Lecture 4 -Research Methods Flashcards
Temporal Resolution
Difference in time between the measurement and brain activity
Spatial Resolution
The size of the brain region we are looking at (neuron, cortical column, entire brain, etc.)
High spatial resolution - we know where a change occurred
Low spatial resolution - we don’t know exactly where but we have a rough idea
Neural Stains
Selective
- Stains some tissue components but not others (nucleus, ribosomes and cell membrane)
Steps for Preparing Brain Tissue for a Neural Stain
1) Perfusion - remove blood with saline solution
2) Hardening - flash freezing or paraffin embedding
3) Slicing - microtome
4) Mounting - albumen
Golgi Stain
Silver chromate stains neurons black Not all neurons are stained Shape and size of neurons No intracellular details First look at the synapse - Cajal, Nobel Prize, Neuron Doctrine (neurons are single cells that are independent and separated by gaps)
Nissil Stain
Penetrates all cells Stains ribosomes Soma visible - Can better quantify (estimate) number of cells First view of structures within neuron Cresyl violet
Electron Microscopy
Greater magnification than light microscope
- Electrons are smaller particles of light
Slices coated with electron-absorbing substance (gold)
- Different parts of neurons absorb gold to different degrees
Electron beam passes through slice and image captured
Minute cellular details
- Difficult to assess overall or general structure
Scanning EM generates 3D images
Anterograde tracing
Where do axons go?
Autoradiography
- Amino acids with radioactive hydrogen isotopes are taken into cell bodies and incorporated into proteins
- Wait for a few days and identify radioactivity in axon terminals
Retrograde tracing
Where do axons come from?
HRP taken up by axon terminals
Wait a few days
Brain stained with HRP substrate to change it to black
- Black identifies originated cell bodies
Contrast X-rays
Effective only if internal structures differ from their surroundings - differences in x-ray absorption
Brain has too many overlapping structures absorbing x-rays to the same degree
To stand out - radioopaque material into structure of interest
- Discovered by Moniz
X-ray passed through brain onto photographic plate
Angiography
Contrast x-ray
Dye injected into carotid artery
Reveals enlarged or displaced blood vessels
Pneumoencelphalography
- Air injected into CSF (lumbar)
- Identify enlarged and displaced ventricles
(Very painful and has fallen out of use)
X-ray Computed Tomography
Early 1970s
Computer assisted x-ray
3D view of the brain
Brain CT composed of 8-9 horizontal sections
X-ray gun and detector rotate in opposition around the head
Not sharp image
- Low resolution axial image
Used to visualize structural abnormalities: tumors, stroke damage, concussion/brain injury
Low spatial resolution, no temporal resolution
Position Emission Tomography (PET)
Highlight metabolically active brain areas
Inject carotid artery with positron emitting radionuclide (2-deoxyglucose)
- 2DG structurally similar to glucose and active cells take up more glucose
- 2DG can’t be metabolized so it accumulates in active cells
Positrons interact with electrons, produce gamma rays (photons)
Scanner detects photons and how many gamma rays are coming from a particular region
Indicates areas of activity during task (reading, speaking, remembering, etc.)
Axial images
No structural information
- Poor spatial resolution
- Co-registered with MRI
Can also identify non-activity measures
- Neurotransmitters, receptors, transporters, ions
Some spatial resolution, some temporal resolution (delay of 40 minutes - not a snapshot)
Magnetic Resonance Imaging (MRI)
High spatial resolution
Horizontal, coronal, and sagittal planes
Expensive
No ferrous metal
Strong magnetic field passed through brain
- Aligns with hydrogen atoms
- Rf pulse causes hydrogen atoms to emit electromagnetic radiation
- Scanner detects emitted radiation
- Neural structures differ greatly in hydrogen atom density
High spatial resolution but no temporal resolution
Functional MRI
Most influential tool in cognitive neuroscience
Uses MRI methods
- Functional, structural and non-invasive
- 3D images of activity over brain
High spatial resolution and poor temporal resolution but better than PET (delay of 2-6 seconds - remember that AP occurs in ms)
BOLD response: activity-related changes in blood flow
- Oxygenated blood accumulates in active areas - oxy and deoxyhemoglobin different magnetic moments
- Detect changes in blood oxygenation - hemodynamics
Warning: pictures do not reflect changes in activity, they reflect changes in BOLD signal
Diffusion Tensor Imaging (DTI)
Identify white matter tracts in the brain
- Connections among structures
- human “connectome” project
Uses MRI technology
Water molecules move in the same direction in white matter - outside of white matter, water molecules diffuse randomly
Example DTI findings:
- Alcohol degrades white matter
- Pre-clinical AD deterioration of white matter (change in white matter of the brain - Alzheimer’s, alcoholics, MS)
Spatial resolution, no temporal resolution
Transcranial Magnetic Stimulation (TMS)
Disrupts neural activity by placing magnetic field under coil positioned over skull
- During cognitive and behavioural tasks
- Assess functions of different cortical areas - establish structure-function and use PET and fMRI to correlate activity to task
TDCS - apply current directly to scalp
Noninvasive
Intervention - mixed results
Current questions: depth of effect, safety and exact mechanism of neural disruption
Good temporal resolution, smeared spatial resolution
One of the few methods used to establish cause and effect in the brain
Scalp Electroencephalography (EEG)
Measure electrical activity from scalp
- Signal is the difference in electrical potential between two large scalp electrodes as a function of time
- In lab, animals EEG recorded inside brain - better spatial resolution than scalp
Signal decays from source over space and time
- Multiple electrodes, correlate signal
EEG waves reflect sum total of all electrical events: clouding underlying nature or neural activity: AP, EPSP, IPSP, eye movements, scalp muscle movements, skin, blood flow
Really, really high temporal resolution, poor spatial resolution
- Evoked potentials/ event related potentials (ERPs)
- need to increase signal to noise ratio
- signal = response to stimulus
- noise = background EEG
- measure repeated ERP to same stimuli and average 0 cancels out background noise
P300 = meaningful needing response
Far field potentials with onset of stimuli, not meaning related, likely brainstem generated - characteristic of an individual detecting and identifying a stimulus
Used in diagnosing epilepsy and developing Brain-Computer Interface (BCI)
EEG Waves
EEG wave forms associated with states of consciousness or brain pathology
Aroused state: low amplitude, fast EEG activity (beta)
Relaxed and awake: high amplitude, slow (10 Hz) EEG activity (alpha)
Deep sleep: high amplitude, slow (1 Hz) - slow oscillations
Epilepsy: spikes (large amplitude, high spiking)
Theta waves: repetitive behaviours
Magnetoencephalography (MEG)
Noninvasive
Measures magnetic fields generated from scalp from neural activity
- Neural currents produce weak magnetic field
Measures ongoing cortical activity
Better temporal resolution than fMRI
Greater spatial accuracy than EEG - localize source of epilepsy
- only as deep as the cortex
Electromyography (EMG)
Measure of somatic nervous system
Measures muscle tension
- Indicator of arousal
Change in voltage between 2 electrodes on large muscle - over skin
Amplitude of signal measures combined with level of muscle tension - integrate raw signals - easier to interpret and height indicates the number of spikes in muscle unit - number of fibers contracting
Clinical uses: diagnoses of neuromuscular problems, biofeedback and man-machine (MASA flying places without keyboards and joysticks)
Important tool for ALS or degeneration of motor neurons/muscle
