Bruno G. Breitmeyer

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.

Simple reaction time as a measure of the temporal response properties of transient and sustained channels.

Psychophysical findings reported by Kulikowski and Tolhurst (1973). Tolhurst (1973), and Breitmeyer and Juksz (1975) have revealed some of the response properties of transient and sustained channels in the human visual system. For instance, transient channels relative to sustained ones are characterized by a poor spatial resolution; i.e. they respond preferably to low spatial frequency stimuli. Moreover, transient channels show a relatively better temporal resolution as revealed by their greater sensitivity to flicker (Kulikowski and Tolhurst, 1973), rapid motion (Tolhurst, 1973) and abrupt stimulus onset (Breitmeyer and Julesz, 1975). These psychophysically determined response charactet%tics have their respective electrophysiological paralkls as revealed by single cell studies of the cat visual system (Fukuda and Saito. 1971; Cleland, Levick and Sanderson, 1973; EnrothCugell and Robson, 1966). Other electrophysiological indices of response properties d@rentiating transient from sustained cells are the tindings that transient channels have a shorter response latency to photic and electric stimulation of the optic nerve (Fukuda. 1973; Fukuda, Sugitani and Iwama, 1973) and that their fibres have a higher conduction speed (Fukuda. 1971; Hoffman and Stone, 1971; Cleland et al., 1973; Hoffman, 1973) than do sustained channels. The possible existence of such latency differences between transient and sustained channels in humans was investigated psychophysically by measuring the simple reaction time of two subjects to vertical sinusoidal gratings of variable spatial frequencies ranging from 0.5 to 1 lQc/deg. After light adapting to a 5 ft-L, uniformly bright stimulus field generated on a Hewlett-Packard 1300A X-Y display (P31 phosphor), reaction times were measured by displaying a vertical grating for 50 msec (onset rise time and offset fall time were each approx 1 msec) on the stimulus screen and requiring the subject to press a switch held in his dominant hand as soon as he saw a grating. The pressing of the switch stopped a msec-time clock whose onset was synchronized with the onset of the grating. The spatial contrast modulations of all gratings used in the first experiment was maintained at 50%. The results of the first experiment are shown separately for each subject in Fig. 1. Mean reaction time is plotted as a function of grating spatial frequency. Each point is based on 30 observations and the bars

Temporal studies with flashed gratings: Inferences about human transient and sustained channels

Abstract The temporal response properties of the human visual system to low and high spatial frequency gratings was investigated by two contrast detection threshold techniques. With the first technique the contrast threshold for detecting vertical sinusoidal gratings at spatial frequencies of 0.5, 2.8 and 16.0 c/deg was determined at exposure durations ranging from 20 to 400 msec. It was found that the critical duration, at and below which reciprocity between contrast and a nonunity power of duration holds, increased from roughly 60 to 200 msec as spatial frequency increased from 0.5 to 16.0 c/deg. The second technique involved subthreshold summation of two, 10 msec flashed presentations of either a 1.0 or 10.0 c/deg grating. The stimulus onset asynchrony (SOA) separating the onsets of the two pulses varied from 0 to 210 msec. The results revealed that the subthreshold interaction of the two flashes at high spatial frequencies can be characterized by monophasic sustained excitation and inhibition; at low spatial frequencies, however, this interaction can be characterized by a multiphasic oscillation of excitation and inhibition superimposed on a monophasic excitatory-inhibitory interaction. The findings are related to properties of transient and sustained channels assumed to exist in human vision.

Feedback Contributions to Visual Awareness in Human Occipital Cortex

It has traditionally been assumed that processing within the visual system proceeds in a bottom-up, feedforward manner from retina to higher cortical areas. In addition to feedforward processing, it is now clear that there are also important contributions to sensory encoding that rely upon top-down, feedback (reentrant) projections from higher visual areas to lower ones. By utilizing transcranial magnetic stimulation (TMS) in a metacontrast masking paradigm, we addressed whether feedback processes in early visual cortex play a role in visual awareness. We show that TMS of visual cortex, when timed to produce visual suppression of an annulus serving as a metacontrast mask, induces recovery of an otherwise imperceptible disk. In addition to producing disk recovery, TMS suppression of an annulus was greater when a disk preceded it than when an annulus was presented alone. This latter result suggests that there are effects of the disk on the perceptibility of the subsequent mask that are additive and are revealed with TMS of the visual cortex. These results demonstrate spatial and temporal interactions of conscious vision in visual cortex and suggest that a prior visual stimulus can influence subsequent perception at early stages of visual encoding via feedback projections.

