Review of Natural Computation
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AI Magazine Volume 13 Number 2 (1992) (© AAAI) Book Reviews BookReviews Neurons, Perception, cussion of viewpoint in the novel! machinery is identified in the brain, Further, the essays themselves stay less and less space is left for general- and Communication firmly within the territory of the purpose computation. authors’ expertise: They are either In “What Does the Brain See? How Philip Swarm overviews of the state of the art or a Does It Understand?” Horace Barlow Images and Understanding: Thoughts piece of work in progress at the time presents the anatomy of the brain, about Images, Ideas about Understand- of invitation. I found that the only visual pathways, cortical maps, and ing, H. Barlow, C. Blakemore, and M. way to make sense of the volume was the structure of receptive fields for Weston-Smith, eds., A collection of to deconstruct it and reassemble the individual neurons. This system essays based on a Rank Prize Fund’s essays according to their original dis- transforms the raw image projected International Symposium, organized ciplines, hence my title and the order onto the retina into a cortical image with the help of Jonathan Miller and that I follow in this review. The stated that has been filtered and restruc- held at the Royal Society in October interdisciplinary claims of the editors tured in many ways. However, as it is 1986, Cambridge University Press, are further weakened by the absence held in the primary visual cortex, this Cambridge, United Kingdom, 1990, of discussion among the participants image is still largely a meaningless 401 pp., ISBN O-521-34177-9 (cloth), themselves; they do not even say if bitwise representation. How does the ISBN O-521-36944-4 (paper). such discussion took place. There- brain find objects and relations in fore, it is left to the reader to judge if the image (rather, in sequences of This volume is a well-written, infor- the whole is greater than its parts, images), or as Barlow puts it, how mative, and thought-provoking col- which is what I try do in this review. does the brain understand what it lection of essays that should interest sees? It is a measure of progress made anyone concerned with the psychology Neurons that this question no longer seems of vision and visual communication. silly, and Barlow’s answer seems plau- The aim of the original symposium Starting with the pioneering (and sible. He suggests that the cells in the was to bring together people from Nobel Prize-winning) work of David primary visual cortex “grow into the arts and sciences who could pre- Hubel and Tosten Wiesel, neurobiol- other cortical areas where they create sent different perspectives on the ogists have made remarkable progress new patterns in which the informa- subject of images and understanding. in uncovering the structure and func- tion is brought together according to The result is an informal tour con- tion of the visual system in higher new principles that are not necessari- ducted by leading specialists (pre- animals. The results of this work ly related to the topography of the dominantly British) that visits both have important implications for our original image” (p. 21). Thus, the pri- famous scientific battlefields and understanding of the brain as a whole. mary area carries out local feature quaint artistic backwaters. Numerous Writing in this collection, Colin detection by way of selective sensitiv- striking pictures enliven the book: Blakemore quotes the steering com- ity to texture, motion, and so on, but Here you can find the sensory somat- mittee of the 1986 Systems Develop- these secondary areas synthesize new ic cortex of a bat, the British miners’ ment Foundation Symposium on higher-level images according to leader Arthur Scargill in full rant, a Computational Neuroscience. The Gestalt-type principles. notation for ballet, a mole used to question under consideration is as Blakemore (“Understanding advertise British Gas, instructions for follows: Is physical locality an essen- Images in the Brain”) tackles the dif- righting a caravan, and many others. tial part of neural computation? ficult question of functional interpre- The 23 essays originated in diverse According to position 1, “Brain func- tation in a way that complements disciplines, among them biology, tion is determined by the logical and Barlow’s presentation. He points out psychology, art history, and linguis- dynamic connection properties of its that just because cells in the visual tics. They are grouped according to neurons. The actual physical struc- cortex are laid out as isomorphic the sessions in which they were pre- ture, location, architecture, and maps of the sensory cells in the sented under thematic titles: The geometry is irrelevant to its logical, retina, it does not follow that the Essence of Images, Movement, Narra- connectionist aspects.” In contrast, brain actually uses the information tion, Making Images, Images and for position 2, the anatomical struc- provided by the map (as