“…Among the many means of ascertaining whether ICSS loci form an integrated system is to determine whether stimulation delivered to pairs of ICSS sites will summate and cause increased lCSS responding. Previous studies demonstrated that response rates elicited under simultaneous or nearsimultaneous stimulation of two ICSS sites were significantly greater than the sum of the response rates elicited when each of the sites was stimulated singly, suggesting that a behavioral interaction between the two ICSS sites occurred (Ackermann , Bodnar, Jackler, Steiner, & Ellman, 1973;Coons, Smith, & Ungerleider, 1976;Ellman, Ackermann, Bodnar, Jackler, & Steiner, 1975;Ungerleider & Coons, 1970). Such ICSS interactions have been demonstrated for the following electrode site combinations: the locus coeruleus (LC) and hypothalamus (HYP), the midventral periaqueductal gray and HYP, the dorsolateral periaqueductal gray and HYP , the substantia nigra and HYP, and bilateral HYP sites (Ackermann et aI., 1973;Coons et al, 1976;Ellman et aI., 1975;Ungerleider & Coons, 1970).…”