Brain Mechanisms

Releasing and directing stimuli for defense. The releasing and directing stimuli for the defensive upright posture have been studied in detail in the rat. Vibrissal or facial tactile stimuli are necessary to release the behavior in naive animals (Thor & Ghiselli 1975), although visual stimuli may suffice to release it in experienced animals (Kanki & Adams 1978). Vibrissal stimuli are necessary for directing the behavior; this is shown by the fact that an experienced animal will continue to show upright posture in response to visual releasing stimuli following removal of the vibrissae, but the posture is not properly oriented with respect to the opponent (Kanki & Adams 1978).

The neural mechanisms of vibrissal and visual-releasing stimuli for the defensive upright posture are known in some detail for the rat. Vibrissal releasing stimuli require only subcortical mechanisms, since they remain effective after bilateral destruction of projections to the thalamic relay nucleus for tactile sensation (Kanki & Adams 1978). Presumably, therefore, the main pathway goes more or less directly from the trigeminal complex to the motor patterning mechanisms of the upright posture, as illustrated in Figure 1.

There is an alternative set of pathways by which visual stimuli can release the upright posture in experienced animals This pathway includes neocortical structures; visual releasing stimuli are unaffected by lesions of the superior colliculus but are rendered ineffective following lesions of the visual cortex (Adams & Severini 1977). The cortical circuitry involved in visually-released boxing includes the ventrobasal thalamic relay nucleus for tactile sensation, since visual stimuli can no longer release the behavior following bilateral destruction of this nucleus (Kanki & Adams 1978). Apparently the visually-released boxing, which is a learned behavior, is elaborated by a cortical system that combines its visual input with the thalamocortical projections of the tactile system.

Although the releasing and directing stimuli of biting in cats during interspecific attack have been studied in some detail (Flynn 1972), the role of these stimuli in the biting in cats or rats during intraspecific defense has not been systematically studied. One may assume, as in the case of the upright posture, that they involve the trigeminal complex, since vibrissal and facial tactile inputs appear to be very important.

The motor pattern of fleeing probably requires releasing stimuli that are processed by a complex forebrain circuitry concerned with a "cognitive map" of a flight path or escape route. Behavioral evidence for this assumption includes observations that animals flee more readily if placed into a familiar place (Metzgar 1967), and that they do not attempt to flee if they know that there is no escape from the test chamber (Blanchard et al. 1976). According to a review of his many studies on this question, Thompson (1978) lists the following forebrain structures as critical for escape: the caudate nucleus and putamen, globus pallidus, entopeduncular nucleus, subthalamus, and ventromedial thalamus (all of which are connected by the lateral forebrain bundle), and the anterior thalamus. Reduction or abolition of escape following midline thalamic lesions has also been noted in both the cat (Mitchell & Kaelber 1966) and the rat (Bohus & deWied 1967). We have found in our laboratory that the midline thalamic lesions that abolish escape behavior in response to shock do not affect upright posture and boxing response to the shock This suggests that the effects of these lesions are specific to the motor patterning mechanism of escape - that is, its releasing and directing stimuli - and do not affect the motivating stimuli for defense in general.

There are no data to indicate that specific releasing or directing stimuli are necessary for activating the motor patterning mechanism for hissing or squealing.

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