 optokinet nystagmu occipit lesion case present invalid rule horizont symmetr optokinet respons indic vascular etiolog patient occipit lobe type hemianopia convers rule experi valuabl observ asymmetri horizont optokinet respons accompani occipit type field defect strong indic mass lesion optokinet nystagmu help corrobor evid diagnos occipit lobe tumor rule 