Abstracts

Rationale: Though it is well known that the alpha component (8-13 Hz) of the awake EEG is dramatically attenuated by eye opening, its relationship to elementary physical variables in the visual input have not been quantitatively estabilished. Eye opening produces large changes in both retinal illuminance as well as contrast. Methods: In order to quantitatively determine how these two variables influence the alpha rhythm we recorded the occipitally derived EEG of 10 normal volunteers as they viewed luminance backgrounds or contrast stimuli on a large luminance and contrast calibrated display. The alpha-band power during different stimulus conditions was determined by performing an Fast Fourier Transform (FFT) on the simultaneously recorded EEG waveform. Results: We find that the alpha-band power is well modeled by an exponentially decreasing function of the display luminance. This relationship was also found to hold over a larger luminance range and display geometry using full-field "Ganzfeld" illumination which eliminates contrast effects from the edge of the display device. At any fixed luminance, the alpha-band power decreases with increasing contrast according to a power law. This holds true for narrowband visual stimuli (sinusoidal gratings presented with counterphase flicker), as well as for broadband visual stimuli (dynamic white noise). Conclusions: The attenuation of the occipital alpha rhythm on eye-opening is thus mediated independently by activation of both luminance and contrast sensitive neuronal populations in the visual cortex. Interestingly, all the effect of luminance on alpha power occurs in the photopic (cone mediated) range of retinal illumination, suggesting that perhaps the rods contribute relatively little to "luxotonic" responses in the visual cortex.