Comparing frontal and lateral viewing in the pigeon. III. Different patterns of eye movements for binocular and monocular fixation.

The presence in the pigeon's retina of two areas of higher cellular density which we have shown mediate different visual functions, suggests the existence of two modes of fixation: a lateral monocular and a frontal binocular one. The participation of eye movements in these modes of fixation remained unexplored. We analyzed oculomotor behaviour in awake head-restrained pigeons by means of EOG and video film. Orienting saccades attaining up to 17 degrees from the resting positions could be elicited by presenting stimuli in different parts of the visual field. Two typical ocular patterns were consistently observed to the sudden presentation of large and novel stimuli: coordinated vergence of both eyes (even with one eye occluded) to stimulation within the frontal binocular field, and uncoordinated ipsilateral saccades to stimuli moving in one lateral field. Results point towards two different and reciprocally exclusive mechanisms of oculomotor control in the pigeon. The relevance of a trident mode of vision correlated to retinal organization and living praxis of some lateral-eyed vertebrates is discussed.