Topic 3 Determining brain function

Question 1

What are the 4 different ways the electrical activity of the brain can be measured?

Answer 1

1) Single-cell recording
2) EEG
3) ERP
4) MEG

Question 2

What are the two different types of electrical activity of brain cells and describe what they represent.

Answer 2

1) Graded potentials:
Voltage of a neuron or receptor that changes with the intensity of the stimulus.
2) Action potentials:
Large and brief reversal in the polarity of an axon.

Question 3

What is the main advantage of single-cell recording?

Answer 3

Know what the cell is tuned to.

Question 4

What are the 5 disadvantages of single-cell recording?

Answer 4

1. Know how it is tuned to the stimuli you thought of
2. Very invasive
3. Hard to record from a cell for extended periods
4. Hard to simultaneously record from many cells and population responses are important
5. Biased to recording from larger cells

Question 5

What do EEG recordings actually measure?

Answer 5

electrical potentials or “brain waves” (typically the sum of rhythmically graded responses of thousands of neurons).

Question 6

What happens to the amplitude and frequency of EEG waves as mental activity decreases?

Answer 6

Decreasing mental activity leads to lower frequencies and variable (higher) amplitudes.

Question 7

What is an ERP?

Answer 7

ERP (Event-related potential)

Brief change in a slow-wave EEG signal in response to a discrete sensory stimulus.

Question 8

Why do you need to average over many trials in an ERP study?

Answer 8

EEG waveform reflects neural activity from all regions of the brain. Most of this activity is not linked to processing the stimulus of interest. So the signal-to-noise ratio is very low.

Question 9

What is the main advantage of an ERP study compared to a behavioural study?

Answer 9

Have a continuous measure, so can be used to investigate the time-course of cognitive processing, as well as the processing of unattended stimuli.

Question 10

What are the advantages of ERP studies compared to other imaging techniques?

Answer 10

1. Less invasive than single-cell recording & PET
2. Better temporal resolution than PET & fMRI
3. Relatively cheap to conduct

Question 11

What is the main advantage and disadvantage of an MEG study?

Answer 11

1. Very good temporal and spatial resolution (much better than EEG/ERP)
2. High cost

Question 12

What are the two types of brain-stimulation techniques?

Answer 12

1) Microelectrode stimulation

2) TMS

Question 13

What are the two main ways to image the structure of the brain?

Answer 13

1) CAT/CT

2) MRI

Question 14

Describe how PET works.

Answer 14

Radioactive molecules are injected into the bloodstream.
Active areas use more blood.
Molecules release radioactive particles that are detected by the PET camera.
A computer reconstructs the variations in the density of the flow of particles, and
produces an image representing areas of high and low blood flow.

Question 15

What are the advantages and disadvantages of PET?

Answer 15

Advantages:

1. Can detect a wide range of radiochemicals
2. Can detect relative amounts of a neurotransmitter, density of receptors, degenerative processes, metabolic activities that occur during learning, brain poisoning.

Disadvantages:

1. Very expensive
2. Measurement of neural activity is indirect
3. Uses a subtraction process
4. Uses radioactive tracer

Question 16

What changes in brain activity do fMRI images directly measure?

Answer 16

fMRI measures BOLD (blood-oxygen-level-dependent) signal.

fMRI detects changes in blood oxygenation associated with neural activity.

Question 17

What are the advantages and disadvantages of fMRI?

Answer 17

Disadvantages:

1. Expensive
2. Magnets are noisy, so hard on the subject.

Advantages of MRI, c/2 CT (static):

1. Much better spatial resolution
2. Better discrimination between grey and white matter
3. Can be adapted to detect changes in blood oxygenation associated with neural activity

Advantages of fMRI, c/2 PET (dynamic images):

1. No radioactive tracer (safer)
2. Better spatial resolution
3. Better temporal resolution

Question 18

An fMRI study seemed to indicate that dead salmon have brain activity. What important aspect of fMRI studies did that study highlight?

Answer 18

Issues with fMRI & PET studies:

1. Design
2. Number of comparison made
3. Run 2 conditions: Baseline & Experimental
4. fMRI does thousands of statistical tests to check whether there’s a difference between the control and the experimental conditions. If you don’t control for the statistical issues you can get false results, just because of the number of tests done.

Question 19

In designing an fMRI study, what two conditions do you need to run? How do you want these two conditions to differ?

Answer 19

1. Baseline
2. Experimental

Baseline task uses all of the perceptual / cognitive components that experimental task has, except for the one of interest.

Question 20

Consequently, what does the final fMRI activity pattern actually represent? That is, describe the subtraction process.

Answer 20

Subtraction process:
Subtract baseline from experimental activity, the active areas remaining are those that do the task of interest. (Stimulation - Control = Difference)

Question 21

A functional imaging study suggests that a cortical region is important in the processing of a task, but a lesion study indicates that it isn’t. What are the four potential reasons for this difference?

Answer 21

1. Activated region reflects a particular strategy of the participants, not essential for doing the task
2. Activated region reflects the recruitment of some general cognitive resource, e.g., increased attention or arousal
3. “Activated” region is being inhibited
4. Lesion study is not powerful enough to detect region

Question 22

An imaging study suggests that a region is not important to a task, but the lesion study indicates that it is. What are the three potential reasons for this difference?

Answer 22

1. The experimental and baseline tasks both critically depend on that region
2. Intrinsically hard to detect activity in this region, e.g. region very small, small change in activity
3. Impaired performance reflects damage pathways going through the region, not the actual region.

Question 23

What is DTI?

Answer 23

Diffusion Tensor Imaging

MRI method that images fiber pathways by detecting directional movements of water molecules in ventricles.