Role of the tectal and posterior commissures in lateralization of the avian brain.

We report a role for the tectal and posterior commissural system (TC/PC) in functional lateralization of the chicken brain. Unlike the corpus callosum of mammals which is thought to generate lateralization, the TC/PC in the chicken brain is shown to suppress lateralization. The TC/PC was sectioned on day 2 post-hatching. The performance of this TC/PC-sectioned group on a battery of monocular, behavioural tests was compared to a sham-lesioned group and an untreated group. The first of these tests measured pecking and fear responses to a small bead. In both the sham-lesioned group and the untreated group there was no change in the pecking response over repeated presentations of the bead, nor was there a left eye/right eye difference in either of these groups. In contrast, the TC/PC-sectioned group showed increased pecking of the bead each time it was presented. This increase in the pecking response was significantly greater for those chicks tested using the right eye than for those tested using the left eye. That is, sectioning the TC/PC caused a lateralized response which was not seen in either of the control groups, demonstrating that the intact TC/PC suppresses lateralization at the tectal level. In the other tasks (latency to peck at a mealworm and a visual discrimination learning task) the untreated controls showed clear left eye/right eye differences while the sham-lesioned group and the TC/PC-sectioned group showed no lateralization. It is suggested that for these two tasks the loss of lateralization is due to lesioning an area in the midbrain but not to sectioning the TC/PC.