ANALYSIS OF VERTEBRATE EYE-MOVEMENTS FOLLOWING INTRAVITREAL DRUG INJECTIONS .4. DRUG-INDUCED EYE-MOVEMENTS ARE UNYOKED IN THE TURTLE

1. Eye movements of awake turtles were measured from both eyes simultaneously using two search-coil contact lenses. Optokinetic nystagmus (OKN) was evoked by full field patterns moving horizontally at different stimulus velocities. Intravitreal injections of either bicuculline or 2-amino-4-phosphonobutyrate (APB) were then made into one eye, after which eye movements were again recorded under similar stimulus conditions. Several days later, eye movements were again recorded and recovery was observed. 2. The effects of these two synaptic drugs on the optokinetic responses of the injected eye were similar to those previously reported in turtles, rabbits, and decorticate cats. APB, which blocks the retinal ON pathway, completely blocked visually evoked responses to any stimulus direction or velocity presented to the injected eye. On the other hand, the uninjected eye was still responsive to optokinetic stimuli. This difference between the eyes is consistent with the nonconjugate nature of OKN in the turtle. 3. After bicuculline application, the injected eye displayed a spontaneous nystagmus with its slow phase in the temporal-to-nasal direction. The movements of the injected eye were independent of stimulus direction or a range of stimulus velocities. During that effect, the eye contralateral to the injection still responded to visual stimuli in a direction- and velocity-dependent manner. For example, if the uninjected eye was exposed to optokinetic stimuli moving temporal-to-nasal, both eyes would then move in their respective temporal-to-nasal directions. This nonconjugate ocular behavior is similar to that seen when each eye of a normal turtle was exposed to its temporal-to-nasal stimulus. 4. When the eye contralateral to a bicuculline injection was occluded, it displayed a small spontaneous nystagmus temporally yoked to the injected eye. Alternatively, when presented with a nasal-to-temporal stimulus, the uninjected eye sometimes responded with an OKN gain above its control value. Thus, to a small extent, a drug-induced excitatory drive did affect the movements of the uninjected eye. This finding is also consistent with the small amount of ocular yoking found in this species during optokinetic stimuli. 5. Thus, after monocular injections of bicuculline or APB, the contralateral eye "behaved normally" in the turtle. After bicuculline, the uninjected eye moved as expected for a normal eye that is contralateral to an eye viewing a temporal-to-nasal stimulus. After APB, the uninjected eye moved as if the opposite eye were occluded. These similarities between drug applications and normal oculomotor behavior support the view that APB and bicuculline selectively affect the neural processing of visual image motion across the retina.