Phoria adaptation to prisms. A cerebellar-dependent response.

Horizontal prism placement in normal subjects results in an apparent change in phoria by an amount equal in power and opposite in direction to the prisms used. However, the measured phoria gradually returns toward the value preceding the placement of prisms. This is termed phoria adaptation. This response was measured in five normal subjects and five patients with cerebellar deficit. The magnitude of the response was significantly reduced in the patient group. Visual acuities and fusional reserves were comparable in the two groups. The mean age was greater, and the stereoscopic acuity decreased, in the patient group. These factors were unlikely to account for the reduced response in patients. The results suggest that the phoria adaptation response in normal subjects depend on the integrity of the cerebellum.