Prehatching visual experience and lateralization in the visual Wulst of the chick

We revealed functional lateralization of the chick visual system by placing injections of monosodium glutamate (0.5 microl, 100 mM) into the left or right Wulst regions of the hemispheres, and examined the effects of light experience before hatching on this lateralization. Following exposure of the left or right eye to light for 24 h beginning on day 18 of incubation, the chick's ability to categorize grain as distinct from pebbles was impaired by glutamate treatment of the Wulst contralateral to the exposed eye. Attack and copulation scores were also elevated. Following incubation of the eggs in darkness or with both eyes exposed to light, treatment of neither the left or right Wulst affected performance on the pebble-grain task: showing that either the left and right Wulst can assume control of this function. Treatment of either the left or right Wulst of these chicks elevates attack and copulation. However, examination of the distribution of attack scores revealed a bimodality in the attack scores of the chicks treated with glutamate in the right Wulst and not those treated in the left Wulst. In summary, light stimulation of one eye during a critical period of embryonic development causes the visual Wulst contralateral to the light exposed eye to develop dominance over its equivalent region in the other hemisphere. Without this lateralized stimulation of light both the left and right Wulst regions are largely but not exactly equivalent.