Effects of Fixation Stability on Multi Focal VEP (mfVEP) Responses in Strabismic and Anisometropic Amblyopes

Purpose:: In previous mfVEP studies, responses to central stimulation of the amblyopic eye were reported to be substantially depressed. However, amblyopes are known to exhibit considerable difficulties in keeping steady fixation. In this study, therefore, we compared the mfVEP responses and the stability of fixation for the same strabismic and anisometropic amblyopes to determine whether the fixation pattern of amblyopes may have any effect on VEP responses. Methods:: Four amblyopes (2 strabismic and 2 anisometropic), who received comprehensive examinations of their visual capacities, served as subjects. The mfVEP recording was made with standard methods in our laboratory. Fixation stability of amblyopes was quantified using a dual Purkinje image eye tracker and visual stimuli identical to those used for the VEP recording experiments. Results:: The mfVEP responses to stimulation of the central 1-2 degrees were substantially depressed in the amblyopic eye of all subjects compared to those in the non-amblyopic eye. In some subjects response latencies were also abnormal in the amblyopic eye. Fixation while viewing with the amblyopic eye was much less stable than while viewing with the better eye, some exhibiting jerk nystagmus and/or saccadic oscillations ranging in amplitude of 0.5 and 2.0 degrees. The magnitude of deficits in mfVEP was largely correlated with that of fixation instability exhibited by these subjects. Conclusions:: Although amblyopes are known to exhibit anomalous responses to central visual stimulation in psychophysical tests, mfVEP recording, and fMRI, the interpretation of results must take the effects of anomalous fixation during measurements into consideration in order to reveal the true nature and extent of sensory neural deficits in amblyopes resulting from early abnormal visual experience