Markov models for social interactions: analysis of electrical stimulation in the hypothalamic aggression area of rats

The analysis of experimental effects on social interactions is extremely difficult since animals usually react to one another's behaviour as well as to the experimental treatment. Methods are described to analyse both treatment effects and social influence, based on a description by a continuous time Markov chain model, where the states represent combinations of acts of the interacting individuals. These methods were applied to agonistic interactions of rats under the influence of subthreshold electrical stimulation at a specific site in the hypothalamus. Above the threshold intensity such stimulation induced attacks in a situation where there is normally no violent behaviour. Subthreshold experiments at different intensities were performed to study the relation between the activated neural mechanism and agonistic interactions without fighting. There were two behaviourally different periods during each observation period. The opponent's behaviour as well as current intensity significantly affected the experimental animal's behaviour in both periods, although the effects were different in the two periods. Both effects were most profound in the second period. In this period increased current intensity made experimental animals more self-directed and yet more reactive to the opponent's behaviour. Furthermore, opponents reduced the amount of contact behaviour shown by the experimental animals. Indeed it appears that their behaviour is aimed at reducing contact.