An examination of VEP response phase.
The accuracy of phase sensitive signal detection (PSD) as applied to swept visual evoked potential retrieval depends upon phase stability. If large phase shifts occur over the course of a swept run, amplitude will be lost in any one PSD channel and threshold estimation may be inaccurate. We were able to demonstrate large latency shifts in the conventional computer averaged VEP and phase shifts in the Fourier-analyzed steady-state VEP, as a function of contrast and spatial frequency. However, the phase changes observed over the portion of a sweep where there is a visually driven response are much smaller than obtained from averaged VEPs to a series of fixed stimulus values. The difference between the observed phase shifts may be caused by a delay in phase response when stimuli are swept. This hypothesis was supported by the finding that the phase of the steady-state VEP response requires from 1 to 2 sec to stabilize following a change in contrast. Sweeping contrast or spatial frequency results in a physiologically different response than when averaged VEP responses are measured to discrete changes in stimulus parameters.
Seiple
W
W
Department of Ophthalmology, New York University Medical Center, NY 10016.
Holopigian
K
K
D000669:Amplifiers, Electronic; D005074:Evoked Potentials, Visual / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D011930:Reaction Time