Brain Mechanisms

Cerebellar contributions to response selection. Adams's target article provides a very valuable and concise summary of an important literature on aggression. Further, the models presented in Figures 1-5, when viewed as conceptual flow charts rather than Markovian processors, provide a useful functional overview of the anatomical-physiological system involved in the control of certain classes of aggressive behavior.

Adams raises an important issue toward the end of his paper concerning the possible mechanisms involved in response selection. Sensory-releasing stimuli are certainly involved in this process, as we have argued elsewhere (Berntson and Micco 1976). The sequential appearance of relevant releasing stimuli within a behavioral chain most probably contributes highly to the serial coherence of motivated behaviors. Beyond this level of integration, however, one is struck by the apparent lack of response conflict between broader classes of motivated behavior, such as hunger and defense, for example. Some mechanism obviously allows an organism to put away concerns over energy balance in the face of a physical threat - even in the presence of releasing stimuli for both classes of behavior [see Toates: "Homeostasis and Drinking" BBS 2(1) 1979]. Adams suggests that such a process may be subserved, in part, by the paleocerebellum. I believe there is merit to this suggestion. Snider and Maiti (1976) and Heath (1976), among others, have documented the widespread functional interactions between the cerebellar fastigial nucleus and limbic mechanisms for motivated behavior. The fastigial nucleus also has been shown to have functional linkage with lower brainstem autonomic and behavioral substrates (Berntson and Paulucci 1979; Miura and Reis 1970; Snider 1975).

Several lines of behavioral research support the involvement of the paleocerebellum in behavioral function. In 1973 I reported that stimulation of the cerebellar fastigial nucleus in the cat could induce robust and coordinated eating and grooming behaviors (Berntson, Potolicchio, and Miller 1973). Comparable responses were independently obtained in Reis's laboratory (Reis, Doba, and Nathan 1973) and Martner's laboratory (Lisander and Martner 1975). Subsequently, similar effects have been demonstrated in the opossum (Buchholz 1976) and the rat (Ball, Micco, and Berntson 1974; Watson 1978a), although in the rat the responses demonstrated less behavioral specificity than in other species. In addition, electrodes in the fastigial nucleus of the rat were found to support self-stimulation (Ball, Micco, and Berntson 1974). These findings, together with reported changes in affective behavior with cerebellar stimulation or ablation (Berman, Berman, and Prescott 1974; Peters and Monjan 1971; Reis, Doba, and Nathan 1973; Zanchetti and Zoccolini 1954), strongly implicate the paleocerebellum in behavioral function.

It is recognized that the cerebellum participates in postural control and motor coordination through a relatively direct action on lower reflex mechanisms. More recently, there has been a great deal of interest in the participation of the cerebellum in the acquisition of skilled movements (Albus 1971; Gilbert 1975; Ito 1972; Marr 1969), perhaps through a response selection process (Eccles 1977). Comparable roles may well be played by other portions of the cerebellum in orchestrating sequences of species-characteristic behaviors and contributing to response selection and inhibition through actions exerted at higher levels of behavioral organization. Some preliminary data from our laboratory, gathered in collaboration with Professor David Hothersall and Kevin Schumacher, are consistent with this view.

Rats having medial cerebellar lesions were tested in an operent DRL (differential reinforcement of low rates) task, which is highly sensitive to behavioral inhibitory processes. In this task the animal must press a bar for food and then wait a specified period before pressing again. A bar press during the time-out period will reset the timing clock and delay the subsequent availability of reinforcement. Thus, the animal must inhibit a high-probability response (bar press) for a period of time after reinforcement. When animals with cerebellar lesions were trained on this task, in the presence of a wood block that allows a collateral or "mediating" behavior to fill the time-out delay, their performance was as good as, or superior to, normal animals.

The fact that lesioned animals could achieve highly efficient performance indicates that timing and learning processes were not disrupted by the lesions. However, if the wood blocks were removed, performance of the lesioned animals deteriorated dramatically and did not recover as in normal animals. Rather, perseverative bar pressing continued at high levels, even though such responding precluded reinforcement. The lesioned animals were apparently deficient in the ability to withhold inappropriate high-probability responses - a deficiency that has previously been suggested to characterize animals with cerebellar damage (Buchtel 1970).

While these data are only suggestive, the robust and widespread behavioral effects of cerebellar manipulations (see Watson 1978b for review), including dramatic alterations in aggressive behaviors, plead for experimental and theoretical attention. In the context of the agonistic behaviors of offense, defense, and submission, the present view might predict specific types of changes after paleocerebellar lesions. While such lesions might alter the overall level of aggressive behaviors, a more interesting prediction would involve interactions among these classes of behavior. For example, it might be predicted that lesioned animals would have difficulty in shifting from offense to, say, defense or submission in accord with changing environmental conditions. Alternatively, components of one pattern may intrude into the behavioral performance of a different pattern. In view of these possibilities, it is likely that the detection of such alterations may require sophisticated behavioral testing and may not be apparent in casual observations. Indeed, it is perhaps for this reason that the cerebellum has not received greater attention in the behavioral literature.