Lesion of the isthmo-optic nucleus impairs target selection for visually guided reaching

Neurons in the isthmo-optic nucleus (ION) of the avian midbrain receive input predominantly from the ipsilateral optic tectum and project to the contralateral retina. Therefore, lesion in the ION removes tectal efferent signals to the retina in birds. In the present study, we trained Japanese quail to reach a target stimulus on a touch-sensor monitor with their beak. ION lesions significantly decreased response accuracy when the target was presented simultaneously with distractors but not when presented alone. Thus the removal of the tectal efferent signals to the retina severely impaired target selection in a competitive visual search as if the ION-lesioning caused visual extinction, which is a neuropsychological deficit seen in human hemispatial neglect patients. We suggest that the avian tectum and its efferent path to the retina improve the accuracy of target selection for visual orienting and visually guided reaching with beak.