Recent models and findings in visual backward masking: a comparison, review, and update.

Visual backward masking not only is an empirically rich and theoretically interesting phenomenon but also has found increasing application as a powerful methodological tool in studies of visual information processing and as a useful instrument for investigating visual function in a variety of specific subject populations. Since the dual-channel, sustained-transient approach to visual masking was introduced about two decades ago, several new models of backward masking and metacontrast have been proposed as alternative approaches to visual masking. In this article, we outline, review, and evaluate three such approaches: an extension of the dual-channel approach as realized in the neural network model of retino-cortical dynamics (Ogmen, 1993), the perceptual retouch theory (Bachmann, 1984, 1994), and the boundary contour system (Francis, 1997; Grossberg & Mingolla, 1985b). Recent psychophysical and electrophysiological findings relevant to backward masking are reviewed and, whenever possible, are related to the aforementioned models. Besides noting the positive aspects of these models, we also list their problems and suggest changes that may improve them and experiments that can empirically test them.

The roles of location specificity and masking mechanisms in the attentional blink.

In a series of four experiments using rapid serial visual presentations of two target letters embedded in numeral distractors, with different numbers of display positions and with or without masking, we show that (1) the nonmonotonic, U-shaped attentional blink (AB) function, which occurs when all items are presented at the same display location, is eliminated in favor of a monotonic function when targets and distractors are presented randomly dispersed over four or nine adjacent positions; (2) the AB monotonicity is maintained with the spatially distributed presentation even when backward masks are used in all possible stimulus positions and when the location of the next item in sequence is predictable; and (3) the U-shaped AB is not due to position-specific forward or backward masking effects occurring at early levels of visual processing. We tentatively conclude that the U-shaped AB is primarily a function of the interruption of late visual processing produced when the item following the first target occurs at the same location. In order for the AB to severely disrupt performance, the item following the first target must be presented at the same location as the target so that it can serve both as a distractor and as a mask interrupting of interfering with subsequent visual processing.

Colored overlays for visual perceptual deficits in children with reading disability and attention deficit/hyperactivity disorder: are they differentially effective?

The Transient Channel Deficit (TCD) model of reading disability was evaluated by examining the effects of color overlays on the reading ability of four groups of children (n = 15 each) with reading disability and comorbid conditions involving math and ADHD. These 60 children were evaluated for reading accuracy and rate on measures of word decoding and reading comprehension under three color transparency conditions (blue, red, no overlay). Results indicated that color overlays did not differentially affect the reading performance of individuals with and without reading disabilities. However, blue transparencies significantly improved reading comprehension in all groups, and reduced reading rate. These findings indicate that the TCD model may need to be reexamined. An alternative hypothesis for the observed effects, involving facilitation of attention processes, was posted.

Adaptive and maladaptive utilization of color cues by patients with mild to moderate Alzheimer's disease.

The ability to utilize color information was investigated in 12 patients with mild to moderate probable Alzheimer's Disease (DAT) and in 12 age- and gender-matched control subjects. All subjects underwent testing of visual acuity and color vision before being tested with a cognitive task consisting of four conditions (no color, color as attention enhancer, color as valid cue, color as distracter). Although the groups did not differ in visual acuity or color vision, patients with DAT were less accurate than controls in all four conditions of the cognitive task. Both groups performed best with color as a valid cue and worst with color as distracter, but condition had a significantly stronger effect on patients than on controls. It is concluded that color is a potent stimulus attribute for patients with DAT.

Discriminability of random-dot stereograms in three-dimensional space.

The discriminability of crossed-disparity (near) and uncrossed-disparity as a function of their location in the upper-left, upper-right, lower-left, and lower-right quadrants of the visual field. Discriminability was assessed using choice reaction-time (RT) and accuracy measures. While near targets were recognized equally well in the upper and lower fields, far targets were perceived more easily in the upper visual field. The discriminability of far targets was particularly poor in the lower left quadrant. These results point to the existence of fundamental asymmetries in perceiving crossed and uncrossed disparities along the vertical and lateral axes.

Visual information processing after severe closed head injury: effects of forward and backward masking.

