Gamma activity in human EEG is related to highspeed memory comparisons during object selective attention

Among the most important processes of the brain in order to correctly perceive the outside world and act within it are binding, attention, and memory. All three functional mechanisms have been associated with brain activity in the gamma frequency range. It needs to be clarified, however, which subprocesses within the gamma frequency range relate to which perceptual or cognitive functions. In a visual discrimination task, we used Kanizsa figures whose constituent inducer disks need to be bound together to perceive the illusory contours. By a variation of the task requirements we manipulated the allocation of object selective attention as compared to a previous study. One out of four objects had to be detected. This detection process requires the comparison of two object dimensions (form and collinearity) with a working memory template. In order to get behavioural and electrophysiological measures, EEG and reaction times were recorded from 16 and 10 subjects, respectively. We demonstrated that the early evoked gamma activity reflects the process of allocating attention to a selected object as early as 50-150 ms after stimulus onset. We propose that the underlying mechanism is a high-speed memory comparison. In addition, we show that this early gamma activity also determines the reaction times needed to respond to the different stimuli.