The existence and role of retinotopic and spatiotopic forms of visual persistence

Abstract Current theoretical conceptualizations of visual persistence fail to address the issue of its functional role. Based on extant experimental data we tentatively identify two general types of visual persistence: one resides in activity along the afferent visual pathway and is retinotopically organized; the other resides at central levels and is spatiotopically organized. Moreover, whereas the former afferent persistence is eliminated via saccadic suppression mechanisms in order to separate successive, retinotopic frames of pattern information, the latter, central one, in contrast, is generated and enhanced via extraretinal signals accompanying saccades in order to preserve phenomenal continuity of a stable spatiotopic representation of the environment from one fixation to the next.

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.

The what and where in visual masking.

A metacontrast mask suppresses the visibility of, without influencing the reaction time (RT) to, the target. We investigated whether this dissociation results from a sensori-motor pathway immune to masking effects or from the characteristics of stimulus timing in mutually inhibitory sustained and transient channels. For target visibility, para- and metacontrast yielded the usual U-shaped functions. Peak paracontrast occurred at stimulus onset asynchronies (SOAs) of -150 to -100 ms. RTs were relatively low for metacontrast and did not show a systematic change as a function of SOA. The RT contribution from contour-masking was greatest at an SOA of -150 ms (paracontrast) and declined to near zero in the metacontrast regime. The dissociation between visibility and RT seen in metacontrast did not occur in paracontrast, rejecting the theory that RTs are elicited by a single sensori-motor pathway immune to masking. The dependence of the dissociation on stimulus timing can be explained by RECOD, a dual-pathway model wherein fast and slow activities interact.

Unconscious Color Priming Occurs at Stimulus- Not Percept-Dependent Levels of Processing

Although past studies have shown that visual information can be processed without awareness, the types and levels of this processing have yet to be determined. We used metacontrast masking to explore unconscious priming effects of white, blue, and green stimuli generated on a color video display. We found that a white prime tends to act more like a green than a blue one. Color confusions among unmasked and masked primes and calibrations of the display phosphors show that physical rather than perceptual properties of the stimuli best explain the white primes effects. We conclude that unconscious color priming in normal observers occurs at early wavelength-dependent levels of processing prior to later color-percept-dependent levels.

Visual masking: past accomplishments, present status, future developments.

Visual masking, throughout its history, has been used as an investigative tool in exploring the temporal dynamics of visual perception, beginning with retinal processes and ending in cortical processes concerned with the conscious registration of stimuli. However, visual masking also has been a phenomenon deemed worthy of study in its own right. Most of the recent uses of visual masking have focused on the study of central processes, particularly those involved in feature, object and scene representations, in attentional control mechanisms, and in phenomenal awareness. In recent years our understanding of the phenomenon and cortical mechanisms of visual masking also has benefited from several brain imaging techniques and from a number of sophisticated and neurophysiologically plausible neural network models. Key issues and problems are discussed with the aim of guiding future empirical and theoretical research.

Visual masking as a probe for abnormal gamma range activity in schizophrenia

BACKGROUND Visual masking procedures assess very early stages of visual perception. Patients with schizophrenia consistently show deficits on visual masking tasks, and these deficits likely reflect vulnerability to schizophrenia. We conducted two experiments to determine whether visual masking procedures can reveal underlying abnormalities in gamma range oscillations in schizophrenia. METHODS In the first experiment, we conducted nonlinear modeling of visual masking performance data from 89 male schizophrenic patients and 20 male comparison subjects. In the second experiment, electrophysiological recordings of event-related gamma activity were taken during a visual masking task in a subset of eight patients and seven control subjects. RESULTS In the first experiment, nonlinear modeling of the performance data revealed evidence of oscillations in the gamma range (30 and 35 Hz) for the comparison group but not patients. In the second experiment, the comparison group, but not the patients, showed a burst of gamma range activity 200-400 msec following target presentation. The difference between patients and comparison subjects in this time period was significant (p <.05). CONCLUSIONS Visual masking procedures can serve as a probe for underlying gamma range activity, which appears to be aberrant in schizophrenia. Perceptual problems in schizophrenia may, at least in part, be due to a failure to establish and/or maintain gamma range oscillations.