Scientific Understanding of Consciousness
Unconscious Motivational Processes
Science 11 May 2007: Vol. 316. no. 5826, pp. 904 - 906
How the Brain Translates Money into Force: A Neuroimaging Study of Subliminal Motivation
Mathias Pessiglione,1,2 Liane Schmidt,2 Bogdan Draganski,1 Raffael Kalisch,1 Hakwan Lau,1 Ray J. Dolan,1 Chris D. Frith1
1 Wellcome Trust Centre for NeuroImaging, Institute of Neurology, University College London, 12 Queen Square London WC1N 3BG, UK.
Unconscious motivation in humans is often inferred but rarely demonstrated empirically. We imaged motivational processes, implemented in a paradigm that varied the amount and reportability of monetary rewards for which subjects exerted physical effort. We show that, even when subjects cannot report how much money is at stake, they nevertheless deploy more force for higher amounts. Such a motivational effect is underpinned by engagement of a specific basal forebrain region. Our findings thus reveal this region as a key node in brain circuitry that enables expected rewards to energize behavior, without the need for the subjects` awareness.
Humans tend to adapt the degree of effort they expend according to the magnitude of reward they expect. Such a process has been proposed as an operant concept of motivation. Motivational processes may be obvious, as when a prospector spends days in extreme conditions seeking gold. The popular view is that motivation can also be unconscious, such that a person may be unable to report the goals or rewards that drive a particular behavior. However, empirical evidence on this issue is lacking, and the potential brain mechanisms involved in converting expected rewards into behavioral activation are poorly understood.
We developed an experimental paradigm to visualize unconscious motivational processes, using functional magnetic resonance imaging.
After correction for multiple comparisons over the whole brain, the only significant activation was located bilaterally in the basal forebrain, bordering several structures encompassing the ventral striatum, ventral pallidum (VP), extended amygdala, and basal nucleus of Meynert. These structures have been conceptualized as forming output channels for the limbic system, which is devoted to emotional and motivational functions. According to fiber tracing studies, reward-related information may access these structures either by a subcortical route via the hippocampus and/or amygdala or by a cortical route via the orbitofrontal and/or anterior cingulate areas.
To improve anatomical localization, we coregistered the statistical parametric map (SPM) with a recent histology-based atlas of the basal ganglia, which was designed to distinguish between functional territories.
To dissociate motivation per se from force production, we next examined brain activity that was linearly related to the amount of force produced. After correction for multiple comparisons over the whole brain, significant activations were found in the supplementary motor area (SMA) and in the primary motor area (M1). Unlike the pallidum, these structures have previously been shown to activate in relation with the amount of force produced. Moreover, M1 activation was observed on the left side, which was consistent with the use of the right hand for the task, whereas pallidal activation was bilateral.
These results indicate that motivational processes involved in boosting behavior are qualitatively similar, whether subjects are conscious or not of the reward at stake. Consistently, the same basal forebrain region underpinned subliminal and conscious motivation. However, differential sympathetic arousal denoted by SCRs argues against an interpretation in terms of mere stimulus-response habit formation, which is known to involve the basal ganglia. More generally, this paradigm offers a potential tool to discriminate between motor and affective components of motivation for financial reward in humans.
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