A review of the visual masking literature in the context of known neurophysiological and psychophysical properties of the visual system's spatiotemporal response reveals that three consistent and typical pattern masking effects-(a) Type B forward or paracontrast, (b) Type B backward or metacontrast, and (c) Type A forward and backward-can be explained in terms of three simple sensory processes. It is hypothesized that sustained channels are involved in the processing of structural or figural information, whereas transient channels are involved in signaling the spatial location or change in spatial location (motion) of a stimulus. In the proposed model, Type B forward masking or paracontrast is mediated via lateral inhibition realized in the center-surround antagonism of the receptive fields of the sustained cells. In apposition to this mechanism of z'ntrachannel inhibition, Type B backward masking or metacontrast is produced by a mechanism of z'nterchannel inhibition, that is, transient cells laterally inhibit the activity of sustained cells. Both mechanisms are assumed to be operating at or prior to the contour-forming levels of visual processing. Type A masking effects are explained in terms of sensory integration of sustained channel information at preiconic and iconic levels of visual processing. The implications of this multichannel model for saccadic-suppression and information-processing approaches to pattern recognition are discussed.The visibility of a briefly displayed target another briefly displayed masking pattern, pattern can be reduced in several ways by Perhaps the most effective way is to display concurrently with the target a spatially over-~~ lapping pattern mask of sufficiently high con-This study was supported in part by National Eye trast whicn part ially or totally obscures the Institute Grant EY01241-02 to the second author.. . TT u -j ^-J.-L. t, • Much of the scholarly and experimental research target. However, besides this rather obvious that contributed to this paper was done while the type of masking procedure, methods of visual first author was on leave of absence from the Uni-pattern masking have been employed in which versity of Houston, between July 1973 and August t h e target and mask displays do not overlap 1974, at the Bell Laboratories, Murray Hill New . j d/ g h fad Jersey. He thanks Bela Julesz of the Bell Labora-. '•" tories for sponsoring and encouraging his research especially interesting because they can be efforts. We also thank Naomi Weisstein for helpful employed to probe the spatiotemporal propcriticisms and suggestions.erties of information processing and, in par-B^K/S^SS^oio^SiSlSS ticular > Pattern-forming operations in the Houston, Houston, Texas 77004. visual system.

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(Bachmann, , 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.Visual masking occurs whenever the visibility of one stimulus, called the target, is reduced by the presence of another stimulus, designated as the mask. Visual masking has been, and continues to be, a powerful psychophysical tool for investigating the steady-state properties of spatial-processing mechanisms

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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.

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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.