Event related beta and gamma oscillatory responses during perception of affective pictures.

Several studies reveal that unpleasant pictures elicit higher beta and gamma responses than pleasant and/or neutral pictures; however, the effect of stimulation design (block or random) has not been studied before. The aim of the study is to analyze the common and distinct parameters of affective picture perception in block and random designs by means of analysis of high frequency oscillatory dynamics (beta and gamma). EEG of 22 healthy subjects was recorded at 32 locations. The participants passively viewed 120 emotional pictures (10 × 4 unpleasant, 10 × 4 pleasant, 10 × 4 neutral) in block and random designs. The phase-locking and power of event related beta (14-28 Hz) and gamma (29-48 Hz) oscillations were analyzed for two different time windows (0-200 ms/200-400 ms). Statistical analysis showed that in the 0-200 ms time window, during the block design, unpleasant stimulation elicited higher beta phase-locking and beta power than the pleasant and neutral stimulation (p<0.05). In the 200-400 ms time window, during the block design, over occipital electrodes unpleasant stimulation elicited higher gamma response power than the pleasant stimulation and neutral stimulation (p<0.05). Unpleasant stimulation did not elicit higher beta or gamma responses in the random design. The present study showed that experimental design highly influences the perception of IAPS pictures. Unpleasant stimulation elicited higher event related beta and gamma phase-locking and power only in block design but not in random design. It seems that longer blocks of aversive pictures affect the brain more than the rapid observation of these pictures.