Linking sense and behaviour to memory

Question 1

Why is the world like a ‘code’?

Answer 1

Lots of sensory information (light, chemicals, pressures) that the brain has to turn into a NEURAL ACTIVITY, DECODE and turn into a MEANINGFUL ACTION (eg. approach, avoid)

Question 2

What is the ‘drift-diffusion model’ used to measure?

Answer 2

The subjective experience of a person or animal through their pattern of choices and response times

Question 3

Describe the ‘drift-diffusion’ model

Answer 3

• Have evidence bounds
• Start at a middle point and go up or down, based upon the sensory information at a point in time
• Eventually reach threshold of ‘match’ or ‘miss match’ - depending on how much evidence accumulated

Question 4

Why is there a distribution of reaction times between different people?

Answer 4

Evidence accumulation is NOISY

Question 5

What is a typical experiment for behaviour of the drift-diffusion model?

Answer 5

Animal decides which way the dots on the screen are moving

After some time - animal flicks eyes in the direction that the dots are moving

Question 6

What is the distribution of reaction times?

How is this recovered?
What does this show?

Answer 6

Skewed to the right (non-Gaussian)

Recovered when scale the X-axis according to the RECIPROCAL of the reaction time (1/reaction time)

Question 7

What is the underlying variable of the distribution of reaction times that has a gaussian distribution?

Answer 7

The RATE of evidence ACCUMULATION

Question 8

What does the equation t=A/r relate to? How?

Answer 8

Height of the decision bound: A
Width (reaction time):t
Rate (slope of the line): r

Question 9

What is reaction time inversely proportional to?

Answer 9

The RATE of evidence ACCUMULATION

Question 10

Describe the experiment where making drosophila make a decision

When is this decision easy? Hard?

Answer 10

• 2 chambers with 2 different odours
• Chose between bad odour and lower concentration of the bad odour
• At the interface where the two odour meet - drosophila must make a decision (decision zone) - carry on walking or turn around

Decision is easy if there is a large difference between the 2 concentrations

Decision is hard if there is a little difference between the 2 concentrations

Question 11

What happens if plot the reaction of the drosophila as a frequency distribution?

Why?

Answer 11

Rightward skew

Easy decisions are made quicker
Hard decisions take longer to make

Question 12

Describe the speed-accuracy trade-off

Answer 12

If have a difficult decision and want to be accurate - takes a long time

If want to make the decision quickly –> less accurate

Question 13

Describe the speed-accuracy trade-off experiment in mice

Answer 13

• Mouse has to choose the water port based upon an odour they smelt
• Reward –> correct water port
• Depending on hoe long give the mice to choose - decide more of less accurately

Question 14

What happens if plot the results of the mouse speed-accuracy trade-off experiment?

Answer 14

Curve that increases gradually and eventually saturates at a certain reaction time

Question 15

What explains why being forced to make a decision too early would make less accurate decisions?

How?

Answer 15

The DRIFT-DIFFUSION model:

• Start at 0
• Can diffuse to 1 (correct decision)
• Can diffuse to -1 (incorrect decision)
• If let the mice decide on their own - almost always get it right
• But it takes time to diffuse to 1
• If cut the decision time short, curve may be below 0 as haven’t gathered enough evidence

Question 16

What adjusts the trade-off between speed and accuracy?

How?

Answer 16

Moving the HEIGHT of the decision bounds

• If decision bounds are far away from 0 - lots of time to integrate evidence
• The closer the decision bounds are to 0, the quicker a decision will be made, however this decision may be LESS ACCURATE

Question 17

When can an animal adjust their decision bound?

Answer 17

Based on context (eg. if a predator is coming)

• Urgency is encoded in the neurons of the brain
• Move decision bounds closer together

Question 18

What happens to the decision bounds over time as the need to make a decision is more urgent?

Answer 18

Over time, decision bounds come closer and closer together (lines are sloped)

Question 19

How does evidence accumulation appear?

Answer 19

In spike rate of certain neurons in the visual cortex

Question 20

What is responsible to moving the eyes of a monkey left or right in a decision experiment?

What is it thought converged on this area?

Answer 20

The INTRAPARIETAL AREA

Sensory input from the eyes and from the visual cortex

Question 21

What happens to the firing rate of the neurons when the task is easier?

What does this cause?

Answer 21

Firing rate increases faster
(Due to the strength of the evidence being stronger)

Spike rate increase faster –> hit the decision bound quicker

Question 22

What do the recordings from the monkey intraparietal area show?

Answer 22

When the monkey moves its eyes to the OPPOSITE side of where the readings are taken from:
- Neuron spike rate DECREASES

• The activity of the neuron correlates with the monkey eventually decides to do (spike rate correlates with the accumulation of sensory evidence in the task)
• Peak just before the monkey makes a decision

Question 23

What happens in a fly with a FoxP mutant?

Why?

Answer 23

Fly takes abnormally long to make a decision:

• FoxP regulates a K+ channel
• KO FoxP –> more K+ channels in the membrane of the KC
• Hyper polarisation of the membrane
• Takes longer to depolarise
• Slows down the rate of evidence accumulation