The effect of visual cortex lesions on vertical optokinetic nystagmus in the cat.

Vertical optokinetic nystagmus (VOKN) was measured before and after visual cortex lesions using the magnetic search coil technique. In normal cats, upward motion elicits higher slow phase gain than downward motion, especially at high stimulus velocities. Following decortication, the gain of upward VOKN may be attenuated by as much as 35% at stimulus velocities of 20% or more. This high velocity deficit in upward VOKN is consistent with the results of single unit studies of the lateral terminal nucleus (LTN) of the accessory optic system which suggest that visual cortical input (via the LTN) makes a direction specific contribution to the motion sensitivity of VOKN.