Cognitive effort and reward. Functional imaging of time on task and the involvement of dopaminergic and cholinergic substrates

Neuroimaging studies have identified the neural substrates associated with sustained cognitive efforts and control and their modulation by rewards. Different lines of evidence implicate the prefrontal cortex (especially the anterior cingulum, ACC), dopaminergic, and cholinergic substrates in this modulation. We studied here the activity of these substrates at increasing time on task (providing increasing levels of cognitive effort) in trial blocks with differing reward levels. In the cortex, while peaking in the ACC, activity associated with time on task was extensive, also including activity decrements outside the default mode network, primarily involving motor and somatosensory regions. Information about reward levels was carried in the ventral tegmental area and ventral striatum, consistent with their motivational role, but did not reflect tradeoffs with increasing efforts during time on task. Instead, parts of the basal forebrain corresponding to the cholinergic Ch4 nuclei increased in activity with time on task and were sensitive to reward levels. This finding is consistent with a cholinergic role in driving compensatory efforts modulated by reward levels, while dopaminergic substrates track levels of expected rewards irrespective of required sustained attention efforts. These findings identify the basal forebrain as a neuroimaging phenotype associated with sustaining task sets and cognitive efforts.