Integration of visual stimuli by the crayfish central nervous system.

It has been found that integration of visual stimuli in optic nerve fibres can take place in ways which, although unexpected, reflect the types of information used by the animal to react to those environmental changes which are of specific importance for it. The findings in the frog by Barlow (1953) and by Lettvin, Maturana, McCulloch & Pitts (1959) were the first which made this principle clear. Previous work of a similar nature in the cat, though showing two types of visual processing, was less in contrast with the view that most integration takes place only at higher levels (e.g. Kuffier, 1953). It may be that in the higher mammals, but not in, for example, the rabbit (Barlow & Levick, 1965), more complex integration is limited to the brain. A study performed in 1954 (Waterman & Wiersma, 1963) of unit responses in the optic nerve of crustaceans showed the presence of very complexly integrating elements. This was later confirmed by experiments in the Hawaiian swimming crab, Podophthalmus (Bush, Wiersma & Waterman, 1964; Waterman, Wiersma & Bush, 1964; Wiersma, Bush & Waterman, 1964). Since the optic nerve in all crustaceans so far studied is a connexion between the brain (supraesophageal ganglion) and four ganglion layers near the retina in the eyestalk, highly complicated reactions can be expected, especially in view of the relatively small number of neuronal elements of the arthropod nervous system (Wiersma, 1957). Recent work using needle electrodes to record from the optic nerve of the crayfish has again revealed many fibre classes, only some of which respond to visual stimuli (Wiersma & Yamaguchi, 1966). Among the types found were primary sensory fibres, visual intemeurones originating in either eye, mechanoreceptor intemeurones with inputs from diverse and often extensive body regions, and highly complexly integrating units. In that paper the differences between the members of any one class and the total number found were presented, together with a preliminary description of their general properties. The purpose of this paper is to give a more extensive description of the properties of several of the visually responsive fibre types, including two optomotor fibres. The descriptions are based on responses obtained from those cases in which the evoked spikes were particularly prominent for a fibre of the class studied.