Field potentials in the human hippocampus during the encoding and recognition of visual stimuli

Intracranial field potentials were recorded from electrodes implanted in the hippocampus in 12 epileptic patients. Potentials were elicited by stimuli presented during a delayed matching-to-sample test. Each trial began with a sample stimulus composed of a 3 x 3 grid of rectangular color patches. The sample was followed by a sequence of similar but task-irrelevant stimuli and the sequential presentation of two test stimuli, one of which was identical to the sample. Patients indicated their recognition of the test stimulus that matched the sample with a button press. High-amplitude negative potentials were consistently elicited by sample and test stimuli. Peak amplitudes occurred 300-500 ms after stimulus onset and were larger for the sample in all cases. The patterns of potential gradients observed between adjacent hippocampal contacts and the locations of maximal amplitudes, as verified by magnetic resonance imaging in seven patients, suggest that these potentials were produced by neuronal activity in posterior hippocampus. These field potentials appear to index a memory storage function engaged in response to events that will later be remembered. The hippocampal contribution to storing declarative memories can thus begin, in some circumstances, within the first half-second after the presentation of a to-be-remembered stimulus.