When during horizontal saccades in monkey does cerebellar output affect movement?

Area of Science:

• Neuroscience
• Oculomotor control
• Cerebellar function

Background:

• The caudal part of the cerebellar fastigial nucleus (CFN) is known to influence horizontal saccades.
• Previous research suggests contralateral CFN activity aids saccade acceleration and ipsilateral CFN activity aids deceleration.

Purpose of the Study:

• To precisely characterize the role of the CFN in saccade control by examining eye rotation during four distinct saccade phases after unilateral CFN inactivation.
• To refine the understanding of how ipsilateral and contralateral CFN activity differentially modulate saccade dynamics.

Main Methods:

• Unilateral inactivation of the caudal fastigial nucleus (CFN) in a model system.
• Quantification of eye rotation during specific phases of ipsiversive and contraversive saccades before and after inactivation.

Main Results:

• Inactivation of one CFN increased total eye rotation during ipsiversive saccades (towards the inactivated side) by approximately 1.8 times normal, with the most significant increase (4.4x) occurring during the later deceleration phase.
• Inactivation decreased eye rotation during contraversive saccades (away from the inactivated side) during early phases (1-3) to 0.7x normal, with normal rotation in phase 4.
• These findings indicate the ipsilateral CFN normally suppresses eye rotation, particularly near the end of a saccade, while the contralateral CFN enhances rotation during acceleration and early deceleration.

Conclusions:

• The caudal fastigial nucleus (CFN) exerts differential control over saccade phases, with contralateral influence dominant early and ipsilateral influence dominant later in the saccade.
• The findings suggest a smooth transition in CFN dominance throughout the saccade, challenging previous discrete functional localization.
• The described pathway involving contralateral interpositus-nucleus-of-the-abducens (IBN) neurons and the abducens nucleus can explain the observed effects of CFN inactivation on saccades.