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

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In order to investigate the roles of the hippocampus in event-related potential (ERP) generation, extracellular neuronal firings in the dorsal hippocampus were recorded together with the ERPs on the cortical surface and at the hippocampus during the auditory discrimination task in awake rats. The major ERP components on the cortical surface in response to the target tone were 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 were 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 sustained 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, and was not affected by the intensity of the tone stimulus. These features in relation to behavioral and paradigm indices were similar to the long-latency ERP components, N2 and P3. Possible involvement of the hippocampus in ERP generation was further suggested by the correlation between the magnitude of the long-latency activation and the amplitudes of N2 and P3 in some hippocampal 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 target and non-target tones. This response was more conspicuous when the rat did not perform the task and its amplitude was positively affected by the stimulus intensity. These findings imply that there are two types of hippocampal 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 activation that may be involved in execution of the cognitive task, and in generation of long-latency ERP components.