Binocular vision and accommodation in prey-catching chameleons

Abstract

We have studied the role of accommodation and binocular convergence in the predatory behaviour of two chameleon species (Chamaeleocalyptratus, C. dilepis). Accommodation measurements support earlier observations that accommodation is the major distance cue. Specifically, accommodation speed (60 D s⁻¹), amplitude (45 D) and precision (no significant under-accommodation) were superior to those of other terrestrial vertebrates. Similar to other vertebrates, accommodation was accompanied by a prominent pupillary constriction (pupillary near response). Accommodation could be coupled or uncoupled in both eyes, depending on the experimental situation or the phase of the predatory behavioural sequence. Uncoupled accommodation occurred:

1. During scanning saccadic eye movements for prey detection. Only one eye accommodated appropriately, the other adopted a hyperopic resting refractive state. Attention switched from one eye to the other at approximately 1-s intervals.

2. During initial stages of distance estimation.

Coupled accommodation only occurred shortly before the tongue shot. Coupling was demonstrated by either covering one eye with a lens or covering one eye with an infrared light transmitting cut-off filter which still permitted refraction to be measured. In both cases the amount of accommodation was identical in both eyes. Search-coil measurements showed that the angle of convergence of both eyes is too variable to permit triangulation or to provide the basic requirement for stereopsis (matching corresponding points). We conclude that coupling of accommodation serves to improve accommodation precision rather than to permit stereopsis.