7 - Is there more than meets the eye?

Summary

In a recent review entitled ‘Why can't the eye see better?’, Horace Barlow (1986) discussed how optical factors, photoreceptor characteristics and the dynamic range of retinal ganglion cells limit visual performance. Barlow's question, like all good riddles, demands a shift in perspective before it can be tackled. In this essay we pose a complementary puzzle, ‘is there more than meets the eye?’ Although there is no doubt that much remains to be discovered about the retina, we question whether the apparent simplicity of retinal function critically underestimates the sophistication of early stages of visual processing.

The complexity of visual coding by retinal ganglion cells is often presented in terms of those receptive field characteristics that may provide the neural substrate for psychophysical phenomena. Thus the centre–surround organization of concentric cells is related to the coding of chromatic or luminance contrast; the On and Off pathways underpin the efficient signalling of increased light and darkness; the spatial and temporal properties of X and Y ganglion cells seem matched to the requirements of pattern vision and motion detection. Although this approach has inherent appeal, the sophistication that psychophysicists and central physiologists demand of retinal function is, in fact, rather limited.

Hierarchical concepts of visual processing deny to retinal function those characteristics that are presently perceived to be intrinsic to cortical function. For example, the long-range horizontal interactions between cortical modules are thought to underlie such diverse processes as vernier acuity and figure–ground discrimination. In the retina, however, the many amacrine connections beyond the classic receptive field are credited with little more than producing the periphery effect.