacontrast masking on detection and spatial-choice judgments: an apparent distinction between automatic and attentive response mechanisms

Several studies of metacontrast masking in the 1960s apparently showed that the latency of simple detection responses was uninfluenced by the phenomenal dimming of the target induced by the mask. More recent studies using more suitable methodologies have clearly shown that such is not the case for situations in which the masking is a monotonically decreasing function of stimulus onset asynchrony. Experiment 1 investigated this issue for the situation in which masking is a U-shaped function of stimulus onset asynchrony. Contrary to the results obtained in monotonic masking situations, simple detection responses were not slowed by the masking. Experiment 2 demonstrated that although detection responses are not slowed in the U-shaped masking situation, spatial-choice judgments are. Experiments 3 and 4 indicated that this masking effect on spatial-choice reaction time is lost relatively rapidly with practice. However, changing the stimulus-response assignments reinstates the effect. The experiments suggest that for the situation in which U-shaped masking functions are obtained, responses that require attention (spatial-choice judgments early in practice or after stimulus-response relationships have been switched) are influenced by the metacontrast-induced phenomenal dimming, whereas responses that are automatic (i.e., detection responses; practiced spatial-choice judgments with consistent stimulus-response mappings) are not.     

Continuous measurement of visible persistence

In the synchrony judgment paradigm, observers judge whether a click precedes or follows the onset of a light flash and, on other trials, whether or not a click precedes light termination. The interclick interval defines the duration of visible persistence. An elaboration of this method consists of two phases: In Phase 1, the luminance of a reference stimulus is psychophysically matched to the peak brightness of the test flash. Five luminance values between .1 and 1.0 of the reference stimulus are used subsequently. In Phase 2, a random one of the five reference stimuli, a test flash, and a click are presented; the observer judges whether the click occurred before or after the brightness of test flash reached the reference value (on onset trials) or decayed below it (on termination trials). This method was validated on 3 subjects with test stimuli whose luminance rises and decays slowly in time, and then was used to trace out the precise subjective rise and decay (temporal brightness response function) of brief flashes.