[Changes in the electric organization of the brain in visual search and discrimination processes of different degrees of difficulty].

EEG activity during visual search processes with different degrees of discrimination difficulty was investigated in 13 male subjects. In accordance with the paradigm used by Neisser, 17 corresponding lists of rounded or angular letters were displayed. The task was to select the target letter Z from each list. The discrimination of the letter Z out of the lists of angular letters is the more difficult task. Spectral analysis shows a significant change in alpha-frequency structure during the more difficult search process. There is a shift of the dominant alpha-frequency from 11.5 to 10 Hz with a significant increase and concentration of power in the dominant frequency. This concentration of alpha-power around 10 Hz is more pronounced in the right occipital region than in the left. Concomitantly, there is a significant increase of power in subharmonic theta-frequencies. By consecutive principal analysis of spectral data these results were further differentiated and confirmed. The changes in the alpha- and theta-band during the more difficult search process are the expression of a complex transformation of EEG-frequency structure and patterning which cannot be interpreted by the usual concepts based on the theory of activation. Moreover, they seem to be the neurophysiological correlate of the accommodation (Piaget) of the neurodynamic processes to the altered perceptual conditions and to their effects on the internal matching process.