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Conditioned flavor aversion and preference in pigs: A SPECT study

The aim of our work was to describe the behavioral consequences of conditioned flavor aversion and preference in pigs, and to investigate the brain circuits involved in the representation of flavors with different hedonic values. The study was realized on eight 30-kg growing pigs and divided into three steps. 1) Animals were negatively conditioned (intraduodenal injection of LiCl) to a F- flavored meal and positively habituated (intraduodenal injection of NaCl) to a F+ flavored meal (F+ and F- being a thyme or cinnamon flavor), after which the behavioral reactions of the animals were recorded. 2) One and five weeks later, animals were subjected to three 30-min two-choice food tests to investigate their preferences between F+, F- and a novel flavor (O: orange). 3) Anaesthetized animals were subjected to three SPECT (Single Photon Emission Computed Tomography) brain imaging (Tc99m): control situation (no flavor), exposure to F+ and F- flavors. Statistical analysis was performed using SPM8 software and cerebral brain flow differences were determined using co-registered T1 MRI and histological atlases. 1) The negative reinforcement induced a physical malaise and visceral illness. After a positive reinforcement, animals displayed behavioral items related to playing or feeding motivation and appeasement (P < 0.05). 2) The F+ flavor was significantly preferred over the O and F- flavors, and the O flavor was significantly preferred over the F- flavor (P < 0.01 for all). 3) Compared to the control situation and contrary to F-, exposure to F+ specifically induced the activation of the dorsal anterior cingulate cortex, the dorsolateral prefrontal and orbitofrontal cortices, the putamen and thalamic reticularis nucleus (P < 0.001). When compared to F- and in addition to the aforementioned regions, F+ also specifically induced the activation of the caudate nucleus and hippocampus (P < 0.05). These neural circuits are involved in sensory associative processes, selective attention, learning and memory, positive emotions, reward and feeding motivation. This study reveals the unconscious cognitive dimension evoked by food flavors according to the individual experience, and highlights the importance of the sensory image of food on hedonism and anticipatory eating behavior.