opposed to Thought, and Images and Meaning. ture of the brain “may represent a the information carried by the indi- However, the desire to have a good major mode of brain function: the vidual cells and their connections). interdisciplinary mix in each session formatting of sensory data in a Indeed, the maps might be a simple renders the already generic session manner which simplifies its further artifact: “an inevitable but useless titles meaningless; for example, John processing” (p. 262). The neurobio- consequence of the fact that nerve Krebs’s contribution on animal lan- logical contributions in this collec- fibres, growing from the sense organs guage is included in the Narration tion all strongly support position 2: to their targets in the brain, tend to session following David Lodge’s dis- As more and more special-purpose preserve the same spatial pattern as SUMMER 1992 97 that of the receptors in the sense principles proposed by Barlow and (p. 107). organ” (p. 267). In other words, rear- Blakemore. David Perret and his co- Although the neurological work ranging the cortical cells in a random authors in “Three Stages in the Clas- has obtained the most spectacular pattern, while maintaining their con- sification of Body Movements by results, evolutionary biologists are nections, would have no effect on Visual Neurons” offer a superb also working on the visual system. In the information carried: Such a prop- demonstration of what the monkey “Tricks of Colour,” John Mollon erty would be characteristic of a con- brain can achieve at the higher levels begins with the instructive history of nectionist model of the brain. of visual processing. They have the theory of color vision. It was the Blakemore is not a connectionist, recorded the response of individual polymath Thomas Young who point- and he offers several lines of argu- neurons in the temporal cortex to ed out in 1801 that trichromacy is a ment to show that the information various kinds of visual stimuli. It property of our visual system, not in the spatial patterns of neurons in turns out that neurons in this area the physical world; he even guessed the cortex is actually used by the are selective for specific kinds of correctly that there are just three brain. His first argument is that there objects, movements, and events; in classes of receptor cell in the retina are cortical maps that are not isomor- other words, visual knowledge is hard tuned to select for specific subranges phic. Edge-detecting cells in the visual wired. By studying the responses of (blue, green, and red) of the visible cortex are one example. Together with hundreds of such cells, the authors spectrum. Recent work has shown the functional connections between established some of the main subpop- that the biology of the color visual maps, this is strong evidence that the ulations. They found that there are system is even more surprising than brain is interpreting the maps them- cells selective for the six canonical its structure: Asymmetries in the selves as well as their constituents. translations in three dimensions (3- trichromatic system suggest that “our Blakemore’s second line of argu- D) and point out that at least in colour vision depends on two rather ment appeals to biological design English, we have names for them and different subsystems, one recently constraints. First, he suggests that only them (up, down, left, right, overlaid on the other” (p. 67). The maps arise naturally from the low- toward, and away). Similarly, there ancient system is dichromatic (blue, level structure of the brain: “Most of are cells selective for one of the six green) and is found in most mam- the connections in the brain are orthogonal views of the head: “Thus mals and some other lower animals; short, local fibres, such that cells are information about body view and it is insensitive to spatial detail and either excited or inhibited by their direction of motion appears to be provides almost purely chromatic close neighbours” (p. 276). Second, segregated into channels of process- information. The more recent trichro- Blakemore claims that topographic ing which use the same three-axes matic system, found in humans and maps facilitate the process of putting system” (p. 96). other Old World primates, is appar- maps together into higher-level struc- Further investigation is revealing ently the result of a duplication of tures. Third, he argues that the genet- even higher levels of specialization. the gene that coded the long-wave- ic specification for building the brain The authors have strong evidence length pigment (green) in the ancient must be small, so the procedural rules that although most neurons are selec- system. Further, the new dimension are likely to be simple; for example, tive according to the viewer’s frame of color is parasitic on an edge- “Make all nerve cells inhibit their of reference, others actually respond detecting system of cells and, conse- immediate neighbors on either side” with respect to an actor- or object- quently, is sensitive to spatial detail.