Dual coding of visual asymmetries in the pigeon brain: the interaction of bottom-up and top-down systems

The pigeon's visual system is an excellent model to investigate the ontogenetic and the neuronal foundations of cerebral asymmetries. Before hatching, lateralized visual stimulation induces structural asymmetries within the tectofugal pathway during a critical time window. Interhemispheric control mechanisms emerge presumably after hatching and stabilize these induced asymmetries. Once established, visual asymmetry in pigeons displays a left hemispheric dominance for complex learning and discrimination tasks and unravels how the interplay between bottom-up and top-down mechanisms generate a lateralized, hemispheric-specific visual analysis. The ascending visual (tectofugal) pathway displays cell size asymmetries and directs more bilateral visual information towards the left hemisphere. This bottom- up system is controlled by telencephalic top-down projections, which affect intra- and/or interhemispheric inhibitory systems in a presumably lateralized manner. Such a flexible organization allows the control of information transfer depending on the visual input and hence adapt the dominant processing mode to environmental requirements.