The effect of central retinal lesions on optokinetic nystagmus in the monkey

In alert monkeys (Macaca mulatta and fascicularis) the effect of central retinal lesions on 'fast' optokinetic responses was investigated during high velocity optokinetic and visual-vestibular conflict stimulation. The 'fast' component of the optokinetic response manifests itself as a rapid rise in the slow-phase eye velocity after light-on, during high velocity optokinetic stimulation; and a sudden drop in eye velocity after light-off. In contrast, the 'velocity storage' component leads only to gradual changes in eye velocity during continuous optokinetic stimulation and after light-off (optokinetic after-nystagmus). Retinal lesions were placed by laser coagulation in and around the fovea. Responses of the normal and lesioned eye were compared. It was found that central lesions up to 12 deg (fovea diameter 6 deg) had only a negligible effect on 'fast' optokinetic responses. With lesions of more than 25-30 deg diameter centered on the fovea definite 'fast' responses could still be obtained, on average reduced to about 50% of the responses of the normal eye. Some monkeys showed initially no 'fast' optokinetic responses and had, therefore, to be excluded from lesion experiments. The results demonstrate that 'fast' optokinetic responses also can be obtained from extrafoveal areas, i.e. areas which are not generally involved in smooth pursuit eye movements. These results are discussed in relation to reports that the 'smooth pursuit' eye movement system is also used to generate 'fast' optokinetic responses.