Abstracts

Activation and Suppression of the posterior alpha rhythm (AR) by eye closure and opening is a commonly known phenomenon. The underlying neurophysiological mechanisms, however, are not completely clear. This study aims to investigate the relationship between AR activation and object fixation., The scalp EEG (10-20 system) was recorded from healthy individuals (n=8) in a dark room. Via a computer screen the subjects were presented black objects on white background for 5 sec each: (1) randomly distributed small squares (0.6 angular degree ([deg]) size, fig. 1A), (2) regularly distributed small squares (0.6[deg], fig. 1B), (3) large squares (6[deg], fig. 1C), (4) large diamonds (6[deg], fig. 1D). All objects were arranged in a way that 25% of the screen were black, with the remaining 75% showing white background. Following presentation of each object, a homogenously light-grey screen was presented for 5 sec (fig. 1E), with the grey value chosen in a way that the average screen luminance remained constant throughout the experiment. The protocol was repeated with visual fixation eliminated by Frenzel lenses. The mean amplitude of the AR (peak-to-peak, derivations O1-T1/O2-T2) was determined. Statistical comparisons were done using Wilcoxon[apos]s test., AR, reactive to eye opening and closure, was present in all individuals. There were no normal alpha variants. During object presentation, AR was suppressed. There were no differences between between the four objects. Disappearance of the objects (presentation of the homogenous grey screen) resulted in AR activation. There was no correlation between AR activation and eye blinks. ARA was more robust after disappearance of the larger objects (fig. 1 C,D), compared to disappearance of the smaller objects (fig. 1 A,B): average amplitude 64 [micro]V, compared to 24 [micro]V (p[lt]0.05). Elimination of visual fixation using Frenzel lenses abolished AR activation., (1) Eye closure is not required for activation of the posterior alpha rhythm. (2) Posterior alpha rhythm can be generated while a subject is looking at a homogenous bright background. (3) Alpha rhythm generation is facilitated by sudden disappearance of structured visual input. (4) Disappearance of large objects leads to more robust alpha rhythm activation than disappearance of smaller objects. (5) Alpha rhythm activation is abolished when visual fixation is eliminated.[figure1],