The Central Code of Sight

Abstract

From bipolar cells of the retina, through the ganglion cells, the neurons of the lateral geniculate nucleus, and the simple-field cortical cells, the neurons of the visual pathway have receptive fields organized into excitatory and inhibitory zones. Between lateral geniculates and the cortex two changes take place: one is the predominance of binocular convergence in the cortex, and the other is the change of the shape of the optimal stimulus from round to elongated (or rectangular). In species which are able to distinguish colors, many of the ganglion cells and of the neurons of the lateral geniculate respond in reciprocal manner to the opponent pairs of wavelengths, red and green, or yellow and blue, respectively. In some rodents (but not carnivores or primates) some retinal ganglion cells respond specifically to stimuli moving in a definite direction. Direction-specific ceUs were found in the visual cortex of all mammals where they have been sought, and in greater abundance in the superior colliculi of the midbrain. In the primary visual cortex there are, beside the simple-field cells the so-called ‘complex cells’ or tilt-detectors.’ In the peristriate area no simple cells are found any more, only those with complex and hypercomplex receptive fields.

In the visual pathway, as in audition, the further one advances in the synaptic cascade, the more restrictive become the ‘feature-detecting’ properties of individual neurons.