8/4/2019 Miyapuram Abstract

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Neuroeconomics of Reward Processing in the Human Brain

Krishna P. Miyapuram1*, Philippe N. Tobler2, Wolfram Schultz3

1Center for Mind/Brain Sciences (CIMeC), Universit degli Studi di Trento, ITALY *e-mail: krishna.miyapuram@cantab.net

2Institute for Empirical Research in Economics, University of Zurich, SWITZERLAND

3Department of Physiology, Development & Neuroscience, University of Cambridge, U.K.

Neuroeconomics is the newly emergent multidisciplinary field combining psychology, economics and

neuroscience studying the phenomenon of 'how people make decisions'. In an uncertain environment, an

adaptive organism must learn to predict the occurrence of future events, which is essential for survival andenables the organism to make optimal decisions and prepare appropriate behavioural responses. Rewardseeking behaviour plays a fundamental role in the survival of a species and depends on successfully

extracting reward information from a large variety of environmental stimuli. Pavlovian conditioning offers a

classic paradigm to investigate the learning of predictive relationship between stimuli and rewards.Neurophysiological studies have identified dopaminergic neurons in the midbrain that respond to prediction

error during associative learning of positively reinforcing stimuli (rewards). Computational models have

emphasised the role of prediction error in learning process. This research concerns about how the

conditioned stimuli represent the rewards after learning the predictive relationship between them. Twoexperiments using event-related functional magnetic resonance imaging (fMRI) are described investigating

responses to conditioned stimuli associated with monetary rewards.

The first experiment investigates the efficacy of different modes ofvisually presented rewards

associated with abstract visual stimuli. Whilefrequent payment (e.g. every trial) of large monetary rewards isnot viable forexperimentation, the intrinsic motivation gained by such small rewards (e.g. 5pence) givenevery trial is usually low. To increase the motivation for individualtrials, we show pictures of U.K. money

bills, coins and correspondingalphanumeric descriptions, while participants were informed that they

wouldtake home only a percentage of displayed money. Increased activation in themidbrain was observed tostimuli predicting larger denomination of visuallypresented money. Increased activation in striatum and

lateral orbitofrontalcortex was observed using a spin echo acquisition acquired together with theusualgradient echo acquisition of fMRI sequences.

The second experiment investigated how conditioned stimuli representimagined rewards.Visual

imagery and perception of external events is known to activate common stimulus-specificbrain areas.

However, it is unknown whether similar mechanisms operate in thereward system. We associated abstractvisual stimuli with either the visual presentationor the imagination of monetary rewards and with non-rewarding, control pictures. Humanmidbrain regions showed activations irrespective of the monetary

rewards beingimagined or visually present. The activations correlated with pleasantness ratings ofabstractvisual stimuli and differentiated monetary rewards from control pictures. Asupport vector machine trained on

the midbrain activation patterns from the visuallypresented rewards predicted with 75% accuracy whether

the participants were to imaginethe monetary reward or the control picture. The observed midbrain

activations suggestcommon brain mechanisms involved in the abstract detection of rewards irrespectiveoftheir visual presence or imagination.

These findings form the first step to investigate the mechanismsunderlying representations of rewardby conditioned stimuli using the insightsfrom animal learning theory.