Examining the paradoxical relation between number of spikes and gaze amplitude in abducens neurons

Abnormal activity of neurons in abducens nucleus of strabismic monkeys

PURPOSE

Infantile strabismus is characterized by persistent misalignment of the eyes. Mounting evidence suggests that the disorder is associated with abnormalities at the neural level, but few details are known. This study investigated the signals carried by abducens neurons in monkeys with experimentally induced strabismus. We wanted to know whether the firing rates of individual neurons are exclusively related to the position and velocity of one eye and whether the overall level of activity of the abducens nucleus was in the normal range.

METHODS

We recorded 58 neurons in right and left abducens nuclei while strabismic monkeys (one esotrope and one exotrope) performed a saccade task. We analyzed the firing rates associated with static horizontal eye position and saccades by fitting the data with a dynamic equation that included position and velocity terms for each eye. Results were compared to previously published data in normal monkeys.

RESULTS

For both strabismic monkeys the overall tonic activity was 50 to 100 spikes/s lower, for every suprathreshold eye position, than what has previously been reported for normal monkeys. This was mostly the result of lower baseline activity; the slopes of rate-position curves were similar to those in previous reports in normal monkeys. The saccade velocity sensitivities were similar to those of normal monkeys, 0.35 for the esotrope and 0.40 for the exotrope. For most neurons the firing rate was more closely related to the position and velocity of the ipsilateral eye.

CONCLUSIONS

These data suggest that strabismus can be associated with reduced neural activity in the abducens nucleus.

Gaze shift duration, independent of amplitude, influences the number of spikes in the burst for medium-lead burst neurons in pontine reticular formation

Changes in the direction of the line of sight (gaze) allow successive sampling of the visual environment. Saccadic eye movements accomplish this goal when the head does not move. Medium-lead burst neurons (MLBs) in the paramedian pontine reticular formation (PPRF) discharge a high frequency burst of action potentials starting ~12 ms before the saccade begins. A subgroup of MLBs rostral of abducens nucleus monosynaptically excites oculomotor neurons. The number of spikes in the presaccadic burst is correlated with the amplitude of the horizontal component of the saccade, and the peak discharge rate is correlated with peak eye velocity. During head-unrestrained gaze shifts, a linear relationship between the number of action potentials in MLB bursts and gaze (but not eye) amplitude has been reported. The anatomical connection of MLBs to motor neurons and the similarity between the phasic motor neuron burst and MLB discharge have raised questions about the usefulness of counting spikes in MLBs to determine their role in eye-head coordination. We investigated this issue using a behavioral technique that permits a dissociation of eye movement amplitude and duration during constant vector gaze shifts. Surprisingly, during gaze shifts of constant amplitude and direction, we observe a nearly linear, positive correlation between saccade duration and spike number associated with a negative correlation between spike number and saccade amplitude. These data constrain models of the oculomotor controller and may further define the time-dependence of hypothesized neural integration in this system.