The signals were subjected to Fourier analysis to extract the amplitude and phase of the evoked response at the first harmonic amplitude or F1 (10 Hz) and second harmonic amplitude or F2 (20 Hz). The analyses determined the asymmetry index and the presence or absence of a “bow tie” by a vector average of at least 10 trials in each eye.
Symmetric and asymmetric mVEPs yield distinct Fourier response spectra. A symmetric mVEP, which reflects equally strong cortical responses to rightward and leftward directions of motion, produces a response spectrum that is composed predominantly of even multiples of the stimulus frequency (e.g., F2, F4, F6). An asymmetric mVEP, which reflects cortical response in only one direction of motion, yields a response spectrum that is composed predominantly of odd harmonic multiples of the stimulus frequency (e.g., F1, F3, F5). Furthermore, the interocular phase of the asymmetric mVEP determines whether each eye responds to the same or different directions of motion. For instance, if both eyes respond to identical directions of motion, they will have the same phase. Conversely, if each eye produces a strong asymmetric mVEP to opposite directions of motion, the interocular phase difference of the F1 response will approximate 180°, and this will appear as a bow tie on a polar plot of amplitude and phase.
The symmetry of the mVEPs was quantified by calculating the amplitude asymmetry index, defined as F1/(F1 + F2). The asymmetry index can range from 1.0 (extremely asymmetric responses dominated by F1) to 0.0 (extremely symmetric responses dominated by F2). In all subjects, the asymmetry index and interocular phase difference in each eye did not differ between viewing with the right or left eye. We herein report the pooled data from either eye viewing. The data reported are amplitude and phase values of the channel with the highest signal-to-noise ratio and the most significant probability for the two eyes. The mean asymmetry index is the mean of the asymmetry indices measured in the left and right eyes.
Patient characteristics were compared using t-tests and Fisher exact tests for proportions. The mean asymmetry index and interocular phase difference were compared among the three groups (early surgery, standard surgery, and normal controls) by ANOVAs, with significance set at P < 0.05. Any significant difference was tested further by post hoc Tukey test. Mean asymmetry index and interocular phase difference of each patient was also compared to the 95% confidence interval of the control group mean.