We have used depth electrodes to record intracranial EEG from several epileptic patients performing two kinds of short term memory tasks. Both tasks followed the same procedure : a series of visual stimulus was presented, half of which had to be remembered by the patients. In one task, the stimuli were letters while in the second task, they were geometric configurations. We quantified the gamma activity induced by the presentation of those visual stimuli and found clear differences between the stimuli to be remembered vs those that didn't have to be remembered. Those differences were particularly strong in the insula.