How the brain translates money into force: a neuroimaging study of subliminal motivation

Abstract

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.

Fig. 1

The incentive force task. Successive screens displayed in one trial are shown from left to right, with durations in ms. Coin images, either one pound (£1) or one penny (1p), indicate the monetary value attributed to the top of the thermometer image. The fluid level in the thermometer represents the online force exerted on the hand grip. The last screen indicates cumulative total of the money won so far.

Fig. 2

SPMs of brain activity. Voxels displayed in gray on glass brains showed a significant effect at P < 0.05 after correction for multiple comparisons over the entire brain. The [x, y, z] coordinates of the different maxima refer to the Montreal Neurological Institute (MNI) space. Axial and coronal slices were taken at global maxima of interest indicated by red symbols on the glass brains. SPMs are shown at a lower threshold (P < 0.001, uncorrected) and were superimposed on the average structural scan to localize significant activations. The images in the left column show regression with the amount of force produced, whatever the condition. The images in the middle column show contrast between conscious pounds and pennies trials (£1 to 1p, 100 ms). For this contrast, SPMs were coregistered with an atlas of the basal ganglia (right column). Caudate, putamen, and accumbens are shown in green; external and internal pallidum are shown in blue, with limbic sectors in violet.

Fig. 3

Main effects of stimulus duration. (A) Incentive force task. Time courses were averaged across trials for the different stimuli (black lines indicate £1 and white lines indicate 1p) and durations (thin, intermediate, and thick lines indicate 17, 50, and 100 ms, respectively). Time 0 corresponds to the moment of stimulus display. The histograms indicate the effect of motivation (£1 to 1p), and the error bars indicate SEM. Pallidal activation is expressed as percentage of blood oxygen level-dependent signal change. Force and skin conductance are expressed in proportion of the highest measure. (B) Perception task. Stimuli were the same as in (A). Possible responses were “seen £1,” “seen 1p,” “guess £1,” and “guess 1p.” A “correct” response means that the subject chose the stimulus that had been displayed. A “seen” response means that the subject perceived all or part of the stimulus. Error bars indicate SEM.