Clinical Review

The consciousness of sight

BMJ 1998; 317 doi: https://doi.org/10.1136/bmj.317.7174.1696 (Published 19 December 1998) Cite this as: BMJ 1998;317:1696
  1. Adam Zeman, consultant neurologist
  1. Department of Clinical Neurosciences, Western General Hospital, Edinburgh EH4 2XU

    We have known since before the first world war that area 17, at the occipital pole of the brain, contains a map of visual space.1 Stimulation of the brain at a given spot in the cortical map reliably excites a visual sensation at a corresponding point in space; damage will reliably extinguish vision there. The American neuroscientists David Hubel and Torsten Wiesel won the Nobel prize for physiology in 1981 for showing that some of the neurones in this map detect the edges and corners which are the elements of visual forms.

    The simple picture of the striate cortex as a visual map that analyses shapes has been greatly embellished since then. Area 17 performs not one visual task but several: blobs of the visual cortex rich in the enzyme cytochrome oxidase are specialised to receive information about colour, while the areas between the blobs separate at least two further streams of information, describing motion, form, and depth. The explanation for so much parallel processing in the visual cortex was supplied by another major discovery.

    Surrounding the striate cortex, and receiving the streams of visual information which flow from it, are upwards of 30 further maps of the visual world (fig 1). This comes as something of a shock, given that our everyday visual experience strikes us as being unified and orderly. Why are they there?

    FIG 1

    Major visual areas of monkey brain. Sulci are arrowed. 11 (middle temporal) corresponds to area V5. Adapted from Douglas et al2

    Summary point

    The visual world is mapped in as many as 30 visual areas beyond area V1, the primary visual cortex

    There is a division of labour among these areas: area V4 specialises in …

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