Motor control by vision and the evolution of cerebral lateralization

Chicks (4 or 5 days old), which are able to use either eye freely, use the right eye (RE) preferentially in approach to a food dish when a lid, which has to be removed, is visible during approach. They use the left eye (LE) instead when no manipulation is required, but the same dish is similarly visible. The RE is also used preferentially in selecting food grains scattered over the floor; RE use in these two contexts is thus associated with visual control which brings the bill in planned contact with a visible target rather than with approach to a site where it is anticipated that feeding will occur. Zebrafish also use the RE preferentially when preparing to bite a target; during purely visual examination of the same target, this preference disappears. This evidence is used together with evolutionary evidence to support a new hypothesis for the origin of cerebral lateralization: paired anterior eyes evolved in filter-feeding ancestors of the vertebrates as part of the acquisition of prey catching. A key use for early vision was to predict likely contact with prey so as to inhibit reflexes of rejection and avoidance normally elicitated by tactile input to the mouth and so to allow ingestion. Innervation of mouth structures by the left side of the CNS caused control of mouth reflexes to become predominantly a left CNS affair. As visual abilities developed this starting condition meant that control of manipulation (which is by the mouth for most vertebrates) remained predominantly with the left side of the CNS.