Prisms bend light toward their base and, when placed in front of the eyes, cause a subject to deviate his gaze to view a target. An arm movement toward the target is in the direction of the gaze, misses the target, and is visually perceived as an error.
Subsequent movements fall closer to the target as the subject adapts.
A study of 27 subjects with neurologic lesions and age-matched controls aided in localizing neural structures involved in prism adaptation. Throwing data were analyzed in a manner to measure independently impairments in performance and in adaptation. Prism adaptation was impaired in humans with lesions in the territory of the posterior inferior cerebellar artery, with generalized cerebellar atrophy, with inferior olive hypertrophy, and with lesions of the ipsilateral inferior or middle cerebellar peduncles or contralateral basal pons. These subjects had little or no incoordination. The data indicate that prism adaptation of throwing is a complex motor adaptation involving dynamic coordination across multiple body segments and joints and is capable of being learned in a manner similar to skill acquisition.