A patient has a right homonymous hemian-opia with saccadic pursuit movements and defective optokinetic nystagmus. The lesion is most likely to be in the

C i.e. Parietal lobe Optokinetic Nystagmus: If a white drum with veical black stripes is rotated in front of the eyes, the patient follows the stripe till it disappears & then switches back suddenly to pick up a new stripe. This reflex is abnormal in patients with congenital nystagmus. One may observe a paradoxical reversal of the optokinetic nystagmus response. Patients with horizontal nystagmus with unilateral hemispheric lesions, especially parietal or parietal-occipital lesions, show impaired optokinetic nystagmus when the drum is rotated toward the side of the lesion. The OKN drum may be used as an estimate of visual acuity. The striped drum is equivalent to a vision of counting fingers when held at a distance of 3-5 feet from the patient. The fuher the drum is from the patient, the better the visual acuity must be to respond normally to the moving drum.