Three tachistoscopic tasks were employed to assess whether survivors of severe closed head injury (CHI) exhibit a disturbance of information processing within peripheral and/or central visual pathways. Twelve survivors of severe CHI and 12 individually matched control subjects completed a recognition threshold (no mask) task, a monoptic, forward masking by visual noise task (to assess processing within relatively peripheral pathways), and a dichoptic, backward masking by pattern task (to assess processing within central pathways). For each experimental procedure, the minimum exposure durations required by subjects to identify correctly single consonants and triple consonants were determined. Survivors of severe CHI showed deficits on all three visual tasks. Both groups also had higher threshold durations for the more complex stimuli (triple v single consonants), but differences in threshold were greater in the patients with CHI. The degree of perceptual impairment exhibited by patients with CHI was highly variable and not consistently related to injury characteristics or residual motor or speech and language impairment.

Effects of background color on reaction time to stimuli varying in size and contrast: inferences about human M channels.

In two experiments we looked at the effects of the color of equiluminant backgrounds on simple reaction time (RT) to increment and decrement spot-stimuli varying in diameter. When comparing both red vs blue and red vs green backgrounds, we found that for the smallest diameter stimuli, RTs tended to be faster with red background; however, as the diameter of the stimuli increased, RTs were faster with the blue or green backgrounds. This trend held only for increment stimuli; no systematic or significant differences between RTs to decrement stimuli presented on red vs either blue or green backgrounds were found. We discuss these results in terms of the effects of diffuse lights of varying wavelength on magnocellular-channel activity.

Metacontrast with masks varying in spatial frequency and wavelength.

In a metacontrast masking paradigm, adult subjects were required to detect briefly presented target lines followed at various delays by a flanking mask varying in spatial frequency and wavelength. Detection accuracy, the dependent measure, was recorded as a function of the delay of the mask. The results showed that long wavelength masks produced maximum masking at a relatively short delay, while short wavelength masks produced maximum masking at a relatively long delay. Results are discussed within the framework of transient/sustained (magnocellular/parvocellular) theory of visual processing, and suggest that low spatial frequency channels respond with shorter latency and/or faster rise time to short wavelength stimuli, and high spatial frequency channels respond with greater sensitivity to long wavelength stimuli.

Effects of isoluminant-background color on metacontrast and stroboscopic motion: interactions between sustained (P) and transient (M) channels.

The magnitudes of metacontrast masking and of stroboscopic motion were investigated as a function of the color of isoluminant backgrounds on which the stimuli consisting of rectangular-shaped luminance decrements appeared. White, green, and red backgrounds were employed. Background isoluminance was obtained by minimal flicker settings with heterochromatic flicker of uniform fields. Both metacontrast and stroboscopic motion were decreased when red as compared to white or green backgrounds were used. Within the context of current sustained-transient channel approaches to visual masking and motion, these results indicate that the activity of transient channels is attenuated by red relative to white or green backgrounds. Moreover, these psychophysical findings may correspond to the suppressive effects of diffuse red light on neural activity in the transient M pathway of monkey.

Effects of reappearance of fixated and attended stimuli upon saccadic reaction time.

The effects of visual attention and fixation upon the distribution of saccadic latencies: express (E-), fast regular (FR-), and slow regular (SR-) saccades were investigated. Extinguishing a fixation or an attention point 200-300 ms before target onset increases the incidence of E-saccades while concurrently decreasing the proportion of SR-saccades. Since this extinction forces a disengaging of attention, these changes in relative proportions of saccades reflect the elimination of one of the steps involved in programming saccades. It is shown that a previously attended stimulus has a favored status relative to other stimuli in the visual field. If, after being turned off, the previously attended fixation point or a peripheral attention stimulus is turned on near the time of the target's appearance, the occurrence of the E-saccades is greatly reduced. However, the appearance of any other stimulus in the visual field at or near the time of the target onset does not inhibit E-saccades. Contrary to the conclusions reached by Posner and Cohen (1984), a stimulus presented at the formerly attended location can attract attention more efficiently than a stimulus presented at another, new location.

The effects of dichoptic and binocular viewing on bistable motion percepts.

