MULTICHANNEL EEG POWER REFLECTS INFORMATION-PROCESSING AND ATTENTIONAL DEMANDS DURING VISUAL ORIENTING

The effects of stimulus complexity and instructions on the EEG orienting response were studied. Temporal and topographical EEG spectral changes from 01, 02, P3, P4, F3, F4, Fz, and Cz were recorded during orienting to, and appraisal of, visual stimuli of varying complexity. Only half the sample of 40 subjects was instructed to attend to the stimuli. Higher stimulus complexity reduced EEG power within beta, mid and high alpha rhythms, whereas the low alpha range was most sensitive to the interaction between stimulus complexity and instructions. Temporal changes in power suggested that the effect of stimulus complexity may be due to late appraisal rather than to early orienting. No single EEG index differentiated between groups. However, when factor scores of relative alpha, beta, and theta power were subjected to discriminant analysis, clear differences appeared between groups. Active attention during visual orienting was associated with a blockade and theta enhancement in posterior leeds, which might reflect two different mechanisms of orienting. Finally, EEG power changes during stimulus exposure predicted performance in a subsequent recognition task. it is argued that the analysis of short-lasting EEG power changes can offer valuable information about the mechanisms of visual orienting, and that a multivariate approach is required in EEG research on attention.