The influence of horizontal convergence on slow oscillatory eye movements during visual fixation.
OBJECTIVE
Slow oscillatory eye movements (SOMs) occur simultaneously with tremor, drifts, and microsaccades during visual fixation. In a previous study, the amplitude of SOM was found to be affected by the visual characteristics of the stimuli. This indicates a perceptual influence on the control of the movement. However, the frequency of SOM did not change. The aim of our study was to investigate how SOM is affected by extraocular muscle tension.


METHODS
In a repeated-measurement experiment, 14 subjects were instructed to maintain fixation for 3 minutes on a bright dot presented at four distances (15, 30, 60, and 120 cm). The level of extraocular muscle tension is assumed to increase with decreased fixation distance due to convergence angle. Eye movements were recorded binocularly using a video eye tracker, and the amplitude and frequency of SOM for each eye were obtained by independently filtering the horizontal and vertical eye position signals with a discrete Fourier transformation.


RESULTS
The results showed no significant differences for the amplitude. However, the horizontal frequency was found to be significantly lower at the closest distance. No significant differences were found for the vertical frequency.


CONCLUSIONS
Based on these findings we conclude that extraocular muscle tension does have an effect on the frequency, but not the amplitude, of the oscillations. The apparent double dissociation between perceptual effects on amplitude versus muscle tension effects on frequency is discussed in relation to the origin and control of SOM.
Department of Clinical Neuroscience, Division of Ophthalmology and Vision, Karolinska Institutet, Stockholm, Sweden.
Bing
B
Zhang
Roberto
R
Bolzani
Gustaf
G
Öqvist  Seimyr
Jan
J
Ygge
Tony
T
Pansell
D000328:Adult; D000368:Aged; D003290:Convergence, Ocular / Q000502:physiology*; D005133:Eye Movements / Q000502:physiology*; D005260:Female; D005403:Fixation, Ocular / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D009801:Oculomotor Muscles / Q000502:physiology*; D009991:Oscillometry; D015348:Vision, Binocular / Q000502:physiology; D055815:Young Adult
extraocular muscle; eye movements; muscle force; ocular convergence