Journey beyond the retina

Few scientists have done more for popularising science than Richard Gregory. His book Eye and Brain, translated into 30 or so languages, has been one of the most readable accounts of vision and his Oxford Companion to the Mind must be on the shelves of every person even remotely interested in philosophy and in the workings of the mind and brain. His exploratorium in Bristol, supported by private funds though not by the government, is a marvel for children and adults alike. It is a great pity that Gregory has not been given a high position in government to deal with scientific affairs.

But that omission may be for the best, for it has allowed him to turn his prodigious erudition to the pursuit of scientific experiments and writing. The range and depth of that erudition are on display in this book - which, ostensibly at least, is about mirrors. Anyone interested in mirrors will find a great deal about the uses to which mirrors have been put through the ages and in different cultures, in art and literature and, of course, in science.

Some of the most exciting chapters, in which Gregory displays an enviable capacity to think originally and write simply, are the ones on the traditional subject of the physics of light. But Gregory gives a more exhilarating overview of this topic than one would find in even elementary textbooks on physics because he is able to look at age-old problems, such as reflection and refraction, in a new way. He manages to intrigue his reader with a host of questions. The reason for this ability to keep the reader interested is simple - Gregory's book is only superficially about mirrors. There is a much more potent thread linking the diverse chapters on mirrors - and that is perception. Perception is what the book is really about. Mirrors are only used as a vehicle for a much broader enquiry that ranges from physics to perception, which gives the book its strength and makes a unified sense of questions as diverse and intriguing as: Why do mirrors reverse left right but not up down? Do monkeys know that they are looking at themselves in mirrors? Are neutrinos left-handed? The reader must be eclectic to appreciate Gregory's intellectual wanderings.

To him, both the elementary seeing we experience all day, as well as the "seeing" of theories in physics, are active processes. It is this wider context which allows him to delve into the physics of light in an approachable way, since even the theory behind the laws of physics - as Gregory argues persuasively with an appeal to Albert Einstein, Richard Feynman and others - is rooted in perception.

The view that seeing is an active process often comes as a surprise to many people owing to a long history of a superficial resemblance between the structure of the eye and that of a camera leading them to equate the two and to suppose that the reception by the brain of the visual "image" imprinted on the retina results in seeing, a passive process like the capture of a scene by a photographic plate. Indeed this was what neurologists believed for the better part of a century, ever since neurology began as a serious scientific discipline in the latter part of the last century. What we now accept as the "seeing" cortex was bitterly fought over for many years and neatly, though without much evidence, separated from the "understanding" cortex.

Artists were probably wiser. It was, after all, Henri Matisse who, long before the modern neurobiologists, said: "Seeing is already a creative act." It is difficult to discern from this book to what extent Gregory dissociates himself from the neurologists and from the artists. I myself adhere to the view that activity in specialised cortical areas leads to understanding and seeing in direct relation to the capacities of the areas, and that there is often no need to appeal to higher faculties of knowledge. In this I disagree with Gregory, and I would have liked to have learned more about the different levels of "top-down" perception that may exist in his schema. The use of illusions may be one way of studying this problem, but I feel there are many more levels of illusion than Gregory allows for here; a notion that would be interesting to consider, even if only to dismiss in the end.

If perception is a backbone, lending strength to this book, it is also the cause of some weaknesses. The topic of nature versus nurture is a highly interesting one, critical to many perceptual problems and philosophical issues, but Gregory's discussion of it is perhaps too brief and one-sided; he says little of the many important conclusions reached through the work of Freud in psychoanalysis, of Harry Harlow and Robert Hinde in experimental psychology and of David Hubel and Torsten Wiesel in physiology. This is a pity because it would have reinforced the book's general view of the active role the brain plays in perception, and would also have taken into account the developmental stages after birth in which the emphasis is on nourishing a genetically endowed and ready-to-use-at-birth apparatus.

One also senses that some of the topics in the last chapter sit uneasily, perhaps because they have been introduced as an after-thought in a climate of opinion where an opinion about consciousness is de rigueur. But the discussion here on consciousness is not satisfactory, as is often the case. Roger Penrose is dismissed yet again, though not very persuasively, and the controversial syndrome of "blindsight" - to see without being aware of having seen - is introduced without any discussion of the intricacies and difficulties that have made the syndrome difficult for many people to accept.

These are, however, minor faults in a new and original way of using mirrors to give an exciting and provocative view of perception. Gregory should be congratulated for producing a book that will bring joy and understanding to many, and which deserves to be placed in every library and on every bookshelf devoted to perception.

Semir Zeki is professor of neurobiology, University College London, and co-head of the Wellcome department of cognitive neurology.