A quantitative study of gamma-band activity in human intracranial recordings triggered by visual stimuli

This paper studies gamma-band responses from two implanted epileptic patien ts during a simple visual discrimination task. Our main aim was to ascertai n, in a reliable manner, whether evoked (stimulus-locked) and induced (trig gered by, but not locked to, stimuli) responses are present in intracranial recordings. For this purpose, we introduce new methods adapted to detect t he presence of gamma responses at this level of recording, intermediary bet ween EEG-scalp and unicellular responses. The analysis relies on a trial-by -trial time-frequency analysis and on the use of surrogate data for statist ical testing. We report that visual stimulation reliably elicits evoked and induced responses in human intracranial recordings. Induced intracranial g amma activity is significantly present in short oscillatory bursts (a few c ycles) following visual stimulation. These responses are highly variable fr om trial to trial, beginning after 200 ms and lasting up to 500 ms. In cont rast, intracranial-evoked gamma responses concentrate around 100 ms latenci es corresponding to evoked responses observed on the scalp. We discuss our results in relation to scalp gamma response in a similar protocol [Tallon-B audry et al. (1996) J. Neurosci., 16, 4240-4249] and draw some conclusions for bridging the gap between gamma oscillations observed on the scalp surfa ce and their possible cortical sources.