| | Representation of Incentive Value in OFC and ABL
While olfactory information appears to be central to processing in rat OFC, processing of other types of information may also be crucial to understanding the role of this region. For example, OFC is also implicated in the processing of information regarding the motivational or incentive value of cues via connections with basolateral amygdala. Lesions of either ABL or OFC in rats result in impairments in discriminating cues associated with different magnitudes of reinforcement (Kesner and Williams, 1995; DeCoteau et al., 1997), and more recently Gallagher and colleagues have found that appropriate responding to conditioned stimuli in a devaluation paradigm depends on intact function in both regions. In this set of two studies, food-deprived rats were trained to associate a light-CS with delivery of a food-US. Prior to training, the test subjects received bilateral neurotoxic lesions of either ABL (Hatfield et al., 1996) or OFC (Gallagher et al., 1999). Rats with lesions of either structure acquired the conditioned response (food cup behaviors) normally as depicted in Figure 1.
Figure 1 
Conditioning of the food cup response in lesioned (LES) and control (CTL) subjects. Both ABL (A) and OFC (B) lesioned rats acquired the food cup response normally when compared to controls. (Figures adapted from Hatfield et al., 1996 and Gallagher et al., 1999)
After the end of training, both test and control subjects were returned to their home cages where half of the rats in each group received paired presentations of the food-US and illness induced by LiCl injections. The other half of the rats received unpaired presentations of the food and illness. Rats that received paired presentations of food and illness developed a conditioned taste aversion as a result of training. The results are shown in Figure 2 for both experiments. By the third trial (day 3), none of the rats (lesion or control) would consume the food in the paired groups; rats that received unpaired presentations continued to consume the food normally.
Figure 2 
Devaluation of the food reinforcer by pairing with LiCl-induced illness in lesioned (LES) and control (CTL) subjects. Filled symbols represent rats that received paired presentations of the food and illness, and unfilled symbols represent rats that received unpaired presentations. Both ABL (A) and OFC (B) lesioned rats showed normal devaluation when presentations were paired. (Figures adapted from Hatfield et al., 1996 and Gallagher et al., 1999)
The rats were then returned to the training environment and exposed to the light-CS. Control subjects that received paired presentations of the food and illness exhibited fewer conditioned responses to the light-CS when compared to rats that received unpaired presentations. These data are shown in Figure 3. In contrast, rats with lesions of either OFC or ABL exhibited the same amount of conditioned responding to the light-CS as did rats that received unpaired presentations. These rats continued to make food cup responses despite the observation that they would not consume the food-US in either the home cage or the training environment. In other words, rats with lesions of either OFC or ABL were unable to alter their behavior to reflect the altered motivational significance of the CS. Similar results have recently been reported in primates with crossed lesions of OFC and ABL (Baxter et al., 2000). These experiments show that connections between OFC and ABL are crucial to the appropriate use of motivational information in some contexts.
Figure 3 
Food cup responses during devaluation testing in lesioned and control subjects. Filled bars represent rats that received paired presentations of the food and illness, and unfilled symbols represent rats that received unpaired presentations. Normal rats exhibit significantly less conditioned responding if they received paired presentations of food and illness; lesions of either ABL (A) or OFC (B) prevent the normal decrease in conditioned responding in the paired group. (Figures adapted from Hatfield et al., 1996 and Gallagher et al., 1999)
Kesner RP, Willams JM. Memory of magnitude of reinforcement: Dissociation between the amygdala and hippocampus. Neurobiol Learn Mem 64:237-244, 1995.
DeCoteau WE, Kesner RP, Williams JM. Short-term memory for food reward magnitude: The role of the prefrontal cortex. Behav Brain Res 88:239-249, 1997.
Gallagher M, McMahan RW, Schoenbaum G. Orbitofrontal cortex and representations of incentive value in associative learning. J Neurosci 19:6610-6614, 1999.
Hatfield T, Han J-S, Conley M, Gallagher M, Holland P. Neurotoxic lesions of basolateral, but not central, amygdala interfere with pavlovian second-order conditioning and reinforcer devaluation effects. J Neurosci 16:5256-5265, 1996.
Baxter MG, Parker A, Lindner CCC, Izquierdo AD, Murray EA. Control of response selection by reinforcer value requires interaction of amygdala and orbital prefrontal cortex. J Neurosci 20:4311-4319, 2000.
Web Content from: Schoenbaum, G. Olfactory Learning and the Neurophysiological Study of Rat Prefrontal Function. In: CRC Series: Methods and Frontiers in Neuroscience. Edited by S.A. Simon and M.A.L. Nicolelis, CRC Press, NY, 2000.
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