Looking at the Brain

COGS 17 – Neurobiology of Cognition

Handout #5, Wednesday, October 29th, 2008

Looking at the brain

Combination of Table 5.1 (p150) and Table 5.2 (p160) and Table 5.3 (p165)

What? / Stands for…? / How does it work? / Best used to look at? / Advantages / Disadvantages
CT / Computed tomography / * X-ray scanner rotated 1 degree at a time over 180 degrees
* Computer reconstructs image / Structural abnormalities, e.g. cortical atrophy or stroke lesions / * Slices brain so we can look at it to find structural problems / * Only horizontal plane
* Radiation exposure dangerous (ionizing)
MRI / Magnetic resonance imaging / * Magnetic field
 H atoms align
* Radio frequency wave
 Excites atomic nuclei
 Relax  emit energy
* Scanner detects H radiation
* Computer reconstructs image / Structural abnormalities / * No ionizing radiation exposure
* Better spatial resolution
* Can study more areas of brain: horizontal, frontal or sagittal planes / * Expensive
* No metal allowed
* Noisy in the machine
PET / Positron emission tomography / * Inject radionuclide (atom with unstable nucleus) that emits positrons
* Positrons interact with electrons to produce photons
* Scanner detects the photons
* Computer determines how many gamma rays (photons) from a particular region
* Map areas of activity (high/low) / Brain activity as a result of radioactive ligands binding to post-synaptic receptors / * Can look at function / * Only 10 mm resolution
* Invasive 
* Radiation is dangerous
fMRI / Functional MRI ^ / * Person performs task in MRI machine
* Measures regional metabolism by detecting levels of oxygen in blood vessels
* Blood oxygen level dependent (BOLD) signal / Both structure and function / * Can relate function to very specific structure
* No injection needed
* Can produce 3D image
* Good spatial resolution
* Short imaging time / Same as for MRI
EEG / Electroencephalography / * Electrodes placed on scalp to measure electrical activity
* Provide isolated stimuli to measure electrical response via one (or more) electrodes / Brain activity as a result of a cognitive process as shown by event related potentials (ERP) / * Good temporal information
* Can locate general area of electrical activity
* Less expensive
* Non-invasive / * Poor spatial information
* Poor resolution
* Cannot locate specific brain area
* Must sit really still so you don’t get a noisy recording
MEG / Magnetic EG ^ / SQUIDs (superconducting quantum interference devices) measure the magnetic fields produced by electrical activity / Brain activity as shown by synchronously firing groups of neurons / See above / See above
TMS / Transcranial Magnetic Stimulation / * Coil placed over target region
* Record cognitive failures / Behavioral response to electrical stimulation (excitation or inhibition) to brain / Used with animals and humans / Magnetic pulse applied over large area so specific area is not known (also see above)
Lesion Methods
E.g. Knife cuts / Damage a specific brain structure thus eliminating function and observing resultant behavior / Relationship between structure (brain) and function (behavior) / * Relates structure and function
* Specifies the exact brain structure / * Can only be used in animals (lesion a human’s brain?! – unethical/illegal!)
* What if lesion has other effects?
Neurohistology
* Nissl Stains (soma)
* Golgi Stain (neurons)
* Myelin Stains (myelin) / * Looks at actual brain tissue
* Fix, preserve, section, stain / Look directly at neuronal components in a brain / Can confirm lesion sites
Electrophysiology
* Intracellular recording
* Extracellular recording / * Electrically stimulate neurons
* Record electrical activity either inside or outside (as ions) / Directly measures electrical activity in the neuron / Directly measures electrical activity in the neuron / Can only be used in animals
Pharmacological methods
* 2-DG autoradiography
* In vivo microdialysis
* Immunocytochemistry
* In situ hybridization / * X-ray of chemical emissions
* Measures chemicals in fluid
* Localize proteins in tissue
* Localize DNA in tissue / Study specific proteins (receptors, transporters, etc.) – study brain at the molecular level / Useful if not studying an actual protein (e.g. look for synthetic enzyme, shows presence of non-protein NT)
Gene knockout / * Normally, gene  protein
* Remove gene  remove protein  lose function / Function of a protein –study brain at the molecular level / Specifies the exact protein (kind of like a small-scale lesion study) / * Only in animals
* What if the knockout gene has other effects?
Animal behavior paradigms / Conditioning…
* Classical conditioning
* Operant conditioning
Learning paradigms…
* Radial arm maze
* Morris water maze / Behavior, conditioning, learning / Don’t need to use human subjects / But animals are different from humans; we want to cross the animal-human barrier