Role of the cerebellar posterior interpositus nucleus in saccades I. Effect of temporary lesions.

The ventrolateral corner of the cerebellar posterior interpositus nucleus (VPIN) contains many neurons that respond during saccades. To characterize the VPIN contribution to saccades, I located this area in three monkeys with single-unit recording and injected the GABA(A) agonist muscimol among saccade-related neurons there to reduce or eliminate neural activity. I compared the size, direction, velocity, and duration of saccades recorded before and after a unilateral injection in all three monkeys. In two of three monkeys, I also examined saccades after bilateral injection. After unilateral VPIN inactivation, upward saccades were abnormally large (avg. across all 3 monkeys = 112% of normal) and downward saccades were abnormally small (avg. across all 3 monkeys = 94% of normal). In the two monkeys tested, bilateral inactivation increased these abnormalities. Upward saccades went from 111% of normal size in these two monkeys after unilateral inactivation to 120% after bilateral inactivation; downward saccades went from 97 to 86%. VPIN inactivation caused changes in saccade gain and did not add of a constant offset to saccades. (The 1 exception was upward saccades in 1 monkey in which both gain and offset changed.) Neither uni- nor bilateral VPIN inactivation consistently affected the size of horizontal saccades (uni- avg. = 101% normal; bi- avg. = 97% normal). In two of the three monkeys, saccades to horizontal targets angled significantly upward after VPIN inactivation (uni- avg. = 3.6 degrees above normal, bi- avg. = 10.3 degrees above normal). The velocities of horizontal saccades were not strongly affected, but downward saccades exhibited abnormally low peak velocities and long durations. Upward velocities were inconsistently changed. I interpret these results to mean that the activity of some VPIN neurons helps drive the eyes downward and the activity of others helps drive the eyes upward. The downward drive outweighs the upward drive. The net effect of VPIN inactivation is to deprive all saccades of a downward component and to slow downward saccades.