Real-Time Monitoring of the Visual Evoked Potential

Subjects can experience loss of consciousness (LOC) during exposure to sufficiently high +G sub z acceleration. LOC is usually preceeded by decreased visual sensitivity, dimming of the visual field, peripheral light loss (PLL) and central light loss. Real-time monitoring of PLL as a measure of + G sub z tolerance on the human centrifuge presently requires an active response by the subject. In contrast, the visual evoked potential (VEP) also reflects the integrity of the visual system and requires only the passive response of viewing a visual stimulus. A preliminary evaluation of the Steady-state VEP for real-time monitoring of visual functioning was accomplished with a computer controlled dual-channel, Fast Fourier Transform (FFT) signal analyzer. Since significant degradation of visual functioning must be recognized in less than approximately four seconds, we required a previously unattained efficiency in producing and measuring the VEP. Using the Coherence Function and the expectation for the variance of a weighted sum of variables we developed analytic methods and instrumentation for reducing the results of the FFT processing to unidimensional measures having a known and maximal signal-to-noise ratio, with measures available at 1 1/2 sec. intervals. Preliminary tests on six subjects indicate that the technique may have adequate stability as a measure of G-tolerance. Stimulus Interruption produces a rapid loss of response, with recovery somewhat slower. However, the measure is considerably more stable for some subjects than for others. Further testing is in progress. (Author) Presented at the Annual Meeting of the Aerospace Medical Association, 10-13 May 82, Bal Harbour, FL.