Motivation is what drives us to act. Many diseases alter motivation.
Damage to some brain areas causes abnormal, misdirected motivation — sometimes leading to compulsive stereotyped behaviours, or unplanned impulsive acts. This can sometimes be dramatic, causing considerable embarassment for patients and their families.
A loss of motivation results in clinical apathy — a disabling disorder that can be difficult to pin down. It is often missed, and is hard to quantify. It carries a huge social and economic burden, and is often frustrating and distressing for carers.
We study what motivation is, in terms of computations. Is it the selection of an appropriate action, given a goal? Or is it the willingness to invest energy, at a cost, in a rewarding course of action? We found that:
- Rewards and dopamine improve motor performance by reducing internal noise. “Reward pays the cost of motor and cognitive control”, Manohar et al. Current Biology 2015. Motivation increases the strength of error-corrective internal feedback, making movements more robust to variability: “Motivation dynamically increases noise resistance by internal feedback during movement” Manohar et al., Neuropsychologia 2019
- People are independently motivated by performance-dependent pay and fixed guaranteed pay: “Distinct motivational effects of contingent and noncontingent reward” Manohar et al. Psychological Science 2017. These two types of motivation are affected oppositely by the neurotransmitter dopamine: “Dopamine promotes instrumental motivation, but reduces reward-related vigour” Grogan et al., eLife 2020. However in healthy people, dopamine agonists energise movements but do not change these two types of motivation. “The effects of pramipexole on motivational vigour during a saccade task: a placebo-controlled study in healthy adults” Au-Yeung et al. Psychopharmacology 2024
- A drug that blocks muscarinic acetylcholine receptors reduces energisation by reward in healthy people “Muscarinic receptors mediate motivation via preparatory neural activity in humans” Grogan et al. eLife 2024.
- Damage to ventromedial prefrontal cortex reduces our tendency to stick with our goals when we are discouraged. “Goal commitment is supported by vmPFC through selective attention” Holton et al. Nature Human Behaviour 2024. This tendency to persist may be a crucial but unexplored part of clinical apathy. “Persistence of effort in apathy” Dallery et al Revue neurologique 2023.
- Damage to specific brian areas increase or decrease responses to incentives. “Human ventromedial prefrontal lesions alter incentivisation by reward”, Manohar & Husain Cortex 2016. This same brain area is important for strategies that rely on confidence. “Reduced decision bias and more rational decision making following ventromedial prefrontal cortex damage, Manohar et al. Cortex 2021.