Degradation of swept-parameter VEP responses by neutral density filters in amblyopic and normal subjects

To determine whether objective visual function, measured by swept-parameter visual evoked potential (sVEP), is preferentially degraded by neutral density filtration (NDF) in normal control and fellow eyes compared to amblyopic eyes, and to determine whether the response to NDF is a function of stimulus type, using grating and vernier stimuli. Monocular Snellen acuity and both grating and vernier sVEP responses were measured in each eye of 23 children or adolescents with amblyopia and 21 visually and neurologically normal children or adolescents. Acuity and sVEP responses were measured with and without a 2.0 log unit neutral density filter before the viewing eye. Suprathreshold sVEP grating responses were more sensitive than vernier to degradation by amblyopia in the unfiltered state and to NDF-induced preferential degradation of responses from fellow and normal control eyes. For threshold measurements, on the other hand, vernier responses were more sensitive to degradation by amblyopia in the unfiltered state and to NDF-induced preferential depression. Threshold vernier responses of amblyopic eyes were paradoxically enhanced by NDF. Neutral density filtration causes preferential degradation of both threshold and suprathreshold sVEP responses in normal control eyes and fellow eyes of amblyopes, compared to amblyopic eyes. The degradation is stimulus specific and dependent upon whether threshold or suprathreshold responses are measured. Grating responses are more likely to identify suprathreshold abnormalities, while vernier stimuli are more likely to detect threshold abnormalities. These findings may be used to optimize the stimulus parameters and design of future studies utilizing evoked potential techniques in amblyopic subjects.