Development and function of lateralization in the avian brain.

The avian brain is functionally lateralized. Different strategies of choice (within and between modalities) are adopted by each hemisphere. Visual lateralization has been studied most but attention to auditory, olfactory and magnetic cues is also lateralized. The left hemisphere (LH) focuses on cues that reliably separate pertinent stimuli from distracting stimuli (e.g. food from pebbles, odour cues from attractive visual cues, magnetic cues from other cues indicating location), whereas the right hemisphere (RH) has broad attention and is easily distracted by novel stimuli. The RH also controls fear and escape responses, as in reaction to predators. Exposure of the embryo to light just before hatching, when the posture adopted occludes the left eye (LE) but not the right eye (RE), leads to the development of asymmetry in the visual projections to the pallium and enhances the ability of the RE/LH to inhibit attention to distracting visual cues and of the LH to inhibit the RH, but has no effect on the RH's interest in novelty. Exposure to light before hatching has both short- and long-term consequences that are important for species-typical behaviour and survival. For example, on a food search task with a predator presented overhead, dark-incubated chicks perform poorly on both aspects of the task, whereas light-exposed chicks have no difficulty. Steroid hormone levels prior to hatching modulate light-dependent development of asymmetry in the visual projections and consequently affect neural competence for parallel processing and response inhibition. Differences between lateralization in the chick and pigeon are discussed.