Book review of Richard Ivry & Lynn Robertson

Richard Ivry & Lynn Robertson:
THE TWO SIDES OF PERCEPTION (MIT Press, 1998)

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The central nervous system of all organisms is remarkably symmetrical. That symmetry ends somewhere inside the mammalian brain, as lesions to the same location in different brain hemispheres have proven, particularly the neurological disorder of aphasia (the loss of language skills, always related to damage to the left hemisphere, a correlation first recognized by the German neurologist Karl Wernicke in 1874, and the loss of spatial skills, related to damage tothe right hemisphere, a correlation first recognized by the British neurologist Hughlings Jackson in 1876). The asymmetric function of the mammalian brain is a puzzle because no physical difference has been observed between the two hemispheres. Split-brain research conducted by Roger Sperry and Michael Gazzaniga showed that both hemispheres are capable of performing the same processing. The American neurologist Lynn Robertson argues that it is a difference in strength rather than in kind. There is a tiny difference in the way the two hemispheres process early perceptual information, the very first data coming from the senses, and that difference gets amplified as it is processed in the two hemispheres. Robertson's theory is that we understand the world at different levels. The world is made of objects that are made of objects. We perceive, recognize and understand objects at all levels of the hierarchy, and we do so in parallel: we do not build a scene from its parts, and we do not split a scene into its parts. Our perception of the world is a cooperative process of both aspects. We recognize both a scene and its parts, because recognition proceeds simultaneously at different scales. Our perception is performed by "frequency-sensitive detectors" in the brain. The right hemisphere is better at analyzing low-frequency information, such as spatial information (objects), whereas the left hemisphere is better at processing high-frequency information, such as language. This asymmetry generates a distributed representation of the world that may have been advantageous for evolutionary purposes. This holds for both vision and audition. THe right hemisphere is better equipped for high spatial-frequency patterns as well as high sound-frequency patterns. And viceversa. The reason there are no visible differences before the two hemispheres is that they perform the same processing on the same patterns of information. The difference is subtle: each hemisphere has a bias for a range of frequencies.

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