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


  • Ken A. Paller,

    Corresponding author
    1. Department of Psychology and Institute for Neuroscience, Northwestern University, Evanston, Illinois
    • Department of Psychology, Northwestern University, 2029 Sheridan Road, Evanston, IL 60208-2710
    Search for more papers by this author
  • Gregory McCarthy

    1. VA Medical Center, Durham, North Carolina
    2. Brain Imaging and Analysis Center, Duke University School of Medicine, Durham, North Carolina
    Search for more papers by this author


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 × 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. Hippocampus 2002;12:415–420. © 2002 Wiley-Liss, Inc.