EEG early evoked gamma-band synchronization reflects object recognition in visual oddball tasks

EEG was recorded in 3 visual oddball experiments during presentation of natural photos of butterflies and plants in order to study the early gamma activity evoked by familiar and novel stimuli. In all three experiments a picture of one specific butterfly served as the target and the subjects' task was to silently count them. In Experiment 1 neutral stimuli were individual pictures of butterflies from other species, in Experiment 2 neutral stimuli were individual pictures of plants and in Experiment 3 both types of neutral stimuli were applied. Phase-locking factor was computed by complex sinusoid wavelet method. Consistent with other studies, significant phase-locked γ-synchronization was found at 80-140 ms post stimulus interval in the 20-50 Hz range at parietal and occipital sites in response to the repeating target. Non-target stimuli did not evoke similar activity in the γ-frequency range. The observed difference can be explained if we assume that the repeated experience of an object may lead to the rapid formation of a neural assembly representing the object causing the repetition priming effect. In our study the single target stimulus was introduced to the subjects before the experiment, whereas individual non-target stimuli were unfamiliar images. Thus, subjects could form a neural representation of the target only. We suggest, that the early phase-locked γ-activity in the 20-50 Hz range might reflect the activation of the neural representation of the familiar target stimulus.