Altering the direction of optokinetic head nystagmus: a lesion study and a hypothetical model.

Optokinetic head nystagmus (OKN) was evoked in frogs in an optokinetic drum with vertically moving horizontal black and white stripes. The nature of the normal OKN was determined, then either one eye was removed or the basal optic root (BOR) was transected unilaterally. Eye removal did not influence the direction of the head movements. After transection of the basal optic root the animals showed oblique or horizontal nystagmic head movements during stimulation in the vertical plane. Transection of the ipsilateral BOR in monocular frogs, or removal of the eye ipsilateral to BOR transection enhanced the occurrence of horizontal head movements. Because in normal animals the pretectum mediates signals for horizontal nystagmic movements of the head, it is concluded that in certain experimental situations the pretectum will drive the optokinetic system, and in spite of stimulation in the vertical plane, horizontal head movements occur.