Our eyes make continuous movements even when attempting to fixate a stationary
object. The involuntary eye movements occurring during fixation are referred to as
fixational eye movements. There is a general agreement on three types of eye
movements occurring during visual fixation: tremor, drift and microsaccade.
Previously, in a 20-minute long recording of eye position during visual fixation, we
observed a slow periodic fluctuation in the eye position signal. The characteristics
of this fluctuation did not match any of the three known components normally
occurring during visual fixation on healthy subjects.
The hypothesis in this thesis has been that the slow fluctuation found in the
recording is a never before described, fourth component of the fixational eye
movement system. The aim of the project has been to test this hypothesis and to
further elaborate the underlying control mechanism of this slow eye movement
component. Study 1: The main purpose of this first study was to prove that the
fluctuation was not an artifact due to recording system, sampling frequency or
filtering technique. The analysis procedure to filter out the fluctuation was
developed and tested. Study 2: The influence of different visual stimuli was
evaluated to investigate the perceptual influence on the oscillation. Study 3: The
effect of increased extraocular muscle tonus was explored during a convergence
task.
The results indicate that a slow oscillatory movement occurs during visual fixation
together with tremor, drift and microsaccade. This slow eye movement has a low
frequency of 0.04 to 0.10 Hz and amplitude of less than 0.50 degrees. The
underlying control mechanisms for amplitude and frequency are different: visual
perception seems to have an effect solely on the amplitude whereas varying muscle
force only changes the frequency.
It has not been the purpose to describe the underlying reason for why the eyes
oscillate and we still have no clear idea of why this oscillation is present in normal
healthy subjects. We speculate that the light energy in the optical focus is
distributed in an oscillatory mode around the fovea that might be a retina
protective mechanism
