Visual acuity and hemispheric asymmetries in pigeons.

Pigeons and domestic chicks tested under monocular conditions in pattern distinction tasks generally show higher discrimination performances with the right eye seeing. At least two different mechanisms could mediate this asymmetry: the dominance of the right eye could either be due to hemispheric differences in the cognitive operations performed during these tasks, or may reflect a lateralization in the spatial frequency discrimination capacity of the left and the right eye system. The aim of the present study was to decide between these two hypotheses. Therefore nine adult homing pigeons (Columba livia) were tested with the left or the right eye seeing in a visual acuity task using high-contrast square wave gratings. Visual acuity, defined as the spatial frequency at which the psychometric function crossed the 75% correct line was virtually identical for the two eyes with 6.6 for the left and 6.4 c/deg for the right eye. Thus, visual lateralization as demonstrated in various pattern discrimination tasks seems not to depend on asymmetries in acuity but probably reflects hemispheric differences in the visual processing mode.