Cerebellar influences on saccade plasticity

Inaccurate saccades adapt to become more accurate. In this experiment the role of cerebellar output to the oculomotor system in adapting saccade size was investigated. We measured saccade adaptation after temporary inactivation of saccade-related neurons in the caudal part of the fastigial nucleus which projects to the oculomotor brain stem. We located caudal fastigial nucleus neurons with single unit recording and injected 0.1% muscimol among them. Two monkeys received bilateral injections and two monkeys unilateral injections. Unilateral injections made ipsiversive saccades hypermetric (gains >1.5) and contraversive saccades hypometric (gains approximately 0.6). Bilateral injections made both leftward and rightward saccades hypermetric (gains >1.5). During unilateral inactivation neither ipsiversive nor contraversive saccade size adapted after approximately 1,000 saccades. During bilateral inactivation, adaptation was either small or very slow. Most intact monkeys completely adapt after approximately 1,000 saccades to similar dysmetrias produced by intrasaccadic target displacement. After the monkeys receiving bilateral injections made >1,000 saccades in each horizontal direction, we placed them in the dark so that the muscimol dissipated without the monkeys receiving visual feedback about its saccade gain. After the dark period, 20-degree saccades were adapted to be 12% smaller, and 4-degree saccades to be 7% smaller. We expect this difference in adaptation because during caudal fastigial nucleus inactivation, monkeys made many large overshooting saccades and few small overshooting saccades. We conclude from these results that: (1) caudal fastigial nucleus activity is important in adapting dysmetric saccades; and (2) bilateral caudal fastigial nucleus inactivation impairs the relay of adapted signals to the oculomotor system, but it does not stop all adaptation from occurring.