Much of the most exciting recent scientific work on consciousness has come from the study of vision. Visual consciousness is many ways the easiest aspect of consciousness to study: it is ubiquitous, highly structured, it has properties that are easy to articulate, it is relatively easy to monitor, and we have a reasonably good understanding of its basis in the brain. The fields of neurophysiology, neuropsychology, and cognitive psychology have all contributed to the recent explosion of work on visual consciousness.

Within neurophysiology, there has been much work on isolating the neural correlates of visual consciousness. The structure of visual pathways is becoming well-understood, and we have reached a point where it is reasonable to ask which structures are most closely associated with consciousness. Research on monkeys and other mammals has focused on the question of whether the correlates of visual consciousness are located in primary visual cortex (V1) or later visual areas. Intriguing recent work on binocular rivalry by Logothetis and colleagues (e.g., Logothetis and Leopold 1998) suggests a special role for inferior temporal cortex (IT). And much attention has been focused on the ventral stream of processing, as discussed below.

Within neuropsychology, there have been intense studies of syndromes that involve a disruption of visual consciousness, and in particular that bring about dissociations between visual consciousness and action. The best-known of these syndromes is blindsight, in which subjects are seen to respond accurately when forced to guess about stimuli that they say they have no conscious awareness of. Syndromes such as visual agnosia and optic ataxia also suggest intriguing dissociations. Knowledge of the neural basis of these syndromes has given several important clues about the brain basis of consciousness.

Within cognitive psychology, there has been much attention to the capacities of unconscious visual perception. More broadly, there has been a good deal of psychophysical work on dissociations between information available for verbal report and that available for control of action, in various implicit processes. And there has been much recent research on the extent of visual consciousness, with a number of studies suggesting that we are not visually conscious of as much as we think we are.

Most of these directions are explored in the chapters in this section. In this way we can see how neurophysiology, neuropsychology, and cognitive psychology complement each other to yield a powerful tool in the study of conscious experience.

The chapter by David Milner and Mel Goodale gives an overview of their important work on visual pathways. It is well-known that vision in the brain involves at least two main pathways, the ventral and dorsal streams. Milner and Goodale (1995) have suggested that the central stream is devoted to cognitive representations of objects and events, which is largely conscious, and that the dorsal stream is devoted to the online control of action, which is largely unconscious. This fits much evidence from neuropsychology, including some dissociation syndromes that Milner and Goodale discuss. It also meshes with work from cognitive psychology on dissociations between motor action and verbal report, and provides a suggestive guide to the neural basis of visual consciousness.

The chapter by Yves Rossetti forms an interesting complement to this work, by discussing psychophysical evidence for "immaculate" motor representations that are not affected by top-down cognitive influences. Insofar as there is such evidence, the case for separate pathways for conscious identification and motor action is strengthened.

The contribution by Kentridge, Heywood, and Weiskrantz discusses the relation between awareness and attention in a subject with blindsight. Intriguingly, it turns that that the subject can direct his attention to stimuli with positive results, even when he is not conscious of the stimuli. This suggests that attention and consciousness may be less closely tied than is often thought to be the case.

At the Tucson III conference, Petra Stoerig presented some recent work on functional magnetic resonance imaging studies of blindsight, showing that there is activity in the ventral as well as the visual stream when a subjectÕs blind field is stimulated. This unconscious ventral activation might be seen as a challenge to the Milner and Goodale hypothesis. In his introduction to the Blindsight and Vision section here (reprinted from Trends in Cognitive Sciences), David Milner addresses Stoerig's work as well as that of Kentridge et al., and discusses how they relate to the Milner/Goodale hypothesis.

Vittorio Gallese discusses the extraordinary recent discovery of "mirror neurons" in area F5 of the monkey premotor cortex. These neurons fire when a monkey performs a certain action, and they also fire when the monkey perceives someone else perform that action. Gallese also discusses evidence that such mirror systems exist in humans, and speculates on possible functional roles for mirror systems in social interaction and the origin of language.

In recent years, there has been a swelling of evidence for what is sometimes called the "grand illusion" hypothesis: that we are visually conscious of much less than we think we are. One source of evidence involves eye movements: our eyes are constantly focusing on different locations, but we usually do not notice this and have the impression of a constant and detailed visual field. In his contribution, Frank H. Durgin gives evidence suggesting that subjects' eye movements can become sensitive to information of which the subject is not consciously aware. This suggests another way in much visual processing is unconscious.

Finally, Lianggang Lou discusses another intriguing relationship between attention and visual consciousness. It seems that in certain circumstances, phenomenology indicates that paying attention to a stimulus can cause it to "fade" from visual consciousness. Lou backs this up with psychophysical evidence that this fading affects attended stimuli rather than unattended stimuli. This counterintuitive phenomenon has implications for various theories of attention, consciousness, and their neural bases, some of which Lou discusses.

References
Logothetis, N. K. and D. A. Leopold. 1998. Single-neuron activity and visual perception. In S. Hameroff, A. Kaszniak, and A. Scott (Eds.) Toward a Science of Consciousness II. MIT Press.

Milner, A. D., and M.A. Goodale. 1995. The Visual Brain in Action. Oxford University Press.