CEREBELLAR CONTROL OF POSTURAL SCALING AND CENTRAL SET IN STANCE

1. The effects of cerebellar deficits in humans on scaling the magnitu de of automatic postural responses based on sensory feedback and on pr edictive central set was investigated. Electromyographic (EMG) and sur face reactive torques were compared in patients with anterior lobe cer ebellar disorders and in normal healthy adults exposed to blocks of fo ur velocities and five amplitudes of surface translations during stanc e. Correlations between the earliest postural responses (integrated EM G and initial rate of change of torque) and translation velocity provi ded a measure of postural magnitude scaling using sensory information from the current displacement. Correlations of responses with translat ion amplitude provided a measure of scaling dependent on predictive ce ntral set based on sequential experience with previous like displaceme nts because the earliest postural responses occurred before completion of the displacements and because scaling to displacement amplitude di sappeared when amplitudes were randomized in normal subjects. 2. Respo nses of cerebellar patients to forward body sway induced by backward s urface displacements were hypermetric, that is, surface-reactive torqu e responses were two to three times larger than normal with longer mus cle bursts resulting in overshooting of initial posture. Despite this postural hypermetria, the absolute and relative latencies of agonist m uscle bursts at the ankle, knee, and hip were normal in cerebellar pat ients. 3. Although they were hypermetric, the earliest postural respon ses of cerebellar patients were scaled normally to platform displaceme nt velocities using somatosensory feedback. Cerebellar patients, howev er, were unable to scale initial postural response magnitude to expect ed displacement amplitudes based on prior experience using central set . Randomization of displacement amplitudes eliminated the set effect o f amplitude on initial responses in normal subjects, but responses to randomized and blocked trials were not different in cerebellar patient s. 4. Cerebellar patients compensated for hypermetric responses and la ck of anticipatory scaling of earliest gastrocnemius activity by scali ng large, reciprocally activated tibialis and quadriceps antagonist ac tivity with the displacement velocity and amplitude. Correlations betw een these antagonist EMG integrals and displacement amplitudes were pr eserved when amplitudes were randomized, suggesting that feedback-depe ndent and not set-dependent mechanisms were responsible for scaling of antagonists by cerebellar patients. Antagonist compensation for initi al hypermetric responses also could be induced in normals when they ov erresponded to unexpectedly small amplitudes of surface displacements. 5. The major effects of anterior lobe cerebellar damage on human post ural responses involves impairment of response magnitude based on pred ictive central set and not on use of velocity feedback or on the tempo ral synergic organization of multijoint postural coordination. Thus th e anterior lobe of the cerebellum appears to play a critical role in m odifying the magnitude of automatic postural responses to anticipated displacement conditions based on prior experience. This study suggests that the midline cerebellum tunes the magnitude of somatosensory loop s for maintenance of stance posture by adjusting the threshold or bias , and not the slope or gain, of automatic postural responses.