Magneto- and electroencephalographic manifestations of reward anticipation and delivery

The monetary incentive delay task was used to characterize reward anticipation and delivery with concurrently acquired evoked magnetic fields, EEG potentials and EEG/MEG oscillatory responses, obtaining a precise portrayal of their spatiotemporal evolution. In the anticipation phase, differential activity was most prominent over midline electrodes and parieto-occipital sensors. Differences between non-reward- and reward-predicting cues were localized in the cuneus and later in the dorsal PCC, suggesting a modulation by potential reward information during early visual processing, followed by a coarse emotional evaluation of the cues. Oscillatory analysis revealed increased theta power after non-reward cues over fronto-central sites. In the beta range, power decreased with the magnitude of the potential reward and increased with reaction time, probably reflecting the influence of the striatal response to potential reward on the sensorimotor cortex. At reward delivery, negative prediction errors led to a larger mediofrontal negativity. The spatiotemporal evolution of reward processing was modulated by prediction error: whereas differences were located in PCC and putamen in the prediction error comparison, in the case of expected outcomes they were located in PCC, ACC and parahippocampal gyrus. In the oscillatory realm, theta power was largest following rewards and, in the case of non-rewards, was largest when these were unexpected. Higher beta activity following rewards was also observed in both modalities, but MEG additionally showed a significant power decrease for this condition over parieto-occipital sensors. Our results show how visual, limbic and striatal structures are involved in the different stages of reward anticipation and delivery, and how theta and beta oscillations have a prominent role in the processing of these stimuli.