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The effects on retinal and cortical visual evoked phenomena of the stimulus intensity/spatial frequency and of the spontaneous, physiological blood glucose/ammonia fluctuations were investigated comparatively in a pilot study on young male volunteers. Flash ERG and flash OPs to 5 different stimulus intensities and pattern VECP to 3 spatial frequencies were recorded at 2-hour intervals during a standard acute pharmaco-EEG experimental session (8 h) with administration of placebo; glucose and ammonia blood concentration levels were assessed concomitantly. The effects of the stimulus intensity/spatial frequency were statistically defined for each amplitude/latency measure by nonlinear regression analysis and were removed by computing the residuals from the regression function, which were then tested separately versus the glucose/ammonia concentration by linear regression. Glucose/ammonia statistically significant effects on the visual system were detected at concentration levels within the range of normality and were representative of a nonnegligeable portion of the data overall variance. These effects were selective on retinal/cortical evoked phenomena and it is conceivable that physiological or pathological metabolic changes might account for a still underestimated source of individual variability in human neuropharmacological studies otherwise adequately controlled.

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