Responses of ipsilateral retino-tectal (type I1) units of the frog (Rana esculenta) to moving configurational bars.

In frogs, retinal information projecting to the ipsilateral optic tectum uses a complex, at least bi-synaptic, route. Ipsilateral visual units recorded at the tectal level correspond to isthmic axon terminals. For a better approach of their visual function, these units have been stimulated with moving (V = 7.6 degrees/sec) configurational stimuli proved earlier to be able to elicit classical behavioural sequences in amphibians. In the presence of W("worm-like")-stimuli of increasing length (2 degrees < L < 20 degrees), the discharge rate of type I1 units remains rather constant. In response to A("antiworm-like")-stimuli, the discharge rate first increases up to L = 5-6 degrees and then decreases continuously. The ability of these units to discriminate bars of equal dimension but of different configuration was defined using the "contrast-like" formula originally proposed by Ewert et al. (1978). The relationship between the discrimination factor D(W, A) and the length of the stimuli is similar in shape to that found in class R3 ganglion cells. Results suggest thus that the classical functional homology between type I1 ipsilateral units and class R2 retinal neurons is inadequate.