L21 - Prediction & Dopamine II

Question 1

How do dopamine neurons respond to “behavioural triggers”?

Answer 1

• Schultz et al. (1986)
  
  • Task recorded dopamine neurons in the ventral tegmental area and substantia nigra using monkeys
  • Monkey has his hand on a key, a box is in front of him which contains an apple and this is revealed when the monkey has his hand on the key release

Question 2

How does dopamine have a particular wave form?

Answer 2

• Mainly in the mid-brain
• Referred to as “wide-waveform” neurons: need to “identify” dopamine neurons and separate them out from the general population of neurons in an area, which will include many different types of neurons
• Generally determine a neuron as being a particular type by the neurotransmitter
  that they release (e.g. dopamine, glutamate and GABA)
• All neurons that were showing the wide waveform are gone with infusion of Casp3: confirms dopamine neurons show this wide waveform
• Positivity and negativity above and below the baseline shows the amplitude
• Can destroy the neurons and see what the profile is of the neurons that remain
• Looking at what the firing of neurons look like is better than injecting

Question 3

How did Schultz et al., (1986) show that dopamine neurons respond to behavioural triggers?

Answer 3

• About half of dopamine neurons fire when door is opened in study
• Many activated by movement execution but some neurons also depressed
• Dopamine neurons are firing at the earliest predictor that they are going to get food
• Firing to different things
• Below shows the spike rate (how many times the neuron is firing a second) - so how active it is at a particular point in time
  
  • Lines indicate when the response was made, dopamine neurons respond before the response was made

Question 4

How do dopamine neurons respond to surprising events?

Answer 4

Delayed response task = harder because the instruction light = tells them which lever to press is separated in time from the light that tells them to make a response. Requires the holding of information in memory
Dopamine neurons response to reward when it is “unexpected”, here that is when monkeys are learning the task (i.e. trigger produces reward 1s later, this is the same across all trial types)
Increase in “spike rate” when reward is delivered early on in the task, but not when they have learnt the task

Question 5

Why do dopamine neurons respond at different points in these tasks when testing surprising events?

Answer 5

• When instruction and trigger come on at the same time = increased spike
• Delayed response task, peak at separate instruction and trigger - still surprised as haven’t learnt prediction

Question 6

What are signal prediction errors?

Answer 6

Signal is at the beginning of the reward-predicted cue
- Decrease in firing when the thing you’re expecting to happen doesn’t happen - negative prediction error
- Even have dopamine firing when initially shown box - as not expecting

Question 7

What is optogenetics?

Answer 7

• Optogenetics = Biological technique to control the activity of neurons or other cell types with light
  
  • Deisseroth, 2011
    
    • microbial opsin genes that safely confer to neurons both light detection capability and defined high-speed effector function in a single readily targetable module
    • Allows us to control neuronal activity with light at very specific timepoints
    • Great for testing at exactly what timepoints particular populations contribute to learning and behaviour
    • Can also target specific neuronal populations
• However
  
  • Optogenetics is likely making neurons do things they are no supposed to do
    
    • e.g. physiologically-relevant firing - where they fire at different rate from one another (not homogenous)
    • Inhibition of these neurons can create a dip in firing which could be interpreted as learning signal when that is not true

Question 8

What is the causal link between dopamine and learning?

Answer 8

• Using optic genetics to drive prediction error
  
  • Tone leads to giving of sucrose
  • Prediction errors tell us we have more learning to do
• Driving dopamine prediction errors can make you learn about what is around you even though you would not normally learn about it
• Surprise is the necessary catalyst for learning

Prediction error - (what happened - what you thought would happen

Dopamine neurons conform to this