Sensitive periods, environmental triggers, and development of a lateralized brain

Lateralization is widespread among vertebrates and the domestic chicken provides an excellent model to investigate the processes involved in its development. Exposure to light just prior to hatching triggers developmental processes that lead to lateralization of the visual pathways and to a range of visual behaviors after hatching. This outcome depends on the orientation of the embryo in the egg (occlusion of the left, and not the right, eye). Chicks hatched from eggs incubated in the dark do not develop visual lateralization. Steroid hormone levels pre-hatching interact with the effect of light (e.g., elevated levels of corticosterone prevent lateralization from developing). These pre-hatching events, occurring during a brief sensitive period, channel development of phenotypes that are likely to enhance survival in different environments. Visually lateralized chicks are able to perform more than one task simultaneously (forage and monitor for predators) with ease, whereas non-lateralized chicks have difficulty in dividing their attention effectively. In a natural context, development of lateralization would be triggered by a series of events beginning with the amount of stress hormone that the hen deposits in her eggs, this itself depending on her social position and other stressors in the environment. Then, just prior to hatching, the embryo's endogenous levels of corticosterone, triggered by stress (e.g., becoming cold), would take effect as too would light exposure, determined by the hen vacating the nest (2 hr is sufficient). Hence, a series of events channel brain development toward lateralization (low stress plus light) or non-lateralization (stress and no light). [Australian Research Council.]