Measuring and Modeling the Interaction Among Reward Size, Delay to Reward, and Satiation Level on Motivation in Monkeys

• Published in: Journal of neurophysiology Vol. 101; no. 1; pp. 437 - 447
• Main Authors: Minamimoto, Takafumi, La Camera, Giancarlo, Richmond, Barry J
• Format: Journal Article
• Language: English
• Published: United States Am Phys Soc 01.01.2009; American Physiological Society
• Subjects: Algorithms; Analysis of Variance; Animals; Conditioning, Operant - physiology; Cues; Data Interpretation, Statistical; Macaca mulatta; Models, Neurological; Motivation; Photic Stimulation; Reinforcement Schedule; Reinforcement, Psychology; Reward; Satiation - physiology
• ISSN: 0022-3077; 1522-1598
• DOI: 10.1152/jn.90959.2008

Laboratory of Neuropsychology, National Institute of Mental Health, National Institutes of Health, Department of Health and Human Services, Bethesda, Maryland Submitted 24 August 2008; accepted in final form 3 November 2008 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. Address for reprint requests and other correspondence: B. J. Richmond, Lab. of Neuropsychology, National Inst. of Mental Health, NIH, Rm. 1B-80, 49 Convent Dr., Bethesda, MD 20892-4415 (E-mail: bjr{at}ln.nimh.nih.gov )