The Eye-Closing Response of Pecking Pigeons

Abstract

The visual control of pecking is believed to be restricted to the fixation pauses which precede the final down-thrust of the head. The latter movement is assumed to be performed under feed-forward control only, a hypothesis that is mainly supported by the finding that pigeons gradually close their eyes when the head moves downward. However, recent findings suggest that visual feedback contributes to the control of pecking motions. Visual information collected during the final down-thrust of the head can modulate the motion in progress. We could further show that, contrary to widespread opinion, the eyelids of pigeons are not completely closed during head motions but that they are narrowed to a slit. The width of this slit is predominantly determined by the optical environment in which the pecks are executed. Visual responses of eyelid slit and pupil-opening are closely interlocked. Here we describe experiments aimed to increase the knowledge of the interactive functioning of these two optical diaphragms. Interactions of slit and pupil responses can be influenced by systemic injections of apomorphine. Central or peripheral dopaminergic mechanisms may be involved in the regulation of the two diaphragms. Atropine application has variable effects on ocular responses depending on the drug concentration and the luminance conditions. One of the most prominent findings is that atropine exclusively affects the diaphragm which regulates the depth of field. It is concluded that pupil opening and eyelid slit are controlled by a shared neuronal network.