Cerebellar-dependent adaptive control of primate saccadic system

Area of Science:

• Neuroscience
• Ophthalmology
• Motor Control

Background:

• The central nervous system exhibits remarkable adaptability in compensating for motor deficits.
• Saccadic dysmetria, characterized by inaccurate eye movements, can arise from peripheral or central nervous system issues.

Purpose of the Study:

• To investigate the role of the cerebellum in the central nervous system's compensation for saccadic dysmetria.
• To examine how cerebellar ablations affect the adaptive mechanisms underlying saccade accuracy and postsaccadic stability.

Main Methods:

• Rhesus monkeys underwent tenectomy to weaken eye muscles, inducing saccadic hypometria and postsaccadic drift.
• Eye movements were monitored during fixation tasks in a rotating magnetic field.
• Cerebellar lesions (total and partial) were performed, and their effects on saccadic behavior and adaptation were assessed.

Main Results:

• Monkeys demonstrated compensation for surgically induced eye muscle weakness, normalizing saccade amplitude and eliminating drift.
• Total cerebellectomy abolished adaptive saccade compensation, leading to persistent hypermetria and drift.
• Partial cerebellar lesions (vermis, paravermis, fastigial nuclei) impaired saccade amplitude adaptation but not drift elimination, suggesting specific cerebellar roles.

Conclusions:

• The cerebellum plays a crucial role in adjusting the neural commands (pulse and step) for accurate saccadic eye movements.
• The midline cerebellum is vital for repairing saccadic dysmetria, while other cerebellar regions, potentially the flocculus, may be involved in correcting postsaccadic drift.