Brain Mechanisms

Motivating stimuli for defense. The following types of motivating stimuli have been identified as activating the defense motivational system in wild muroid rodents: pain, sudden noise, sudden visual movement, dorsal tactile stimulation, restraint, certain olfactory stimuli (including defense pheromones), and the stimuli that evoke neophobia (Adams, submitted for publication). Experimental analyses have been hampered by the fact that some stimuli are not as effective in laboratory animals as in wild animals, especially sudden noise, sudden visual movement, and neophobia. This may be due to genetic differences in favor of docile animals created by selection pressure in the laboratory, as well as to artificial laboratory rearing practices which frustrate the normal ontogenetic development of the neural mechanisms that process these stimuli (Clark & Galef 1977).

Many of the motivating stimuli for defense do not require forebrain mechanisms. In the rat, startle and freezing may be elicited by loud sounds, and noxious tactile stimulation can produce jumping, vocalization, biting, struggling, urination, and defecation in chronic preparations from which the entire forebrain has been removed (Lovick 1972; Woods 1964). In cats, defense can be obtained in response to pain, dorsal tactile stimuli, and restraint after complete removal or transection of the forebrain (Woodworth & Sherrington 1904; Bazett & Penfield 1922; Keller 1932; Magoun et al. 1937; Kelly et al. 1946; Bard & Macht 1958).

Pain as a motivating defense stimulus was considered by classical neurologists to reach the midbrain tegmentum and central gray by way of the ventrolateral columns of the spinal cord (Woodworth & Sherrington 1904) and the paleospinothalamic tract in the brainstem (Mehler 1969). More recently, however, the mechanisms responsible for the sensory filtering of pain have turned out to be more complex than previously thought, and they may involve projections to the central gray from the dorsal columns and medial lemniscus as well (Liebeskind & Mayer 1971).

Auditory stimuli that motivate defense may reach the central gray directly by way of the lateral lemniscus without being relayed through the inferior colliculus. Lesions that destroy the inferior colliculus do not abolish an escape response to noise in the rat (Lyon 1964), while lesions that destroy the ventral portion of the central gray do abolish the response. Central gray neurons in the rat are responsive to click stimuli, as has been shown by evoked potential (Hara et al. 1961) and single-unit recording techniques (Adams 1968). Central gray neurons in the rat that fire maximally during shock-elicited fighting are also facilitated by auditory stimuli (Pond et al. 1977); handclaps also produced upright defensive posture and boxing in the absence of shock stimulation in these animals.

Visual stimuli that activate defense may reach the central gray by way of the pretectum. This is suggested by the results of Schneider (1969), who found that undercutting the pretectum abolishes the freezing response of the hamster to overhead visual movement, while undercutting the superior colliculus, if anything, enhances the freezing response. Support for this also comes from Schaefer's (1970) findings that electrical stimulation of the pretectum produces flight in rabbits. This neural system may be homologous to the one that has been systematically studied in the pretectum of the toad, by which visual stimuli of a certain large size, movement velocity, and contrast produce escape behavior (Ewert 1970; Ingle 1976). The anatomy of the system in mammals is not clear, however; while there are projections from the central gray to the pretectum in the cat (Hamilton & Skultety 1970), reciprocal connections have not been reported (Berrnan 1977).

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