Normal Aging does Not Impair Orbitofrontal-Dependent Reinforcer Devaluation Effects

Responding to preconditioned cues is devaluation sensitive and requires orbitofrontal cortex during cue-cue learning

The orbitofrontal cortex (OFC) is necessary for inferring value in tests of model-based reasoning, including in sensory preconditioning. This involvement could be accounted for by representation of value or by representation of broader associative structure. We recently reported neural correlates of such broader associative structure in OFC during the initial phase of sensory preconditioning (Sadacca et al., 2018). Here, we used optogenetic inhibition of OFC to test whether these correlates might be necessary for value inference during later probe testing. We found that inhibition of OFC during cue-cue learning abolished value inference during the probe test, inference subsequently shown in control rats to be sensitive to devaluation of the expected reward. These results demonstrate that OFC must be online during cue-cue learning, consistent with the argument that the correlates previously observed are not simply downstream readouts of sensory processing and instead contribute to building the associative model supporting later behavior.

Enhanced performance of aged rats in contingency degradation and instrumental extinction tasks

Normal aging in rats affects behavioral performance on a variety of associative learning tasks under Pavlovian conditions. There is little information, however, on whether aging also impacts performance of instrumental tasks. Young (9-12 months) and aged (24-27 months) Fisher 344 rats were trained to press distinct levers associated with either maltodextrin or sucrose. The rats in both age groups increased their lever press frequency at a similar rate, suggesting that the initial acquisition of this instrumental task is not affected by aging. Using a contingency degradation procedure, we then addressed whether aged rats could adapt their behavior to changes in action-outcome contingencies. We found that young and aged rats do adapt, but that a different schedule of reinforcement is necessary to optimize performance in each age group. Finally, we also addressed whether aged rats can extinguish a lever press action as well as young rats, using 2 40-min extinction sessions on consecutive days. While extinction profiles were similar in young and aged rats on the first day of training, aged rats were faster to extinguish their lever presses on the second day, in spite of their performance levels being similar at the beginning of the session. Together these data support the finding that acquisition of instrumental lever press behaviors is preserved in aged rats and suggest that they have a different threshold for switching strategies in response to changes in action-outcome associations. This pattern of result implies that age-related changes in the brain are heterogeneous and widespread across structures.

Orbitofrontal cortex volume in area 11/13 predicts reward devaluation, but not reversal learning performance, in young and aged monkeys

The orbitofrontal cortex (OFC) and amygdala are both necessary for decisions based on expected outcomes. Although behavioral and imaging data suggest that these brain regions are affected by advanced age, the extent to which aging alters appetitive processes coordinated by the OFC and the amygdala is unknown. In the current experiment, young and aged bonnet macaques were trained on OFC- and amygdala-dependent tasks that test the degree to which response selection is guided by reward value and can be adapted when expected outcomes change. To assess whether the structural integrity of these regions varies with levels of performance on reward devaluation and object reversal tasks, volumes of areas 11/13 and 14 of the OFC, central/medial (CM), and basolateral (BL) nuclei of the amygdala were determined from high-resolution anatomical MRIs. With age, there were significant reductions in OFC, but not CM and BL, volume. Moreover, the aged monkeys showed impairments in the ability to associate an object with a higher value reward, and to reverse a previously learned association. Interestingly, greater OFC volume of area 11/13, but not 14, was significantly correlated with an animal's ability to anticipate the reward outcome associated with an object, and smaller BL volume was predictive of an animal's tendency to choose a higher value reward, but volume of neither region correlated with reversal learning. Together, these data indicate that OFC volume has an impact on monkeys' ability to guide choice behavior based on reward value but does not impact ability to reverse a previously learned association.

Normal aging alters learning and attention-related teaching signals in basolateral amygdala

Normal aging has been associated with an increased propensity to wait for rewards. When this is tested experimentally, rewards are typically offered at increasing delays. In this setting, persistent responding for delayed rewards in aged rats could reflect either changes in the evaluation of delayed rewards or diminished learning, perhaps due to the loss of subcortical teaching signals induced by changes in reward; the loss or diminution of such teaching signals would result in slower learning with progressive delay of reward, which would appear as persistent responding. Such teaching signals have commonly been reported in phasic firing of midbrain dopamine neurons; however, similar signals have also been found in reward-responsive neurons in the basolateral amygdala (ABL). Unlike dopaminergic teaching signals, those in ABL seem to reflect surprise, increasing when reward is either better or worse than expected. Accordingly, activity is correlated with attentional responses and with the speed of learning after surprising increases or decreases in reward. Here we examined whether these attention-related teaching signals might be altered in normal aging. Young (3-6 months) and aged (22-26 months) male Long-Evans rats were trained on a discounting task used previously to demonstrate these signals. As expected, aged rats were less sensitive to delays, and this change was associated with a loss of attentional changes in orienting behavior and neural activity. These results indicate that normal aging alters teaching signals in the ABL. Changes in these teaching signals may contribute to a host of age-related cognitive changes.

