The effect of abnormal fixational eye movements upon visual acuity in congenital nystagmus.
OBJECTIVE
The purpose of this study was to investigate the role that abnormal eye movements play in the degradation of visual acuity.


METHODS
Visual acuity was measured monocularly in 10 normal subjects (26.7+/-4.3 years) and 5 subjects with congenital nystagmus (34.9+/-8.8 years), using Regan Repeat Letter charts (RRL) and a logMAR based test (LogMAR Crowded Acuity Test (CAT)) while eye movements were continuously recorded using a commercially available infrared limbal eye tracker (Type 54, Optoelectronic Developments, UK). The eye tracker was controlled via a virtual oscilloscope (Viewdac, Keighly Instruments, UK) on an IBM PC clone (Opus Technology 486).


RESULTS
The mean visual acuity obtained with RRL was significantly higher than that obtained by CAT in the subjects with congenital nystagmus. A significant correlation was found between the root mean square value of the nystagmus waveform and the angular extent of CAT. Linear regression analysis revealed a correlation between the duration of the foveation periods and the linear acuity of the subjects with congential nystagmus. The nystagmus waveforms also demonstrated increased amounts of high frequency components (HFC: >3.0 Hz) when compared to the normal eyes.


CONCLUSIONS
The results of this study reveal 1) RRL charts provide a measure of a subjects resolution limit which is less dependent on eye movements; 2) the duration of foveation periods has a significant effect on visual acuity measurements obtained using a linear test chart format; 3) the predominance of high frequency components in the congenital nystagmus waveforms lead to short foveation periods adding to the degree of fixation instability.
Simmers
A J
AJ
Visual Perception Unit, Department of Psychology, University of Essex, Colchester, UK. anitas@essex.ac.uk
Gray
L S
LS
Winn
B
B
D000328:Adult; D005133:Eye Movements; D005260:Female; D005403:Fixation, Ocular; D006801:Humans; D008297:Male; D009759:Nystagmus, Pathologic / Q000151:congenital* / Q000503:physiopathology; D015835:Ocular Motility Disorders / Q000503:physiopathology*; D014787:Vision Tests; D014792:Visual Acuity / Q000502:physiology