Gamma responses and ERPs in a visual classification task

Objective: We examined event-related potentials (ERPs) and gamma range EEG activity in a visual classification task to assess which variables affect these responces. Methods: Ten subjects silently counted the occurrence of rare Kanizsa squares (targets) among Kanizsa triangles and non-Kanizsa figures (standards). By applying a time-frequency analysis to the data and selectively calculating topographical maps of certain frequencies. Results: We were able to find 3 different types of gamma responses to Kanizsa figures: an early phase-locked gamma response at 40 Hz in the N100 time range, late phase-locked gamma activity (200–300 ms) at 40 Hz and a continuous phase-locked gamma response at 80 Hz due to the monitor refresh frequency. The two 40 Hz responses were significantly higher for Kanizsa figures than for non-Kanizsa figures and within the Kanizsa figures were higher for the target figure than for the non-target. Conclusion: The phase-locking of these two responses, previously found also as non-phase-locked activity, could be synchronized due to the monitor flicker frequency. Also, our findings suggest that the gamma responses are not solely associated with the binding of stimulus features, but reflect some processes related to target processing.