Visual-motor adaptation. Quantitative demonstration in patients with posterior fossa involvement.

Short-term visual-motor adaptation to magnifying spectacle lenses was studied in normal subjects and in patients with nonacute posterior fossa lesions. When normal subjects, looking through magnifying lenses, pointed open loop to targets without viewing their hands, they initially underestimated the distance (magnification effect). After a 20-minute close-loop training or adaptation exposure period during which they viewed the performance of their hands, a modified visual-motor scheme evolved, compensating for about half of the lens-induced pointing error (adaptation effect). Removal of the lenses after adaptation caused open-loop, overshooting pointing errors (adaptation after-effect). Four patients with remission of cerebellar signs showed normal visual-motor adaptive performance, evidence of ability to recalibrate gain. One patient with persisting cerebellar ataxia was unable to recalibrate gain during close-loop visual-motor training. His history of transient palatal myoclonus implicates a role for the cerebellar-olivary system in calibration of visual-motor gain.