The relationship between stimulus reactivity and heart rate in two inbred strains of Mus musculus

In previous studies combining multivariate and behavior genetic analyses, we found that locomotor activity can be distinguished experimentally from stimulus reactivity and is under separate genetic control. The present study is the first in a series designed to investigate the physiological correlates of stimulus reactivity (the initial responses of an animal to novel/complex stimuli). Heart rate was monitored for six males and six females each of two strains of inbred mice-DBA/2J and LG/J-during testing of stimulus reactivity. DBAs had low stimulus reactivity scores (long latencies to approaching novel stimuli and low exploration scores), although they maintained fairly high activity scores. Heart rate associated with initial and later approaches to novel stimuli in the DBAs was highly unstable and high in magnitude. LGs, on the other hand, had high stimulus reactivity scores accompanied by low and stable heart rates throughout the testing situation. Physiological strategies suggested by the work of Lacey and of Obrist are discussed.