Regrowth of optic fibers and behavioral recovery after optic chiasm transection.

We studied the effect of optic chiasm midline transection on visually guided behavior and retinotectal fiber regrowth in frogs. After complete transection, frogs do not respond to visually presented prey and looming stimuli. Beginning about 2 months later there is recovery of visual function. However, unlike recovery after optic nerve transection, animals respond as if the stimulus were not at its actual position, but at the symmetric position in the contralateral field. For instance, if a prey stimulus is located 5 cm away from the recovered frog at an eccentricity of 40 degrees to the left of the midline, the animal will respond as if the stimulus were 5 cm away at 40 degrees right. Further, these animals typically respond to looming stimuli not by jumping away from the stimulus, but by either colliding with the stimulus or jumping toward the side from which the stimulus approaches. These behaviors persist throughout the testing period, up to 17.5 months postlesion. Electrophysiological recordings reveal that visual activity in the optic tectum is retinotopically organized but driven primarily by stimuli to the ipsilateral eye. HRP histochemistry reveals that some regenerated retinal fibers are found to cross at the midline of the chiasm. Thus, the midline is not impenetrable to crossing retinal fibers. Frogs with cut of 3/4 of the chiasm respond normally to prey stimuli initially but later respond as if the stimuli are at mirror image locations. In these animals most retinotectal fibers project to the ipsilateral tectum despite the presence of intact contralaterally projecting retinotectal fibers during the recovery period.