Trends in Cognitive Sciences
Volume 4, Issue 9, 1 September 2000, Pages 345-352
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Review
What’s new in visual masking?

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Abstract

A brief display that is clearly visible when shown alone can be rendered invisible by the subsequent presentation of a second visual stimulus. Several recently described backward masking effects are not predicted by current theories of visual masking, including masking by four small dots that surround (but do not touch) a target object and masking by a surrounding object that remains on display after the target object has been turned off. A crucial factor in both of these effects is attention: almost no masking occurs if attention can be rapidly focused on the target, whereas powerful masking ensues if attention directed at the target is delayed. A new theory of visual masking, inspired by developments in neuroscience, can account for these effects, as well as more traditional masking effects. In addition, the new theory sheds light on related research, such as the attentional blink, inattentional blindness and change blindness.

Section snippets

The standard view

Visual masking is typically divided into two types, based on the spatial relationships that exist between the contours of the target and mask patterns. Masking that involves spatial superimposition of contours is commonly referred to as ‘pattern masking’, while masking that involves closely adjacent but nonoverlapping contours it is called ‘metacontrast’. Typical examples of stimuli used in each of these types of masking are shown in Fig. 1a,b.

Pattern masking presents the visual system with two

Nagging problems for the standard view

Although these standard views account for a large portion of the data on visual masking, there are several persistent findings that complicate the picture. First, consider the perceptual fate of masked targets. In the standard view, backward masking terminates the processing of the target at a pre- categorical level7, 8, 9. However, a phenomenon known as ‘masked priming’ suggests that processing of masked targets continues to lexical and even semantic levels. In conventional priming, where the

New forms of masking that defy the standard view

In our laboratory, we have been exploring two new forms of masking that are difficult to reconcile with current theories. Here, we provide only a brief introduction to give the reader a flavor of the phenomena that require explanation. These masking effects can be experienced first hand on the internet (http://www.interchange.ubc.ca/enzo/osdescr.htm) and can be read about elsewhere15, 28, 29.

The first form of masking (Fig. 1c) occurs when a briefly presented target is followed by four dots that

Masking based on cortical re-entrant processing

Our novel view of masking is based on recent advances in neuroscience and psychophysics. Our starting point was the principle that communication between two brain areas is never unidirectional: if a source area sends signals to a target area, then the target area sends signals back to the source area through re-entrant pathways30, 31, 32. It has been suggested that the architecture of cortical re-entry might be used to test for the presence of specific patterns in the incoming signals33, 34.

Object substitution and the role of attention in perception

One of the most important practical implications to arise from our understanding of object substitution is that backward masking does not simply terminate the processing of a target. Rather, the perceptual mechanisms of conscious perception appear to be actively engaged in perceiving the mask. An example of this principle is illustrated in Box 1 for a phenomenon known as ‘the attentional blink’.

We believe that masking by object substitution also has direct relevance to the recently popularized

Acknowledgement

The research described in this article was supported by grants from the Natural Sciences and Engineering Research Council of Canada to each author.

Outstanding questions

  • What is the true relationship between focused spatial attention and early visual processing? The computational model for object substitution assumes that early visual processes operate somewhat independently of the focus of attention. The limiting factor is the length of time that elapses before attentional mechanisms interact with the early processes in a given location. However, is it possible that the mechanisms of attention themselves alter the operations of these early processes? What are

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