Masking of and by tactile pressure stimuli

Masking of and by tactile pressure stimuli was investigated in six Ss as a function of stimulus intensity (force) and stimulus onset asynchrony. Increase in the force of the masked stimulus and decrease in the force of the masking stimulus were inversely related to the magnitude of masking, as defined by either a relative or an absolute decrease in sensitivity. The introduction of stimulus onset asynchrony produced both forward and backward masking, the latter being of somewhat larger magnitude. Comparisons are made with results obtained in visual metacontrast masking.     

Parallel but temporally displaced visual half-field metacontrast functions

Using a classic letter-ring metacontrast paradigm, left and right visual field meta-contrast functions were separately determined. The parallel U-shaped recognition functions for both half-fields were found to interact differentially with stimulus onset asynchrony, the left visual field function being displaced by 13 ms toward longer test stimulus-masking stimulus separations. This result was consistent with the hypothesis of longer processing time requirements for verbal stimuli delivered to the right than to the left hemisphere. This indicates that the neural locus (loci) responsible for left visual field verbal processing delay is (are) capable of mediating metacontrast phenomena. It was tentatively concluded that a relative processing delay within the right hemisphere underlies the differing visual half-field metacontrast interaction with stimulus onset asynchrony.     

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.     

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.     

Visual recogition of dot-pattern bigrams: an extension and replication.

The study examined visual recognition of bigrams, each formed from a pair of "random" dot patterns, as a function of stimulus offset asynchrony and duration. The results replicate and extend those of an earlier study by showing that the effect of backward masking in vision, where the mask is actually a part of the preceding composite target, is limited to about 250 to 300 msec. This time interval is suggested as that required to complete the processing of that composite target. The results may be understood in terms of an interruption hypothesis, with selective attention and/or discontinuity detectors as mechanisms possibly involved in the masking process.     

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.     

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.     

U-shaped backward contour masking during stroboscopic motion.

Two stationary and spatially separated visual stimuli, presented briefly and successively in time, are known to produce stroboscopic motion whose vividness is a U-shaped function of the stimulus onset asynchrony. Contour masking is also known to occur under such stimulus conditions. The findings show that the contour masking is confined to only the first stimulus and that it, like metacontrast, is a backward U-shaped function of the stimulus onset asynchrony. A simple model, based on known psychophysical and neurophysiological properties, is proposed to explain these results.     

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.     

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.     

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.     

Pathways in type-B (U-shaped) metacontrast

Rod and cone targets were crossed, in every combination, with rod and cone masks in flanking-bars metacontrast. Strong type-B (U-shaped) metacontrast was obtained in each condition, contrary to the claim that rod and cone masking are independent. In each condition, visibility declined steadily with stimulus-onset asynchrony (SOA) in trials in which target and mask appeared to be simultaneous, and increased with SOA in trials in which they appeared to be successive. The 'U' results from collapsing across these different types of trials, which may reflect distinct monotonic processes in masking. Under the light adaptation conditions used the time, Tmax, at which metacontrast was at a maximum was delayed by about 25 ms if rods, rather than cones, detected the target. Whether rods or cones detected the mask hardly altered Tmax.     

Attend to it now or lose it forever: selective attention,metacontrast masking,and object substitution

Metacontrast masking occurs when the visibility of a brief target stimulus is decreased by the subsequent appearance of another nearby visual stimulus. Early explanations of the phenomenon involved low-level mechanisms, but subsequent studies have suggested a role for selective attention. The results of three experiments presented here extend previous findings to the metacontrast paradigm. It is shown that the strength of metacontrast masking increases with the number of distractor items in a display, decreases when the target location is validly but not invalidly precued, and is eliminated when search for the target is efficient (pop-out search) but not when search is inefficient (serial search). A connection between metacontrast masking and object substitution masking is considered.     

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.     

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.     

Visual recognition as a function of stimulus offset asynchrony and duration

The stimuli consisted of two complementary dot patterns that formed a bigram when they were flashed simultaneously; impairment of letter recognition developed when one of the patterns was briefly extended beyond the termination of the other (stimulus offset asynchrony). However, if the ratio of stimulus offset asynchrony to bigram duration remained constant, the probability of a correct recognition response also remained constant as duration varied over a 50- to 100-msec interval. When percent stimulus asynchrony increased, the impairment increased. An interaction between bigram letter position and each of bigram duration and percent stimulus asynchrony was observed with recognition accuracy greater in general for the letter in the left half of the field.     

Transient and sustained effects of an auditory accessory stimulus in a visual go/no go task

The present study examined the facilitation effects of an auditory accessory stimulus that was irrelevant to a visual reaction time (RT) task as a function of stimulus onset asynchrony between the accessory stimulus and the visual target stimulus. Results of the present experiment showed that the auditory accessory stimulus caused two variations of RT, short-term and long-term, that were distinguished on the basis of stimulus onset asynchrony. This finding suggested that effects of an accessory stimulus consisted of two qualitatively different facilitations. The Transient Facilitation appeared instantly after the onset of the accessory stimulus and then soon decayed, and the Sustained Facilitation increased and decreased more gradually than the former.     

Age differences in peripheral perceptual processing: a monoptic backward masking investigation

Peripheral processes in vision were investigated in two experiments involving monoptic backward masking with random noise. For young and old subjects, peripheral processing time (represented by stimulus onset asynchrony of target and mask) was characterized as a power function of target energy. Although processing time for both age groups showed a similar rate of decline with increasing target energy, old subjects processed targets more slowly at all energy levels. Results were independent of education, sex, and criterion differences between young and old. Target duration was related to critical interstimulus interval, such that stimulus onset asynchrony between target and mask was approximately constant for a given target energy within each age group. Evidence suggests that peripheral processing begins with target onset and that processing time is best characterized by a power function relating stimulus onset asynchrony of target and mask to target energy.     

Spatial factors in metacontrast

Different mask ring widths, intercontour distances, mask durations, and interstimulus intervals were varied in a parametric manner. Results show decreasing amounts of metacontrast with increases in intercontour distance, and no significant effects of mask width or mask duration. The results provide support for monotonic metacontrast functions with maximum metacontrast at the offset of the target.     

On the use of metacontrast to assess magnocellular function in dyslexic readers

It has been proposed that dyslexia is the result of a deficit in the magnocellular system. Reduced metacontrast masking in dyslexic readers has been taken as support for this view. In metacontrast, a masking stimulus reduces the visibility of a spatially adjacent target stimulus when the target stimulus precedes the masking stimulus by about 30–100 msec. Recent evidence indicates that the latency difference between the magnocellular and parvocellular subcortical pathways is at most 20 msec and may be as small as only 5 msec, or even less. This makes it difficult to attribute the latency in metacontrast to the latency differences between the magnocellular and parvocellular systems. It is therefore problematic to attribute reduced metacontrast masking to a deficit in the magnocellular system.