4 - Investigating the Brain Flashcards
Frequency Banding of electric impulses in the brain (found in EEG)
Delta (0-4Hz)
- large delta amplitude is associated with sleep
- found in frontal areas
Theta (4-8Hz)
- linked to maximal alertness and altered states of consciousness, such as meditation
- found widespread in the brain
Alpha (8-13Hz)
- mental effort is associated with decreased alpha amplitudes, increased alpha is associated with relaxation
- posterior regions such as the parietal and occipital cortices
Beta (13-30Hz)
- beta waves are reduced by motor movement
- most present in the frontal locations
Gamma (30+ Hz)
- gamma waves are linked to cognition
- found across the somatosensory cortex
EEG: pros and cons
Electroencephalogram
- direct method of brain study
- electrodes are connected to the head to record electrical brain activity
Pros:
- very fast measurements
- cheap
Cons:
- The Inverse Problem
> EEG can’t tell you exactly where the electric signal came from, these pass out of the brain through the skull via the path of least resistance which is the sutures of the skull
> thus the problem is you have to predict the source from the signal
- Spatial Resolution
> the information is difficult to narrow down to a specific area
MEG: pros and cons
Magnetoencephalography
- direct method of brain study
- MEG machine detects the magnetic field produced by electric currents in the brain
> since currents have a magnetic field at 90 degrees from the direction of current
Pros:
- as fast as EEG
- better spatial resolution than EEG because there is no magnetic resistance (works best at the surface of the brain
Cons:
- the machine can’t detect magnetic fields on the exact horizontal plane
> this is a theoretical limitation since this is not really seen in the brain
- expensive (squids have to be cooled with liquid nitrogen)
fMRI: pros and cons
Functional Magnetic Resonance Imaging
- indirect method of brain study
- allows you to look at function (rather than MRI’s looking at structure)
- creates a 3D structure of the brain made of boxels via the HDR
Haemodynamic Response (HDR):
- neurons fire, then 7 seconds later, oxygenated blood flows to the area
- 15 seconds after they fire, blood flow stops and oxygen levels drop below baseling
- Oxygenated blood and Deoxygenated blood have different magnetic strengths, so fMRI measures this and produces voxels
- this is called the BOLD signal:
> Blood
> Oxygen
> Level
> Dependent
Pros:
- getting cheaper
- very good spatial resolution
- reasonable temporal resolution
Cons:
- not comfortable, noisy (can’t be used for auditory stuff)
- slow technique because it uses the HDR
- doesn’t really show you electrical activity, just blood
PET: pros and cons
Positron Emission Tomography
- indirect method of brain study
- Administer a radioactive substance to the patient, normally one that is found in the body (13N, 15O, 18F) or radioactive glucose
- the substance is in the blood, when neurons fire in the brain, blood flows there and as the substance decays it releases positrons, which interact with electrons, causing Annihilation
- this releases gamma rays which are detected
Pros:
- can use radioactive substances that we know bind to certain receptors, to determine how much natural NT is released
- reasonable spatial resolution
Cons:
- slow, since dependent on HDR and radioactive decay
- expensive
- uses radioactive material (limitations)
DTI: pros and cons
Diffusion Tensor Imaging
- in an MRI machine
- detects the magnetism of water
Pros:
- not as fast as EEG or MEG but faster that fMRI and PET
- good for pathways
Cons:
- expensive
- noisy, (MRI)
Validity in animal testing (3)
Face Validity
- does the animal present with characteristics similar to the human condition
Construct Validity
- does the issue affect the same biological systems in the animal as it does in the human?
Predictive Validity
- do antidepressants make the animal less depressed?
- it’s rare to have a perfectly valid model
Behavioural testing in animals
Locomotive activity
- how much does a rat stick to the side / general movement? (anxiety)
Morris water maze
- pool with opaque liquid, with a hidden platform, task is to find the platform and see how long it takes for them to learn the location
- can manipulate the cues: where the rat is put in, and external cues (objects)
> tests memory and is used in Alzheimer’s research
5 choice serial reaction time chamber
- a main chamber with 5 small chambers with lights in
- light comes on in one chamber, rat goes to it and a reward comes behind it
- can measure general behaviour, reaction time and learning
Experimental Lesions (animal testing)
Non-selective lesions: - RF or electrical lesion > irreversible > removes all tissue around the tip of the electrode > often used to mark the recording site
- Excitotoxic Lesion
> irreversible
> removes only the cell bodies
> can be induced with any agonist to the main glutamate receptors (NMDA, AMPA, Kainate) - Aspiration
> irreversible
> removes all tissue in the area (sucked away) - Local anaesthetic
> reversible
> suppresses activity in the area
> can be hard to judge the extent of supprssion
Selective Lesions:
- irreversible
- selective toxins target certain types of cells via certain neurotransmitters
- E.g. 6 Hydroxydopamine (6OHDA) is used to model Parkinson’s Disease as it selectively removes dopaminergic neurons in the SNc (substantia nigra pars compacta) and reduces dopamine in the striatum
Electrophysiology in animal testing
- not limited to EEGs (as with humans) so can insert electrodes directly into the brain
Acute
- temporary insertion of electrode
Chronic
- permanent electrode
Intracellular - electrode is within the cell > patch clamping is one option Extracellular - electrode is just outside the cell
- if the input is normal but the output is changed, it indicates a problem with structure
- it the input is wrong but the output is normal, it indicates a corrective attempt
Neurochemistry
Microdialysis
- introduce fluid and measure the difference
Voltammetry
- measure the voltage effects of different chemicals
Perfusion Fixation
using the animals circulatory system to pump away the blood for preservation and post-mortem examination
Immunohistochemistry
post-mortem process that uses antigens, antibodies and fluorescent markers to detect cells of interest
