Critical limiting factors in the design of the eye and visual cortex

The main factors limiting the performance of the peripheral parts of the visual system can be specified, and doing this clarifies the nature of the interpretive tasks that must be performed by the central parts of the system. It is argued that the critical factor that hinders development of better resolving power is the difficulty of confining light within the waveguide-like outer segment, and that for sensitivity this critical factor is the thermal decomposition of photosensitive pigments. Knowledge of these limits makes many surprising details of the eye intelligible. Understanding the difficulties posed by the narrow dynamic range of nerve fibres may give similar insight into the coding of the retinal image for transmission to the brain. Our level of understanding changes when we come to the visual cortex, for although we do not lack good anatomical and neurophysiological data, these do not make the principles of operation self-evident in the way that the structure of the eye immediately suggests that it is an image-forming device. The cortex converts the representation of the visual field that it receives into reliable knowledge of the world around us, and the trouble may be that we lack good models of how this can be done. A system that can respond to single quanta and resolve almost to the diffraction limit is unlikely to employ grossly inefficient methods for those higher functions upon which its whole utility depends, and so it is worth seeking out the limiting factors. The quality of human performance at certain higher perceptual tasks is high compared with the limit of reliable statistical inference; hence mnuch of the sample of information available in a visual image must be effectively utilized. But there are strong limitations on the connectivity in the cortex, so that one is forced to consider how the relevant information can be collected together. Three stages of dealing with the visual image are proposed: the improvement of the cortical map in primary visual cortex by processes analogous to spatial and temporal interpolation; the detection of linking features in this map; and the concentration of this information by non-topographical mapping in adjacent visual areas.