[Visual evoked potentials and hemispheric differences: on the functional significance of homologous regions]

A total of 18 right handed subjects participated in an experiment, in which visually evoked potentials were recorded from four scalp electrodes (01, 02 and P3, P4) when words or flashes of diffuse light were presented as stimuli. The evoked potentials were subdivided into three components, an early component (Komp1, 0-150 ms) and two late components (Komp2, 150-300 and Komp3, 300-450 ms). For each of these components and for each subject, correlations were computed between different recording sites. The results show that homologous correlations (01:02, P3:P4) yield values, which are significantly higher than the values of the intra- (01:P3, 02:P4) and diagonal interhemispheric (01:P4, 02:P3) correlations. This result, which holds true for all of the three components in a similar way, is independent from the type of presented stimuli. In contrast to homologous correlations, intra- and diagonal interhemispheric correlations do vary as a function of stimulus type and different components as well. Here, the results show that the degree of correlation between different recording sites is positively related to the degree of cognitive load: with increasing cognitive load, the degree of synchronous neural activity increases. Homologous correlations are independent from the type of stimuli (words and pictures) and from the type of VEP-components (Komp1, 2, and 3) as well. Thus, it can be concluded that homologous correlations reflect attentional rather then cognitive processes.