Neuronal firing activity in the dorsal hippocampus during the auditory discrimination oddball task in awake rats: relation to event-related potentialgeneration

In order to investigate the roles of the hippocampus in event-related poten tial (ERP) generation, extracellular neuronal firings in the dorsal hippoca mpus were recorded together with the ERPs on the cortical surface and at th e hippocampus during the auditory discrimination task in awake rats. The ma jor ERP components on the cortical surface in response to the target tone w ere the P2, N2, and P3 with the latencies being approximately 100, 200 and 450 ms, respectively. For the non-target tone, the N2 and P3 components wer e not clearly observed. Local ERP at the hippocampus exhibited similar wave -forms to that on the cortex in response to the target and non-target tones . In the hippocampus, 11 of 21 neuronal firings showed a long-latency susta ined activation from 113.6 +/- 89.7 ms to 539.1 +/- 208.6 ms with the peak being 281.6 +/- 167.4 ms after the target tone onset. This increase was not observed after the non-target tone, and was not prominent when the rat did not perform the task. It was not time-locked to lever pressing behavior, a nd was not affected by the intensity of the tone stimulus. These features i n relation to behavioral and paradigm indices were similar to the long-late ncy ERP components, N2 and P3. Possible involvement of the hippocampus in E RP generation was further suggested by the correlation between the magnitud e of the long-latency activation and the amplitudes of N2 and P3 in some hi ppocampal firings. On the other hand, seven neuronal firings showed a short -latency transient activation with the peak at 36.3 +/- 14.4 ms accompanied by the early components of local ERP in the hippocampus both after the tar get and non-target tones. This response was more conspicuous when the rat d id not perform the task and its amplitude was positively affected by the st imulus intensity. These findings imply that there an two types of hippocamp al neuronal activation during the auditory discrimination oddball task. One is the short-latency activation that is related to information processing of the exogenous stimulus property. The second is the long-latency activati on that may be involved in execution of the cognitive task, and in generati on of long-latency ERP components. (C) 1999 Elsevier Science B.V. All right s reserved.