Willingness to Wait and Altered Encoding of Time-Discounted Reward in the Orbitofrontal Cortex with Normal Aging

Abstract

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 mo) and aged (22-26 mo) 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 differently-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.

Neurophysiology of Reward-Guided Behavior: Correlates Related to Predictions, Value, Motivation, Errors, Attention, and Action

“…Understanding how these circuits work to produce behavior allows us to look for alterations in neural signals in animal models of human disorders, such as models of psychological disorders, drug addiction, and aging (Hernandez et al 2015; Roesch et al 2007b, 2012a, b; Gruber et al 2010; Stalnaker et al 2009). Future therapeutic methods (behavioral, psychological, pharmacological, etc.)…”

The Affective Neuroscience of Aging

“…In addition, the animal literature shows consistent decreases in delay discounting with age (e.g., Roesch et al, 2012, Simon et al, 2010), although a critical difference is that delay discounting tasks for animals require the animal to learn about the delays and rewards via repeated experience. With humans, most delay discounting tasks simply describe the amount of money and delay in text format on each trial and do not present outcomes until the end of the session.…”

“…With humans, most delay discounting tasks simply describe the amount of money and delay in text format on each trial and do not present outcomes until the end of the session. One interesting finding to note is that, when two options differ only in delay (not in reward amount) older rats are impaired at learning to avoid the longer delay option, but that when the two options differ in reward amount (and not in delay), they are not impaired (Roesch et al, 2012). Thus, learning about differences in time duration may be more impaired in aging than learning about differences in amount (c.f., Zanto et al, 2011).…”

Age-related differences in discounting future gains and losses.

“…The relationship of the prefrontal cortex to intertemporal choice would explain on an evolutionary level why animals that have less developed frontal cortices, such as rodents (see Roesch, Bryden, Cerri, Haney, & Schoenbaum, 2012) and non-human primates (see Woolverton, Myerson, & Green, 2007), discount the future at a substantially greater rate than humans (Mazur, 2001). The prerequisite of developed and functioning prefrontal cortices in order to make more patient intertemporal choices is also consistent with the observations that children discount future rewards at a greater rate than adults (Green, Fry, & Myerson, 1994).…”