Measures of the autonomic nervous system are often used to infer mood or temporally extended affective states in humans, yet little is known about this relationship in other animals. Studies in animals have predominantly assessed instantaneous and short-lived behavioral and neural responses to either brief aversive or positive stimuli. No studies in nonhuman animals have probed the relationship between autonomic activity and temporally extended affective states that extend over minutes or hours, durations typical of mood states in humans. To address this issue, we analyzed EKG signals recorded from three animals while they performed a two-alternative forced-choice task for fluid reward. On each trial, animals chose between images that led to the delivery of different amounts of fluid. Importantly, the amount of fluid reward varied within each session as the amount of reward available on each trial was randomly selected. This meant that subjects experienced periods of high and low reward intake, which we predicted would modulate their affective state. To test this possibility, we examined the relationship between changes in reward rate over the course of each session and the subjects instantaneous heart rate over different time scales (10-60 seconds). For all three subjects, there was a consistent negative correlation between heart rate and reward rate. This effect was not driven by changes in breathing as a result of reward consumption and was statistically significant in a high proportion of sessions (p<0.01). Thus, our data suggest that heart rate may reflect a temporally extended affective state in nonhuman animals. Tastes and their associated values drive food consumption and influence choice behavior. To understand the neural mechanisms underlying reward-guided decision making and value learning, we characterized the gustatory system in subjects with fMRI while measuring licking to control for oral movements, and to assess preferences. To identify taste-responsive cortex, we delivered small quantities of sucrose, citric acid, or distilled water in random order without any predictive cues (e.g., visual stimuli) to subjects while using event-related fMRI. In addition, we used an MRI-compatible lick sensor to measure subjects licking during the scans. fMRI signals associated with licking in the absence of fluid delivery were used to mask responses to fluid delivery/receipt and associated licking. Licking in the absence of fluid delivery and fluid receipt times for each tastant were incorporated into a general linear model to analyze fMRI data. By contrasting BOLD responses to sweet and sour tastes with those from distilled water, we identified taste responses in primary gustatory cortex area G, an adjacent portion of the anterior insular cortex, and area 12o. Choice tests run outside the scanner revealed that all three subjects strongly preferred sucrose over citric acid or water. BOLD responses in the ventral striatum and amygdala reflected subjects preferences, with greater BOLD responses to sucrose than citric acid. Finally, we examined the influence of hydration level by contrasting BOLD responses to receipt of fluids when animals were thirsty and after ad libitum water consumption. BOLD responses in area G and area 12o in the left hemisphere were greater following full hydration, which may reflect an influence of internal state on taste processing. By contrast, BOLD responses in portions of medial frontal cortex were reduced after ad libitum water consumption. Thus, activation in this region, which is implicated in value-based decision making, is modulated by internal state. These findings highlight brain regions involved in representing taste, taste preference and internal state.
