title: Measuring and modeling the interaction among reward size, delay to reward, and satiation level on motivation in monkeys.
abstract: Motivation is usually inferred from the likelihood or the intensity with which behavior is carried out. It is sensitive to external factors (e.g., the identity, amount, and timing of a rewarding outcome) and internal factors (e.g., hunger or thirst). We trained macaque monkeys to perform a nonchoice instrumental task (a sequential red-green color discrimination) while manipulating two external factors: reward size and delay-to-reward. We also inferred the state of one internal factor, level of satiation, by monitoring the accumulated reward. A visual cue indicated the forthcoming reward size and delay-to-reward in each trial. The fraction of trials completed correctly by the monkeys increased linearly with reward size and was hyperbolically discounted by delay-to-reward duration, relations that are similar to those found in free operant and choice tasks. The fraction of correct trials also decreased progressively as a function of the satiation level. Similar (albeit noiser) relations were obtained for reaction times. The combined effect of reward size, delay-to-reward, and satiation level on the proportion of correct trials is well described as a multiplication of the effects of the single factors when each factor is examined alone. These results provide a quantitative account of the interaction of external and internal factors on instrumental behavior, and allow us to extend the concept of subjective value of a rewarding outcome, usually confined to external factors, to account also for slow changes in the internal drive of the subject.
authors.affiliation: Laboratory of Neuropsychology, National Institute of Mental Health, National Institute Health, Department of Health and Human Services, Bethesda, MD 20892-4415, USA.
authors.forename: Takafumi
authors.initials: T
authors.lastname: Minamimoto
authors.forename: Giancarlo
authors.initials: G
authors.lastname: La Camera
authors.forename: Barry J
authors.initials: BJ
authors.lastname: Richmond
mesh_terms: D000465:Algorithms; D000704:Analysis of Variance; D000818:Animals; D003216:Conditioning, Operant / Q000502:physiology; D003463:Cues; D003627:Data Interpretation, Statistical; D008253:Macaca mulatta; D008959:Models, Neurological; D009042:Motivation; D010775:Photic Stimulation; D012055:Reinforcement Schedule; D012054:Reinforcement, Psychology; D012201:Reward; D012527:Satiation / Q000502:physiology*