Mark C Price

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University of Bergen

As a classroom demonstration, visual masking always gets a gasp. A visual stimulus such as a word is first presented for a few milliseconds on its own. Despite its brevity, the word is easy to identify. However when the word is presented for exactly the same period of time, but is followed after a blank delay by a random jumble of letters (Figure 2.1), most observers consciously perceive only the second stimulus. The preceding word seems to have completely disappeared. If a longer delay between the two stimuli is used, the masking effect of the second stimulus, or mask, is reduced. The first stimulus might now be perceived as a vague flash, even though none of the letters are

Figure 2.1. Examples of stimulus displays in a masking experiment. The left panel depicts a target word, and the right panel depicts a mask composed of overlapping letters. Each display might appear in the same location for about 10 milliseconds. Depending on the exact conditions of presentation, and on individual variation between subjects, effective masking which precludes conscious awareness of the stimulus might typically be achieved with an interval of 20 - 50 milliseconds between displays.

Figure 2.1. Examples of stimulus displays in a masking experiment. The left panel depicts a target word, and the right panel depicts a mask composed of overlapping letters. Each display might appear in the same location for about 10 milliseconds. Depending on the exact conditions of presentation, and on individual variation between subjects, effective masking which precludes conscious awareness of the stimulus might typically be achieved with an interval of 20 - 50 milliseconds between displays.

discernible. A yet longer inter-stimulus delay will typically allow the observer to identify some letters. If a long enough delay is used, the entire word will again become visible.

Similar effects can be obtained using a variety of masks. These can simply be a light flash or a random dot pattern, in which case they are referred to as noise masks. Alternatively the mask can consist of letter fragments, letter strings or a real word. These are referred to as pattern masks because they are visually similar to the target. Although the example of masking described above is a backward effect of a mask on a preceding target, forward masking in which the mask interferes with a succeeding target can also occur. Masking also takes place if masks flank or surround the spatial location occupied by the target, rather than occurring in the same position. This is referred to as "paracontrast" masking when the masking is forwards, and "metacontrast" if backwards. An example is the masking of a small solid black disc by a slightly larger black circle; all the subject sees is an empty circle.

Whatever the exact details of the type of masking used, it is clear that the rapid succession of two stimuli in some sense oversteps the temporal resolution of the visual system. The mask appears to interfere with processing of the target somewhere between peripheral sensory registration of the target and its emergence into subjective awareness. Exactly how and where such interference takes place has been the subject of much debate. Perhaps the point on which there is most agreement is that masking embraces not one but many varieties of interaction at several levels of stimulus processing. Although part of the story appears merely to involve the temporal summation of the target and mask in peripheral processing channels between the retina and cortex, much higher level processes are also involved.

One of the most striking pieces of evidence for this comes from studies of nonconscious perception in which the meaning of a masked stimulus is shown to be processed, despite the fact that the stimulus is not perceived consciously. For example, an undetectable masked word (e.g., BREAD) can speed up reaction times to make a judgment about a subsequently presented word to which it is semantically associated (e.g., BUTTER). The fact that masking can prevent conscious awareness of sensory input, while leaving nonconscious perceptual and recognition processes at least partially intact, has been influential in leading some theorists (Bachmann, 1993; Marcel, 1983b) to propose that masking may interfere with brain processes that are critically involved in the emergence of consciousness itself.

In this chapter I discuss three ways in which masking contributes to the study of how neurocognitive events in the brain could give rise to conscious experience. Although masking occurs in the auditory and somatosensory modalities as well as in vision, this discussion will be confined to vision since it is here that the vast majority of empirical and theoretical work in masking has been conducted. Not all of the methodological and theoretical issues that will be raised are exclusive to masking, but few other phenomena in psychology bring one so readily and so comprehensively into contact with them.

The first section of the chapter describes the salutary lesson that masking provides for the way we operationalize and measure consciousness. In particular, masking studies highlight the difference between subjective introspective measures of what somebody is conscious of, and more objective behavioral measures. While the latter are commonly used to avoid the methodological pitfalls of introspective techniques, they may confound conscious and nonconscious processing. I argue that an introspective approach is possible, extremely informative, and at times necessary. Masking experiments also force us to distinguish subtly different ways in which various aspects of stimulus information can become manifest in consciousness, and therefore sharpen our definition of what it means to be conscious of something. For example, awareness of masked stimuli can range from awareness of stimulus presence without awareness of stimulus meaning, to counterintuitive instances of awareness of meaning without awareness of stimulus presence.

The second section examines what masking studies can tell us about the limits of stimulus processing without conscious awareness. In order to identify the brain processes which give rise to subjective awareness, we need to identify which processes can occur in the absence of consciousness, and which cannot (the approach labelled as "contrastive analysis" by Baars, 1997). One way to do this is to compare the qualitative differences between conscious and nonconscious perception (see Merikle & Daneman, 1998). Studies involving masking have formed a major part of such research, illustrating both the sophistication and the limitations of perception without awareness and exposing some complex but poorly understood interactions between conscious and nonconscious processes.

In the last section, I turn in more detail to theoretical accounts of the mechanisms that underlie masking. Some theories propose that masking prevents consciousness by disrupting early perceptual analysis. However theories of this kind are difficult to reconcile with the kind of nonconscious perception effects described in the second section, and I therefore concentrate on those theories which hold that masking directly disrupts higher processes involved in giving rise to consciousness.

Throughout the chapter a recurring and unifying theme will be the importance of phenomenological data, and I repeatedly argue that failure to address such data can impoverish or even mislead our theoretical understanding.

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