Topic 3 Determining brain function Flashcards
What are the 4 different ways the electrical activity of the brain can be measured?
1) Single-cell recording
2) EEG
3) ERP
4) MEG
What are the two different types of electrical activity of brain cells and describe what they represent.
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.
What is the main advantage of single-cell recording?
Know what the cell is tuned to.
What are the 5 disadvantages of single-cell recording?
- Know how it is tuned to the stimuli you thought of
- Very invasive
- Hard to record from a cell for extended periods
- Hard to simultaneously record from many cells and population responses are important
- Biased to recording from larger cells
What do EEG recordings actually measure?
electrical potentials or “brain waves” (typically the sum of rhythmically graded responses of thousands of neurons).
What happens to the amplitude and frequency of EEG waves as mental activity decreases?
Decreasing mental activity leads to lower frequencies and variable (higher) amplitudes.
What is an ERP?
ERP (Event-related potential)
Brief change in a slow-wave EEG signal in response to a discrete sensory stimulus.
Why do you need to average over many trials in an ERP study?
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.
What is the main advantage of an ERP study compared to a behavioural study?
Have a continuous measure, so can be used to investigate the time-course of cognitive processing, as well as the processing of unattended stimuli.
What are the advantages of ERP studies compared to other imaging techniques?
- Less invasive than single-cell recording & PET
- Better temporal resolution than PET & fMRI
- Relatively cheap to conduct
What is the main advantage and disadvantage of an MEG study?
- Very good temporal and spatial resolution (much better than EEG/ERP)
- High cost
What are the two types of brain-stimulation techniques?
1) Microelectrode stimulation
2) TMS
What are the two main ways to image the structure of the brain?
1) CAT/CT
2) MRI
Describe how PET works.
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.
What are the advantages and disadvantages of PET?
Advantages:
- Can detect a wide range of radiochemicals
- Can detect relative amounts of a neurotransmitter, density of receptors, degenerative processes, metabolic activities that occur during learning, brain poisoning.
Disadvantages:
- Very expensive
- Measurement of neural activity is indirect
- Uses a subtraction process
- Uses radioactive tracer
What changes in brain activity do fMRI images directly measure?
fMRI measures BOLD (blood-oxygen-level-dependent) signal.
fMRI detects changes in blood oxygenation associated with neural activity.
What are the advantages and disadvantages of fMRI?
Disadvantages:
- Expensive
- Magnets are noisy, so hard on the subject.
Advantages of MRI, c/2 CT (static):
- Much better spatial resolution
- Better discrimination between grey and white matter
- Can be adapted to detect changes in blood oxygenation associated with neural activity
Advantages of fMRI, c/2 PET (dynamic images):
- No radioactive tracer (safer)
- Better spatial resolution
- Better temporal resolution
An fMRI study seemed to indicate that dead salmon have brain activity. What important aspect of fMRI studies did that study highlight?
Issues with fMRI & PET studies:
- Design
- Number of comparison made
- Run 2 conditions: Baseline & Experimental
- 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.
In designing an fMRI study, what two conditions do you need to run? How do you want these two conditions to differ?
- Baseline
- Experimental
Baseline task uses all of the perceptual / cognitive components that experimental task has, except for the one of interest.
Consequently, what does the final fMRI activity pattern actually represent? That is, describe the subtraction process.
Subtraction process:
Subtract baseline from experimental activity, the active areas remaining are those that do the task of interest. (Stimulation - Control = Difference)
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?
- Activated region reflects a particular strategy of the participants, not essential for doing the task
- Activated region reflects the recruitment of some general cognitive resource, e.g., increased attention or arousal
- “Activated” region is being inhibited
- Lesion study is not powerful enough to detect region
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?
- The experimental and baseline tasks both critically depend on that region
- Intrinsically hard to detect activity in this region, e.g. region very small, small change in activity
- Impaired performance reflects damage pathways going through the region, not the actual region.
What is DTI?
Diffusion Tensor Imaging
MRI method that images fiber pathways by detecting directional movements of water molecules in ventricles.
