Seeing can be misbelieving

Is our experience of the visual world all that we think it is? Recent work in the philosophy of consciousness has drawn on empirical work in psychology to argue that there is a dissociation between the sensory experience of the visual world and our conscious experience of the same visual world. Contrary to many people's belief (or so we are asked to believe), our (visual) sensory experience is patchy and incomplete. Yet we become aware of these deficiencies in our sensory experience only in rare, contrived or experimental situations. This is the grand illusion to which the title of this collection of 14 articles by philosophers of cognitive science refers.

One popular demonstration of the deficiencies of conscious experience is change blindness, the fact that observers can fail to notice quite dramatic changes in a scene or situation, such as the colour or style of an actor's hair, even when they are attending to parts of that situation. What makes change blindness interesting is that people are frequently surprised when it is demonstrated to them. Daniel Levin refers to this as blindness to change blindness, and argues that it is just one aspect of visual metacognition. In reality, metacognitive tasks of all kinds (for example, relating to memory or judgement, as well as vision) frequently lead to surprising results. Jonathan Cohen suggests that the grand illusion - that our conscious experience of the visual world diverges from our perceptual experience - is more of a "banal surprise". I sympathise with this view.

The problem with my opening question is it assumes that there is a consensus on what we think our experience of the visual world is.

Proponents of the grand illusion view appear to assume that we agree that our conscious experience of the visual world is a rich, detailed reflection of the environment, and that change blindness shatters this illusion. Those who disagree take exception to the assumption of conscious experience being rich and detailed. Daniel Dennett is right to argue that the surprise expressed by naive participants when they encounter change blindness supports the assumption. Does that surprise also support a grand illusion, or is it more banal?

The real issue is whether change blindness and related phenomena have any bearing on questions concerning the nature of consciousness. Several authors suggest that they do, arguing that these phenomena support one or other variant of Dennett's multiple drafts theory of consciousness. In fact, there are arguments either way. Change blindness could easily be viewed as supporting a Cartesian Theatre view of consciousness, in which the homunculus happens to be looking at (that is, attending to) the wrong part of the screen at the time the change occurs. What change blindness confirms is simply that our visual experience is not uniformly rich and detailed. At best, it is only the part we are attending to that is rich and detailed.

While change blindness is the preoccupation of most contributors, several subsidiary themes relating to the nature of consciousness emerge. Some authors pay particular attention to the notion of a stream of consciousness. Susan Blackmore argues that there is nothing "stream-like" about consciousness. Others caution against the use of first-person reports. Eric Schwitzgebel notes the lack of evidence pointing to any correlation between self-reported visualisation abilities and abilities with tasks that are supposed to involve visualisation (such as imagining a scene and performing some manipulation of the imagined scene). His conclusions reiterate the unreliability of first-person reports, and lead to a position approaching neo-behaviourism.

Consciousness remains a contentious topic. It is only in recent years that it has re-emerged as scientifically respectable, but that re-emergence has itself been contentious. Many contributions to consciousness debates have been ill informed or overly subjective. This volume does not suffer from these deficiencies. The common thread of change blindness ensures that all contributions are grounded in at least one empirically robust effect. There is no doubt that this grounding advances the quality of debate about the nature of visual experience. It is less clear if the contributions advance the consciousness debate itself. Certainly the lack of any form of consensus, even on which questions are of primary concern and which questions have been answered, remains a worry - a worry that the collection fails to address.

Richard Cooper is reader in cognitive science, Birkbeck College, University of London.