Response of the infant rat to light prior to eyelid opening: mediation by the superior colliculus.

Although the influence of light on the infant rat before Day 14, when its eyelids open, are presumed to be minimal, we have shown that as early as postnatal Day 6, male and female albino rats demonstrate negative phototaxis. Bilateral enucleation eliminates negative phototaxis, whereas unilateral enucleation does not. Rearing in the dark does not prevent the expression of the behavior. Knife cuts on postnatal Day 5, which undercut the dorsal midbrain involving the superior colliculus, prevent the expression of this response to light. Unilateral dorsal midbrain knife cuts and posterior cortex lesions have no effect. These results call attention to an early period in the postnatal development of the rat when it is capable of exhibiting negative phototaxis. The superior colliculus in the dorsal midbrain appears to be crucial for demonstrating this behavior.