Willingness to wait and altered encoding of time-discounted reward in the orbitofrontal cortex with normal aging

Normal aging has been associated with cognitive changes, including shifts in responding for time-discounted rewards. The orbitofrontal cortex, an area previously associated with aging-related cognitive changes, is critical for normal discounting. Previously we have shown in a choice task that rats prefer immediate over delayed reward and that neural representations of delayed reward in orbitofrontal cortex were attenuated, whereas immediate reward elicited strong responses. Changes in choice performance were correlated with changes in firing rate in orbitofrontal neurons, suggesting that these reward representations were critical to the rats' ability to wait for reward. Here we asked whether age-dependent changes in discounting behavior were related to changes in the representation of delayed reward in the orbitofrontal cortex. Young (3-6 months) and aged (22-26 months) rats were trained on the same discounting paradigm used previously. We found that aged rats showed less sensitivity to increasing delay preceding reward delivery, shifting behavior away from the delayed reward more slowly than younger rats. This sensitivity was specific to delay, since choice performance did not differ between the two groups when delay was held constant and reward size varied. Aged rats exhibited a corresponding increase in the prevalence of neurons that fired more strongly for delayed reward. Again this change was specific to delay; there was no change in encoding of different-sized rewards. These results suggest that natural aging results in altered representations of reward in orbitofrontal cortex. These changes may relate to the increased ability to delay gratification and reduced impulsivity associated with aging.

Risk, reward, and decision-making in a rodent model of cognitive aging

Impaired decision-making in aging can directly impact factors (financial security, health care) that are critical to maintaining quality of life and independence at advanced ages. Naturalistic rodent models mimic human aging in other cognitive domains, and afford the opportunity to parse the effects of age on discrete aspects of decision-making in a manner relatively uncontaminated by experiential factors. Young adult (5–7 months) and aged (23–25 months) male F344 rats were trained on a probability discounting task in which they made discrete-trial choices between a small certain reward (one food pellet) and a large but uncertain reward (two food pellets with varying probabilities of delivery ranging from 100 to 0%). Young rats chose the large reward when it was associated with a high probability of delivery and shifted to the small but certain reward as probability of the large reward decreased. As a group, aged rats performed comparably to young, but there was significantly greater variance among aged rats. One subgroup of aged rats showed strong preference for the small certain reward. This preference was maintained under conditions in which large reward delivery was also certain, suggesting decreased sensitivity to reward magnitude. In contrast, another subgroup of aged rats showed strong preference for the large reward at low probabilities of delivery. Interestingly, this subgroup also showed elevated preference for probabilistic rewards when reward magnitudes were equalized. Previous findings using this same aged study population described strongly attenuated discounting of delayed rewards with age, together suggesting that a subgroup of aged rats may have deficits associated with accounting for reward costs (i.e., delay or probability). These deficits in cost-accounting were dissociable from the age-related differences in sensitivity to reward magnitude, suggesting that aging influences multiple, distinct mechanisms that can impact cost–benefit decision-making.

Age-related differences in the anticipation of future rewards

The present study examined the anticipation of future reward in 7-mo- and 26-mo-old Fischer 344/Brown Norway rats. Young and old rats were divided randomly and assigned into one of two conditions. In the Contrast Condition, subjects were given a water solution containing 2% sucrose for 3 min followed immediately by a water solution containing 32% sucrose for 3 min. In the No-Contrast Condition, subjects were given a water solution containing 2% sucrose for 3 min followed immediately by a water solution containing 2% sucrose for 3 min. Across 10 days of testing in the Contrast Condition, young rats showed significantly less intake of the less preferred 2% sucrose solution, whereas old rats showed increased intake of the 2% sucrose solution. Young rats showed a significant increase in intake of the preferred 32% sucrose solution compared to aged rats across the 10-day testing period with the exception of days 8-10 where intake did not differ between groups. In the No-Contrast Condition, there were no significant differences between young and old rats, with both groups consuming significantly more of the first 2% solution than the second 2% solution. Therefore, these data suggest that age-related changes may impair the ability to anticipate future rewards.