Certain parts of the brain related to autonomic activity and emesis become active following administration of an emetic US, and an analysis of such brain activity is improving our understanding of what constitutes the US, UR, and CR in food aversion studies (Bernstein, 1999). The nucleus of the solitary tract (NTS) in the hindbrain receives much afferent input from both the abdominal viscera and the gustatory system, and an injection of LiCl, the US, in rats produces a characteristic pattern of neural activation in NTS as measured by expression of the immediate early gene c-Fos. Intraoral infusions of a preferred solution such as sac charin or of a noxious tastant such as quinine do not by themselves induce c-Fos expression in the NTS, but once saccharin has been paired with LiCl, an intraoral infusion of saccharin alone, the CS, thereafter evokes activity in the NTS as if LiCl had been given instead (Swank & Bernstein, 1994). Thus, the CR as measured by c-Fos expression in the brain resembles the UR. This activation of the NTS results from descending information from the forebrain, suggesting a learning interpretation, and does not result simply from conditioned fear or autonomic arousal (Bernstein 1999; Schafe, Fitts, Thiele, LeDoux & Bernstein, in press).
The neural substrates of conditioned food aversions differ considerably depending on the procedures used to elicit them. Lesions in the amygdala abolish aversions conditioned by intraoral infusion of a CS but do not disrupt those conditioned by drinking the CS from a bottle (Schafe, Thiele, & Bernstein, 1998); lesions of the inferior olive disrupt aversions conditioned by a concurrent acquisition procedure but not by a sequential procedure (Mediavilla, Molina, & Puerto, 1999); and lesions of the area postrema eliminate aversions conditioned by emetic agents such as LiCl (Ritter, McGlone & Kelley, 1980) but not those conditioned by amphetamine or apomorphine (Berger, Wise & Stein, 1973; Van der Kooy, Swerdlow, & Koob, 1983). Clearly, conditioned food aversions are not a unitary phenomenon but represent several different neural processes that may reflect different kinds of learning (i.e., classical and instrumental conditioning).
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