Exploring distinct and joint contributions of the locus coeruleus and the substantia nigra/ventral tegmental area complex to reward and valence processing using high-resolution fMRI

Abstract

Dopaminergic neurons in the substantia nigra and the ventral tegmental area (SN/VTA) are classically viewed as key mediators in reward processing, while noradrenergic cells in the locus coeruleus (LC) are thought to modulate (negative) saliency processing. However, this conventional distinction is being revised by more recent research in animals. To explore the respective contributions of both the LC and SN/VTA in reward and valence processing in humans, we assessed fMRI data during stimulus encoding and response phase of a rewarded emotion-discrimination task (n=38). Participants responded significantly faster to reward-predicting and negative valence stimuli compared to their non-salient counterparts. LC activity was overall higher during trials involving reward prospect, and in particular for reward trials featuring positive valence, demonstrating an additive effect of reward and positive valence in LC. Moreover, LC activity was differentially increased for negative compared to positive valence in the response phase, indexing its role in invigorating responses to negative events. The SN/VTA showed increased activity in the response phase of reward trials (neutral valence) and negative valence trials (no-reward), which aligns with coding relative saliency of these events in their respective contexts. LC modulations were accompanied by covariations in occipital cortex, suggesting noradrenergic contributions to visual prioritization of salient events. The findings underscore the sensitivity of both LC and SN/VTA to reward prospects and negative valence, challenging the dominant view of SN/VTA's involvement in merely positive events and emphasizing their essential role in action invigoration above and beyond mere stimulus encoding. The intricate roles of the DA and NA system in reward and emotional valence processing in humans warrant further exploration and validation, given the limitations inherent to neuroimaging of deep brain structures.