Behavioral indices for neural systems involved in pain and fear responses of monkeys

Abstract

Monkeys were trained to choose between an escape and an avoidance response by pressing one of two levers. An escape response terminated electric shock across one foot and indexed “pain”. An avoidance response terminated light or buzzer cues and indexed “fear”. Electrodes were then aseptically implanted in the mesencephalic-diencephalic junction of each barbiturate-anesthetized monkey while gross potentials evoked by electrical stimulation of the extremities were recorded at each site. Lever pressing and motor responses to electrical stimulation at each brain site (ESB) were monitored. The effect of lesions at these sites on the escape and avoidance responses was observed. The ESB of the lateral spinothalamic tract (LST) and the terminal muclei of the medial lemniscus (ML) and LST systems produced much lever pressing, whereas ESB elsewhere did not. The type and location of the electric shock used during training contributed to the patterns of these lever press responses. Evoked potentials did not correlate with lever press responses to ESB, but did correlate with motor (“orienting”) responses to ESB. The data indicate that (a) the use of several physiological techniques in a subject trained to make more than one type of response is a useful approach to the study of sensory function in the nervous system; that (b) interaction of information coded by ML and LST fibers relative to pain sensation is different in each of the terminal nuclei; and that (c) there may be functional differences between VPI and VPL.