TASK-RELEVANT LATE POSITIVE COMPONENT IN RATS - IS IT RELATED TO HIPPOCAMPAL THETA-RHYTHM

Long-latency components of event-related potentials (like the P300 or P3) correlate with the ability of subjects to detect and process unexp ected, novel or task-relevant events. Task-relevant late positive comp onents were recorded in the neocortex and hippocampus of rats performi ng an auditory discrimination task, similar to the ''odd-ball'' paradi gm used in human experiments. Surface and depth electrodes were implan ted in anaesthetized rats at frontal, temporal and anterior occipital neocortical regions and the hippocampus. After recovery from surgery r ats were trained to discriminate two auditory signals, a frequent irre levant tone and a rare tone related to water reward. In response to th e task-relevant tone but not the irrelevant tone, P300-like late posit ive components (mean latency of 274 ms) were recorded throughout the s urface of the neocortex. The largest amplitudes were found at the ante rior occipital cortex situated above the hippocampal CA1 region. The a mplitude of the task-relevant positive component increased further wit h cortical depth without reversing its polarity. An amplitude maximum was found in the CA1 region with a polarity reversal at the pyramidal cell layer and the largest negative amplitude in stratum radiatum. Pow er spectra of differences between responses evoked by task-relevant to nes and those evoked by irrelevant tones revealed peaks in the theta r ange (4-12 Hz). It is suggested that the P300-like component in rats c orresponds to a theta wave out of a burst of hippocampal theta cycles. (C) 1996 Wiley-Liss, Inc.