Greater frontal-parietal synchrony at low gamma-band frequencies for inefficient than efficient visual search in human EEG.

In a study on monkeys, [Buschman, T.J., Miller, E.K., 2007. Top-down versus bottom-up control of attention in the prefrontal and posterior parietal cortices. Science 315 (5820), 1860-1862.] reported more frontal-parietal neuronal synchrony in a low gamma-band (22-34 Hz) for inefficient than efficient visual search, but a reverse effect in a higher gamma-band (36-56 Hz). We examine whether this difference in top-down versus bottom-up influence on visual search also occurs in humans using scalp EEG. Ten participants identified the location of a target item (coloured, oriented rectangular bar) in search displays also containing similar distractors. For the efficient search condition, in which response time was less dependent on set size (two or four items), distractors had no feature in common with the target. For the inefficient search condition, in which response time increased with set size, distractors shared one feature with the target. Analysis of phase-locking values revealed significantly greater synchronization between frontal-parietal electrode pairs in the lower frequency band around 160-480 ms post-stimulus for inefficient search. No significant difference was observed in the higher frequency band. These results partly correspond to [Buschman, T.J., Miller, E.K., 2007. Top-down versus bottom-up control of attention in the prefrontal and posterior parietal cortices. Science 315 (5820), 1860-1862.], suggesting that top-down control is mediated by neuronal synchrony at lower frequencies. The failure to observe a bottom-up effect may be due to stimulus familiarity - monkeys require weeks of training in contrast to the few minutes given to humans.