Locus coeruleus neurons encode the subjective difficulty of triggering and executing actions

Abstract

The brain stem noradrenergic nucleus locus coeruleus (LC) is involved in various costly processes: arousal, stress, and attention. Recent work has pointed toward an implication in physical effort, and indirect evidence suggests that the LC could be also involved in cognitive effort. To assess the dynamic relation between LC activity, effort production, and difficulty, we recorded the activity of 193 LC single units in 5 monkeys performing 2 discounting tasks (a delay discounting task and a force discounting task), as well as a simpler target detection task where conditions were matched for difficulty and only differed in terms of sensory-motor processes. First, LC neurons displayed a transient activation both when monkeys initiated an action and when exerting force. Second, the magnitude of the activation scaled with the associated difficulty, and, potentially, the corresponding amount of effort produced, both for decision and force production. Indeed, at action initiation in both discounting tasks, LC activation increased in conditions associated with lower average engagement rate, i.e., those requiring more cognitive control to trigger the response. Decision-related activation also scaled with response time (RT), over and above task parameters, in line with the idea that it reflects the amount of resources (here time) spent on the decision process. During force production, LC activation only scaled with the amount of force produced in the force discounting task, but not in the control target detection task, where subjective difficulty was equivalent across conditions. Our data show that LC neurons dynamically track the amount of effort produced to face both cognitive and physical challenges with a subsecond precision. This works provides key insight into effort processing and the contribution of the noradrenergic system, which is affected in several pathologies where effort is impaired, including Parkinson disease and depression.

Natural locus coeruleus dynamics during feeding

“…Early electrophysiology studies in rats (5,8) and monkeys (9) noted inhibition of LC neurons during liquid consumption of sucrose or juice. In contrast, LC neurons are endogenously activated by cues predicting reward availability, and this activation is modulated by reward size and effort (10)(11)(12). Whether these endogenous LC responses play a causal role in the regulation of feeding behaviors remains unknown.…”

Covariations between pupil diameter and supplementary eye field activity suggest a role in cognitive effort implementation

“…As for neural implementation, phasic locus coeruleus (LC) activity is known to transmit feedforward information to the SEF via ascending projections to the prefrontal cortex (PFC), providing a plausible pathway for such a bottom-up influence. Recent work has shown that LC neurons can reflect both cognitive and physical efforts with a subsecond precision [ 37 ].…”

Insights into the role of noradrenaline in effortful decisions

“…In this new study, Bornert and Bouret [ 4 ] highlight important properties of LC neurons compatible with the notion of subjective difficulty to overcome both physical and cognitive challenges.…”