Hysteresis and slow drift in abducens unit activity.

Area of Science:

• Neuroscience
• Ophthalmology
• Motor Control

Background:

• The abducens nucleus controls horizontal eye movements.
• Understanding neural activity during saccadic eye movements is key to motor control.
• Hysteresis in neural firing rates has been debated.

Purpose of the Study:

• To quantify hysteresis in abducens motoneurons during saccadic eye movements.
• To investigate the variability of neural firing rates over time.
• To explore the implications of hysteresis for oculomotor control.

Main Methods:

• Recording of abducens unit activity in trained monkeys during horizontal saccades.
• Eliciting standardized 20-degree saccades to the primary eye position.
• Analyzing steady-state firing rates in relation to saccade direction and history.

Main Results:

• Averaged steady-state firing rate depended on saccade direction, indicating hysteresis.
• Population hysteresis measured 5.4 spikes/s (5.5% of primary position rate).
• Slow, correlated variations in firing rates were observed across units, possibly due to cocontractive noise.

Conclusions:

• Hysteresis in abducens neurons exists and is quantifiable.
• The observed hysteresis is small and may be negligible for many applications.
• The oculomotor system likely employs compensatory mechanisms for this neural hysteresis.