The two hemispheres of the avian brain: their differing roles in perceptual processing and the expression of behavior

Abstract

The hemispheres of the avian brain are specialized to carry out different functions. Since each eye sends its input mainly to the contralateral hemisphere, birds respond differently to stimuli seen with the left eye than they do to stimuli seen with the right eye. The right hemisphere attends to novel stimuli, which easily distract it from ongoing functions. It assumes control in emergency or stressful conditions. The left hemisphere attends to learnt categories and controls behavior in routine, non-stressful situations. This division of function extends to processing of auditory, olfactory and even magnetic stimuli. Evidence for this comes from a number of avian species, and has been shown in both laboratory and field tests. Knowledge of these specializations is relevant to understanding the behavior of birds in the wild since birds respond in different ways to stimuli on their left and right sides (e.g. preferential response to predators and conspecific on the left side and to prey on the right side) and they choose to view different stimuli with the left or right eye. Individual differences in the strength of visual lateralization are determined by exposure of the embryo to light, versus incubation in the dark, and by the levels of steroid hormones in ovo. The importance of these influences on lateralization is discussed in terms of behavior in the natural habitat. The potential importance of hemispheric dominance in the welfare of birds is also considered.