LONG-LATENCY EVENT-RELATED POTENTIALS IN RATS - EFFECTS OF TASK AND STIMULUS PARAMETERS

Animal models of event-related potentials have recently been developed in rats in order to gain further understanding of the psychobiologica l variables which underlie these waveforms. In the present study, unan esthetized male Wistar rats, chronically implanted with electrodes, we re utilized in order to: (i) compare event-related potentials recorded following the presentation of passively presented auditory stimuli fr om different neocortical, hippocampal and perihippocampal sites; (ii) test the effects of changes in stimulus probability and loudness on ev ent-related potentials recorded from those sites; and (iii) record eve nt-related potentials from rats who were actively performing in a tone discrimination task. The results of these studies showed that in all electrode sites (frontal cortex, parietal cortex, entorhinal cortex, h ippocampus) a series of large amplitude potentials in the 10-200 ms la tency range could be recorded in response to passively presented stimu li. Late positive potentials in the 300-400 ms range were only identif ied in recordings from the posterior cortex, entorhinal area, and dors al hippocampus. Some of these late positive components were also found to be sensitive to changes in stimulus probability. A similar series of waves were detected in a paradigm where rats were required to activ ely discriminate between two tones; however, the morphologies of the w aveforms were found to be more distinct. These studies suggest that ra ts may be good subjects for the exploration of the neural origins of e vent-related potentials. These studies demonstrate that rats performin g in an auditory discrimination task can generate electrophysiological potentials which are time locked to the onset of a ''cognitively rele vant'' stimulus (event-related potentials). These potentials can be re corded in limbic (hippocampus and amygdala) and cortical (parietal cor tex) brain sites. The event-related potentials recorded in rats respon d to changes in stimulus parameters in a similar fashion to those prev iously described in monkeys and human subjects. The identification of a rat model of event-related potentials provides an opportunity to fur ther explore the neural origins of event-related potentials, to estima te the role of genetics in determining individual variation in wavefor ms, as well as to provide electrophysiological assays of the effects o f various drugs on neurosensory and cognitive processing.