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 t he VPIN contribution to saccades, I located this area in three monkeys with single-unit recording and injected the GABA(A) agonist muscimol among sacc ade-related neurons there to reduce or eliminate neural activity. I compare d the size, direction, velocity, and duration of saccades recorded before a nd after a unilateral injection in all three monkeys. In two of three monke ys, I also examined saccades after bilateral injection. After unilateral VP IN inactivation, upward saccades were abnormally large (avg. across all 3 m onkeys = 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 i nactivation caused changes in saccade gain and did not add of a constant of fset to saccades. (The 1 exception was upward saccades in 1 monkey in which both gain and offset changed.) Neither uni- nor bilateral VPIN inactivatio n consistently affected the size of horizontal saccades (uni- avg. = 101% n ormal; bi-avg. = 97% normal). In two of the three monkeys, saccades to hori zontal targets angled significantly upward after VPIN inactivation (uni- av g. = 3.6 degrees above normal, bi-avg. = 10.3 degrees above normal). The ve locities of horizontal saccades were not strongly affected, but downward sa ccades exhibited abnormally low peak velocities and long durations. Upward velocities were inconsistently changed. I interpret these results to mean t hat the activity of some VPIN neurons helps drive the eyes downward and the activity of others helps drive the eyes upward. The downward drive outweig hs the upward drive. The net effect of VPIN inactivation is to deprive all saccades of a downward component and to slow downward saccades.