The Role of Locus Coeruleus and Substantia Nigra/Ventral Tegmental Area activity in Reward and Salience Processing: A High-Resolution fMRI Study
Dopaminergic neurons in the substantia nigra/ventral tegmental area (SN/VTA) are classically associated with reward processing, whereas noradrenergic neurons in the locus coeruleus (LC) have been shown to support salience processing more broadly. However, rewarding stimuli are also inherently salient, and in most paradigms reward and salience are conflated, for example when rewarded targets are also rare and/or visually distinct. As a result, it remains unclear in how far these systems are sensitive to reward-specific properties or more basic salient properties of the stimulus. Using a visual oddball paradigm with high-resolution fMRI (n = 45), we compared oddballs that were rewarded, visually salient, or neutral, while keeping stimulus probability and motor requirements constant across conditions. Participants responded more accurately to rewarded than to neutral oddballs, and faster to rewarded than to both neutral and visually salient oddballs. At the neural level, SN/VTA activity was increased for rewarded oddballs compared with both neutral and visually salient oddballs, whereas LC activity was increased for rewarded compared with visually salient oddballs. Visual salience alone did not increase activity in either region relative to neutral oddballs. Together, these findings suggest that both the noradrenergic and dopaminergic systems are preferentially engaged by motivationally relevant stimuli, beyond more basic forms of salience.