Nonvisual complex spike signals in the rabbit cerebellar flocculus

Area of Science:

• Neuroscience
• Ophthalmology
• Vestibular System

Background:

• Floccular complex spikes (CSs) are known to convey retinal image slip signals.
• Emerging evidence suggests CSs also carry nonvisual information relevant to eye movements.

Purpose of the Study:

• To investigate the nature and characteristics of nonvisual signals carried by floccular Purkinje cell CSs in rabbits during vestibular stimulation in darkness.
• To determine how these nonvisual signals interact with visual and vestibular inputs in the context of the vestibulo-ocular reflex (VOR).

Main Methods:

• Recorded eye movements and Purkinje cell CS activity in awake rabbits undergoing sinusoidal oscillation in the dark.
• Stimulated with frequencies from 0.2 to 1.2 Hz and velocity amplitudes from 6.3 to 50°/s.
• Analyzed CS modulation amplitude and phase response relative to head velocity and position.

Main Results:

• Over 75% of Purkinje cells exhibited nonvisual CS modulation, with relatively constant amplitude across stimulus ranges.
• The phase of nonvisual CS modulation differed from VOR in light and shifted systematically with frequency and head velocity, indicating nonlinearity.
• Nonvisual CS modulation appears to represent an internally derived eye movement error signal.

Conclusions:

• Floccular CSs encode both visual (retinal slip) and nonvisual (internally derived) error signals.
• The interplay of these signals in the inferior olive contributes to a net error signal for VOR adaptation.
• The role of CSs in VOR adaptation models must be broadened to include these nonvisual error components.