Linking Senses and Behaviour to Memory

Question 1

What are David Marr’s 3 levels of analysis?

Answer 1

• Computational problem
• Algorithm
• Physical implementation

Question 2

What is the drift-diffusion model?

Answer 2

Accumulation of evidence until you reach the upper or lower boundary where you come to a decision

Question 3

How can decision making be studied in primates? (4)

Answer 3

• Look at moving dots and decide what is the direction of motion
• Distribution of the reaction times was skewed right
• Normal distribution is restored if you plot 1/reaction time (reciprocal)
• Reaction time is proportional to 1/rate of evidence accumulation

Question 4

How can decision making be studied in flies? (3)

Answer 4

• Give flies the choice between a high concentration of a bad odour and a low concentration of a bad odour and they decide quickly which side to stay on
• Give flies the choice between 2 very similar concentrations of the bad odour and they take longer to decide which side to stay on
• Again shows a right skewed distribution in reaction times

Question 5

Why is the distribution of reaction times skewed right? (2)

Answer 5

• Rate of evidence accumulation (r) = height of decision bound (A) / reaction time (t)
• Therefore t=A/r, r has a normal distribution so the t distribution only looks normal if you plot 1/t

Question 6

How can decision making be studied in mice? (3)

Answer 6

• Let a mouse sample an odour and choose a water port based on which odour it smelled, reward for correct one
• The longer you allow the mouse to sniff the odour the more accurate the decision
• Premature decision making leads to inaccurate decisions because not enough evidence has been accumulated to reach the correct decision bound

Question 7

How do you adjust the trade-off between speed and accuracy? (2)

Answer 7

• Move the decision bound
• Make the decision bound come down over time as the decision becomes more urgent

Question 8

Which level of analysis does the drift-diffusion model correspond to?

Answer 8

Level 2 - algorithm

Question 9

Which area of the monkey brain is responsible for generating saccades?

Answer 9

Lateral intraparietal area (LIP)

Question 10

What is the evidence for evidence accumulation in monkeys? (3)

Answer 10

• Easier to tell which direction the dots are moving in = LIP neurons ramp up and reach a decision faster
• Neurons reach threshold just before the monkey executes the decision action = evidence for physical implementation of drift-diffusion model evidence accumulation
• Rate of evidence accumulation differs depending on stimulus strength but the threshold is always the same

Question 11

What is the evidence for evidence accumulation in drosophila? (3)

Answer 11

• FoxP (transcription factor) mutants take abnormally long to decide between 2 odours
• FoxP mutants have excess K+ channels = ‘leaky’ neurons
• Takes longer to reach threshold to depolarisation = rate of evidence accumulation is slowed

Question 12

What is the physical implementation of the algorithm in David Marr’s 3 levels of analysis?

Answer 12

Evidence to suggest that it is the ramping up of neuronal activity to a threshold