Motivation in the motor system - Coggle Diagram
Motivation in the motor system
Focus on positive reinforcement to increase/maintain behaviour
Positive motivational (rewarding) factors: Intrinsic and extrinsic
How does reward impact motor behaviour? Increase vigour/engagement and behavioural changes through reinforcement-based learning
Speed-accuracy trade-off: the faster you are the less accurate you are
Manohar et al. 2015: Participants made eye movements towards targets. Looked at the trade off in selecting the action and executing the action. Reward breaks the speed-accuracy trade-off, making participants faster, ore accurate and the ability to inhibit the distractor target. (Money was reward)
Summerside et al. (2018): Reaching movements. No accuracy demands just measuring how fast participants would reach the reward zone. Reward increases behavioural vigour (Money was reward)
Codol et al., 2020: combining the above two studies with reaching movements were measured through selection and execution. Reward again broke the trade-off. Also looked at punishment (negative reinforcement) moving faster meant losing less money, this had the same effect on performance.
Reward, effort and dopamine
For every behaviour our brain evaluates benefit vs cost (the faster we move the more energy spent)
Why don't we move fast and accurately all the time? (w/o reward)
Reward pays the cost for this additional effort
Shadmehr et al. 2016: participants made reaching movements to one of 8 different directions, found participants preferred to make movements with directions that just needed to move the elbow and move faster than directions that required moving the whole arm. This suggests participants took the mass of the arm into consideration as the whole arm is heavier and so requires more energy to move, and chose the action that was less effortful.
Termed as the 'utility' of movement- the worth of a particular movement. Increasing reward leads to an increase in utility as the effort is seen as more worthy.
Humans make decisions on the effort and reward.
When moving fast, the amount of noise and variability in our actions increase. Increased stiffness through muscle contraction allows us to reduce noise. This is highly energetically demanding compared to normal movement.
This is probably why the brain needs reward to use this mechanism
Codol et al. 2020: reaching movements- measured stiffness in arms, the bigger ellipses the more co-contraction/stiffness there is the arm. The area of the ellipses was bigger in the reward trials, suggesting
reward enhance motor behaviour by inducing mechanism which increases the stiffness of our arms.
Carroll et al. 2019: To be accurate and faster the brain needs to make more online corrections.
Reward leads to greater online corrections by paying for the attentional and metabolic costs of the corrections.
Dopamine and the Basal Ganglia: dopamine cells project to the striatum (d1 and d2 receptors) which excites the direct pathway and inhibits the indirect pathway this allows movements to be initiated.
Two forms of dopamine firing:
- dopamine released independently of neuron activity, low level, constant activity.
- large amount of dopamine released in spikes of neuron activity
There is a clear dissociation between tonic and phasic dopamine and the roles of dopamine in increased vigour and reinforcement-based learning
Tonic dopamine is related to our sensitivity to reward and effort
- high levels of dopamine means you are more sensitive to reward and less sensitive to cost, making it more likely that you will increase speed an accuracy when reward is present. Low levels of dopamine means you will be less sensitive to reward and so movements are less vigorous (like in Parkinson's)
Reward, effort and Parkinson's disease (low levels of dopamine)
Low levels of dopamine in the striatum meaning that individuals have a weak direct pathway and an over excited indirect (inhibitory) pathway, preventing the production of movement- akinesia and bradykinesia
The relationship between reward and effort is altered: less sensitive to the effects of reward and more sensitive to the effort of a task
Manohar et al. 2015: In Parkinson patients, reward has little effect on execution and selection. Suggests they have less vigour, suggesting they have increased sensitivity to effort and reward has less effect.
Do they have reduced effort or are they just not able to move as fast due to PD?
Mazzoni et al. 2007: PD patients can move with speed and accuracy, similar to controls, if forced to do so HOWEVER they choose not to. It took PD patients more trials to do the task successfully, so they can but they choose not to.
Salimpour et al. 2015: Similarly PD patients also show altered sensitivity to effort. Suggesting tonic dopamine is crucial in determining our sensitivity to effort. Using tDCS researches altered cortical excitability, using a cathode reduced PD sensitivity to effort. The results are unclear due to the cathode making it harder for the area to be sensitive BUT PD patients were on medication and this can alter the effects of cathodal and anodal tDCS, with cathodal tDCS causing an increase in dopamine production in the brain
Chang et al. (2015): Dopamine medication enhances PD patients willingness to produce effort compared to PD without medication