Two competitive percepts are produced from a bistable stroboscopic motion display. In this display two frames, each containing three horizontally arrayed elements are presented alternately for several cycles. At short interstimulus intervals (ISIs) element or end-to-end motion responses are obtained when the two inner, spatially overlapping elements are seen as stationary and the third element moves back and forth from one end to the other end. Group motion responses are obtained at longer ISIs when the three elements are seen to move back and forth as a group. The dominance of these two percepts across ISIs was controlled by the manipulation of (1) element size, (2) frame duration, and (3) viewing conditions. Under both binocular and dichoptic viewing, element motion responses increase as element size and frame duration decrease. By maximizing pattern persistence substantial element motion responses were obtained dichoptically as well as binocularly. Instead of supporting the existence of two separate, low-level and high-level, motion systems, our data suggest that there is a single, high-level mechanism for motion whose output can be modulated by pattern persistence.

A choice reaction time analysis of spatial frequency discrimination.

Simple reaction time to the onset of sinewave gratings was measured as a function of spatial frequency in two observers. These results are compared to the choice reaction time required for the observer to correctly discriminate the spatial frequency of two gratings flashed sequentially. Grating contrast was either 0.75 or 1.5 logarithmic units above the detection threshold for each spatial frequency tested. The spatial phase and contrast of the reference and test gratings were varied from trial to trial by small random amounts to eliminate fixed cues other than the difference frequency. The spatial frequency difference between the reference and test grating was either 0.125, 0.25 or 0.5 octave. As has been earlier reported, simple reaction time increases with increasing spatial frequency. Contrary to this, choice reaction time first increases (up to 4 c/deg) and then decreases. We derived the time required by the observer to make a spatial frequency judgment by subtracting the simple reaction time from the choice reaction time for a given spatial frequency and contrast. The maximum decision time occurs in the medium spatial frequency range (between 1 and 4 c/deg), at which frequencies we are most sensitive. The time required to make a correct spatial-frequency discrimination decreases with increasing spatial-frequency difference. The decision time is, however, fairly invariant over a large range of suprathreshold contrast levels. The findings suggest that the decision time for spatial frequency discrimination increases with the number of mechanisms involved.

Relationship between directed visual attention and saccadic reaction times.

Saslow (1967) and Fischer and Ramsperger (1984) found that saccadic reaction time (SRT) depends on the interval between the fixation point offset and the target onset. Using a continuously visible fixation point, we asked whether a similar function would be obtained if subjects attended to a peripherally viewed point extinguished at variable intervals before or after the target onset. The interval was varied between -500 ms (i.e., attention stimulus offset after saccade target onset = overlap trials) and 500 ms (i.e., attention stimulus offset before saccade target onset = gap trials). The results show a constant mean SRT of about 240 ms for overlap trials, and a U-shaped function with a minimum of 140 ms, at a gap duration of 200 ms, for gap trials. These findings suggest that saccadic latencies do not depend on the cessation of fixation per se, but rather on the disengagement of attention from any location in the visual field. The time required for subjects to disengage their attention is approximately 100 ms. This disengaged state of attention--during which short latency (express) saccades can be made--can be sustained only for a gap duration of 300 ms. At longer gap durations mean SRTs increase again.

The role of visual pattern persistence in bistable stroboscopic motion.

Two alternating frames, each consisting of three square elements, were used to study bistable stroboscopic motion percepts. Bistable percepts were obtained which depend on the interstimulus interval (ISI) between the alternating frames. At short ISIs only end-to-end element motion was observed; and at higher ISIs only group motion was perceived. It was found that the progressive ISI-dependent transitions from element to group motion depended on element size and frame duration. These dependencies are predictable from the systematic influence which these variables are known also to exert on visual pattern persistence, indicating that such persistence contributes to determining which precept dominates during bistable stroboscopic motion sequences. These findings bear relevantly on recent attempts to conceptually relate bistable motion percepts to short-range stroboscopic motion processes.

Stationary patterns suppress the perception of stroboscopic motion.

Two spatially separated vertical bar stimuli briefly flashed in temporal sequence produced strong sensations of stroboscopic apparent motion; particularly at intermediate onset asynchronies. The sustained presence of two additional stationary vertical bars flanking the two movement-inducing bars during their presentation significantly decreased the rated magnitude of the sensation of stroboscopic motion. Control experiments rule out contrast reduction of the movement-inducing bars by the stationary flanking bars as a source of the decrease of the rated magnitude of stroboscopic motion. These results are related to similar effects observed in metacontrast and suggest that sustained channels responding to stationary patterns inhibit transient channels responding to brief or rapid image displacements giving rise to perception of stroboscopic motion.

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.