Visual masking by object substitution in schizophrenia.

BACKGROUND: Schizophrenia patients demonstrate impairment on visual backward masking, a measure of early visual processing. Most visual masking paradigms involve two distinct processes, an early fast-acting component associated with object formation and a later component that acts through object substitution. So far, masking paradigms used in schizophrenia research have been unable to separate these two processes. METHOD: We administered three visual processing paradigms (location masking with forward and backward masking, four-dot backward masking and a cuing task) to 136 patients with schizophrenia or schizoaffective disorder and 79 healthy controls. A psychophysical procedure was used to match subjects on identification of an unmasked target prior to location masking. Location masking interrupts object formation, four-dot masking task works through masking by object substitution and the cuing task measures iconic decay. RESULTS: Patients showed impairment on location masking after being matched for input threshold, similar to previous reports. After correcting for age, patients showed lower performance on four-dot masking than controls, but the groups did not differ on the cuing task. CONCLUSIONS: Patients with schizophrenia showed lower performance when masking was specific to object substitution. The difference in object substitution masking was not due to a difference in rate of iconic decay, which was comparable in the two groups. These results suggest that, despite normal iconic decay rates, individuals with schizophrenia show impairment in a paradigm of masking by object substitution that did not also involve disruption of object formation.

Local foveal inhibitory effects of global peripheral excitation.

Global excitation produced by oscillating a peripheral square-wave grating back and forth through one-half cycle inhibits the visibility of an incremental test flash only when the flash is presented in the foveal region of the visual field. This finding is discussed in the context of the neurophysiological periphery effect and shift-effect and their possible role in saccadic suppression.

Dynamic random-dot stereograms reveal up-down anisotropy and left-right isotropy between cortical hemifields.

With the use of dynamic random-dot sterograms (which are devoid of all monocular depth cues), the temporal duration for detecting a small, briefly presented test square of different depth than the surround varied as a function of its location in the central portion of the visual field. Test squares presented in the upper hemifield were detectable at consistently shorter durations than those in the lower hemifield when the fixation marker was in front of the surround, and vice versa when the marker was behind. No such anisotropy was found for left and right hemifield. Esploratory studies suggested a similar up-down anisotropy and left-right isotropy in spatial resolution. Thus, the upper hemifield representation at the cortex shows a general superiority over the lower one for vinocular detectors tuned to uncrossed disparitites, and the lower hemifield shows superiority for those tuned to crossed disparities.

Backward masking performance in unaffected siblings of schizophrenic patients. Evidence for a vulnerability indicator.

BACKGROUND: Visual masking is a procedure that is used to assess the earliest components of visual processing. In backward masking, the identification of an initial stimulus (the target) is disrupted by a later stimulus (the mask). The masking function can be divided into an early component (e.g., up to about 60 ms) that reflects the involvement of sensory-perceptual processes, and a later component that reflects susceptibility to attentional disengagement as the mask diverts processing away from the representation of the target. Schizophrenic patients show anomalies on both masking components. It is not known whether backward masking deficits reflect enduring genetic vulnerability to schizophrenia. METHODS: We assessed 32 unaffected siblings of schizophrenic patients and 52 normal control subjects on the early and late components of 4 masking conditions. The conditions differentially involved the sustained and transient visual pathways. RESULTS: The unaffected siblings showed poorer overall performance than control subjects on the masking procedures. More specifically, siblings showed anomalies on the early, sensory-perceptual component, but not on the later, attentional disengagement component. CONCLUSIONS: The backward masking performance deficits that have been observed in schizophrenic patients appear to reflect enduring vulnerability to the disorder rather than only the symptoms of the illness. This vulnerability appears to be associated with early, sensory-perceptual processes.

Backward masking in unmedicated schizophrenic patients in psychotic remission: possible reflection of aberrant cortical oscillation.

OBJECTIVE: Patients with schizophrenia consistently show performance deficits on measures of visual backward masking, but the nature of these deficits is not well understood. Performance deficits on backward masking tasks may indicate an underlying predisposition instead of the presence of illness, because deficits are present in unaffected first-degree relatives. Performance deficits in remitted patients would constitute converging support for this hypothesis. METHOD: Eleven patients with recent-onset schizophrenia who were in a period of no medication use during remission of psychosis were compared with a matched normal group on three visual masking conditions. These conditions included target identification tasks with a high-energy mask, a low-energy mask, and a blurred target. RESULTS: Patients in psychotic remission showed significant deficits across all conditions. In addition, trend analyses revealed significant group differences in the shape of the masking functions: the comparison group showed an oscillating performance pattern across all masking conditions, whereas the patients did not exhibit this pattern on any condition. CONCLUSIONS: These data from patients in well-documented psychotic remission add converging support for the hypothesis that deficits on backward masking procedures are indicators of vulnerability to schizophrenia. Because visual masking procedures may reflect underlying neural oscillations of 30 to 70 Hz in the visual cortex, the pattern of results is consistent with the theory that visual masking deficits in schizophrenia stem from an underlying failure to establish cortical oscillations.

Mechanisms of visual attention revealed by saccadic eye movements.

This paper summarizes recent data on the initiation of saccadic eye movement in relation to the mechanisms of visual attention. In particular, the occurrence of express saccades, defined by their extremely short reaction times, is discussed on the basis of the observation that these saccades do not occur when the subjects (man or monkey) are attending to either a fixation point or to any other visual stimulus in the periphery of their field of view including the "future" saccade target location. It is concluded that the system of visual attention can be in two states: engaged or disengaged. In order to generate a saccade or to move attention from one point to another visual attention must be in the disengaged state. The disengagement takes some time which is or is not included in the saccadic reaction time depending on whether or not visual attention is engaged at the time of the onset of the saccade target. During engaged visual attention saccades are inhibited thereby providing steady central fixation or the absence of saccades during directed peripheral attention.

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.

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.

Metacontrast investigations of sustained-transient channel inhibitory interactions.

The metacontrast paradigm was exploited to study possible inhibitory interactions between sustained and transient channels. Variations and measures of target disinhibition or target recovery. The former type of target recovery is taken as evidence for interchannel sustained-on-transient inhibition; the latter, as evidence for intrachannel transient-on-transient inhibition.

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.

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.

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.

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.

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.

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.

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.