Electrophysiological and behavioral correlates of arousal

Abstract

The relationships among levels of activity, responsiveness, exploration, and cortical arousal were assessed in a group of similarly reared male rats from a strain that was highly inbred to maximize genetic homogeneity. In Experiment 1, 22 subjects were tested in a complex, compartmentalized open field (i.e., a closed field), and those falling approximately 1 SD above or below the group activity mean (eight were eliminated) were designated as high- or low-active, implanted with electrodes, and tested electrophysiologically to ascertain the level of cortical arousal as determined via measurements of visually evoked potential afterdischarge amplitude. High-active subjects demonstrated a significantly lower afterdischarge amplitude, indicating that behavioral arousal may be indexed by a readily quantifiable electrophysiological measure. A second group of 14 rats (Experiment 2) was tested and retested after 45 days in the closed field, estabhshing that high- or low-behavioral responsiveness is a stable characteristic. Subjects then received 2 days’ exposure to one of two dissimilar compartments of an exploratory apparatus and were tested to determine amount of time spent in the familiar vs. novel compartments. High-active rats were found to demonstrate significantly shorter initial response latencies. However, no other differences were found. These rats were next tested in a completely open field. No significant activity differences were discovered. These findings show that a high level of arousal as a response to a highly complex environment or intense stimulation is not related to generalized activity elicited in a low-complexity environment (e.g., the open field). These findings also indicate that similarly reared rats may be differently influenced by novel, complex, or intense stimuli, such that some subjects become highly responsive and cortically aroused, whereas others respond with low levels of behavioral and cortical activation. The findings are discussed in terms of cortical and subcortical mechanisms mediating arousal.