Effect of unilateral cerebral cortical lesions on ocular motor behavior in monkeys: saccades and quick phases

Abstract

Saccades and quick phases of vestibular and optokinetic nystagmus were quantitated using the magnetic-field search coil technique before and during 1 yr after unilateral decortication in three rhesus monkeys. Saccades were examined during several different behavioral conditions: spontaneous saccades made in the light and in the dark; intentional saccades including visually guided saccades to a target light, predictive saccades to a target light stepped to a predictable location, and target-searching saccades when the monkey was rewarded to find and fixate the target light located in the defective visual hemifield; and reflexive saccades made to novel visual, auditory, and tactile stimuli. Quick phases of nystagmus and spontaneous saccades could be initiated immediately postoperatively, although those initiated away from the side of the lesion were reduced in amplitude and rarely moved the eyes into contralateral craniotopic space. Intentional and reflexive saccades could not be initiated away from the side of the lesion during the first postoperative week. Visually guided saccades and reflexive saccades to stationary or moving visual stimuli in the defective visual hemifield never recovered. Target-searching and predictive saccades directed away from the lesioned side recovered but were generated in a staircase pattern; those saccades from orbital positions further into craniotopic space on the side opposite the lesion had progressively higher latencies and smaller amplitudes. The amplitudes of visually guided saccades to targets stepped into the normal visual hemifield were increased in amplitude by approximately 15% but slowly returned to near preoperative values by 20 wk. Pure vertical visually guided saccades to targets stepped or moved in the vertical direction were not generated throughout the postoperative period. Instead, the animals generated oblique saccades, tilted 10-15 degrees toward the side of the lesion. Velocities of saccades and quick phases were significantly reduced at all amplitudes both away from (approximately 37%) and toward (approximately 22%) the side of the lesion. This deficit diminished with time but velocities were still low one yr postoperatively. Our results suggest that cortical areas in one hemisphere are involved in the initiation of contralaterally directed intentional and reflexive saccades but not in the initiation of spontaneous saccades or quick phases. In time, other structures can initiate contralaterally directed intentional and reflexive saccades, except those guided by vision.(ABSTRACT TRUNCATED AT 400 WORDS)