Differential contributions of the two visual pathways to functional lateralization in chicks

The contribution of the two visual pathways to lateralization of visual behaviour in chicks was assessed using unilateral injections of 0.5 microliters of 100 mM monosodium glutamate into localized regions of the forebrain. Chicks treated with glutamate in the left visual hyperstriatum made more errors in a visual discrimination task (pebble-floor test) than did chicks treated in the right visual hyperstriatum. Glutamate injection into the left visual hyperstriatum also elevated attack and copulation scores, but this did not occur following injection of the right visual hyperstriatum. The performance of chicks treated in the right visual hyperstriatum did not differ from that of sham-operated controls. Thus, only the left visual hyperstriatum is involved in the control of these three visually guided behaviours. By contrast, glutamate injections of the left ectostriatum affected only the attack behavior and not performance in the pebble-floor test or copulation responses. Glutamate treatment of the right ectostriatum had no affect on any of the behaviours tested and this was also the case for glutamate treatment of both the left and right neostriata. Although injecting glutamate in a larger volume that allows glutamate to spread over a wide area of the left hemisphere is known to retard auditory habituation, localized injection of glutamate in the areas chosen for this study had no effect on auditory habituation. The results suggest that the tectofugal and thalamofugal pathways have different roles in the lateralization of visual functions. The forebrain region which receives input from the thalamofugal visual system has a lateralized role in categorising pebbles as different from food grains, and also a role in controlling attack and copulation responses. The forebrain region which receives input from the tectofugal visual system is involved in the control of attack responses only.