This very good and important book confirms the arrival of a major new scientific discipline: cognitive neuroscience. The authors of its 92 chapters are among the most eminent in their fields, which include not only anatomy and physiology but also experimental psychology, linguistics, computer modelling and philosophy. The aim of cognitive neuroscience is to explain how the brain achieves its purpose of enabling an animal to interact successfully with the world. The term cognition has come to refer to that level of description that links brain physiology, on the one hand, to behaviour (and experience) on the other.
What entities do we talk about in the domain of the cognitive? One of the main functions of the brain is to "represent" things. Collections of representations in the brain are often referred to as "maps". A typical Ordnance Survey map is a good metaphor for a brain map. Such a map contains information about where things are by presenting the same spatial arrangements in miniature and also about what things are by using arbitrary signs for churches and trees. It has long been known that there is a map of the body spread along the primary motor cortex from the legs (at the top) to the mouth (at the bottom). Likewise, there is a map of the image on the retina to be found in the primary visual cortex. It is now clear that there are hundreds of maps within the brain, each representing something different. Many of the early chapters are concerned to show where in the brain a particular map is, what it represents and how this representation is achieved. The nature of these maps is determined by the nature of the animal in whose brain the map is found. For example, the barn owl hunts by sound. To catch its prey, the owl must be able to locate a sound very precisely in space. We are not surprised to learn that there is an area in the barn owl's brain where different neurons respond to sounds from different locations in space. These neurons represent the location of the sound, But how is this representation achieved? Like us the owl hears sounds through its two ears. Thus the only clues to the source of the sounds come from the difference in loudness and time of the sound reaching each ear. The map of the position of each sound is constructed from this information.
Given that perceptual maps are made from such minimal clues it is not surprising that there are many situations where what we see does not correspond to reality. In the Kanizsa diagram (see illustration below) we see a white triangle (point uppermost) on top of the other figures. This triangle appears to be whiter than the surround giving three distinct white on white edges. In reality these edges do not exist. They are illusory contours. There is now clear evidence that these illusory contours are represented in the brain. Neurons in the visual cortex of the monkey become more active when the part of the figure containing the illusory contour is moved across their receptive fields as long as the rest of the figure is being "seen" by other neurons. We also have a fairly good idea of how the system "computes" these illusory contours by bringing together representations from several earlier maps. Since the illusory contours are represented in the monkey's brain we can then infer that the monkey also "sees" illusory contours. Brain activity provides us with a window on the consciousness of others.
A major problem for cognitive neuroscience is that many functions cannot be studied in animals. Three key higher order functions are: mentalising (the ability to predict the behaviour of others from beliefs and intentions); language; and consciousness. There is good evidence that neither mentalising nor language can be observed in animals (other than humans) except in the most rudimentary form. Whether or not animals are conscious remains controversial largely because consciousness has so many different meanings. Study of the brain processes underlying these abilities has depended on finding patients with specific impairments caused by brain lesions. In the case of language, for example, there is much evidence from psychological studies of brain-damaged patients that there are separate representations associated with phonology, syntax and semantics. However, the associated lesions are typically large and ill defined and it has proved extremely difficult to show that these representations are associated with specific brain regions. Now that it is possible to study brain function in healthy volunteers using imaging techniques there are likely to be considerable advances in delineating human brain maps.
On the basis of the final section of this book there is a long way to go before consciousness can be "explained" in terms of brain function. However cognitive neuroscience has made enormous advances towards such an explanation. It is now possible to pose sensible questions about the relationship between brain and consciousness. From my reading of this book there are two fundamental alternatives. First, all representations within the brain are potentially available to consciousness. In this case we can ask about the nature of the extra process that renders them conscious. Second, only a subset of representations in the brain can ever become conscious. In this case we can ask about the difference between these two classes of representation. Answers to these questions must help to explain the riddle of the evolutionary purpose of consciousness.
On the basis of this book I am convinced that cognitive neuroscientists can and will answer these questions.
Chris Frith is professor of neuro-psychology, Institute of Neurology, London.
The Cognitive Neurosciences
Author - Michael S. Gazzaniga
ISBN - 0 262 07157 6
Publisher - MIT Press
Price - £64.95
Pages - 1,447