Class 5,6 Flashcards
Function x is disrupted by lesion to brain region Y, than Y supports function x (ex. fineas gage, HM, Broach (patient TAN))
Brain lesioning logic
Function x is disrupted by lesion to brain region Y, than Y supports function x
suggests a causal role of brain region Y in function X
(ex. fineas gage, HM, Broach (patient TAN))
Human vs non-human lesion studies
Limitations of Lesioning
Is the brain region critical for the task?
Disconnection syndrome
Disconnection syndrome
brain region X may not directly participate in function A, but may disconnect two brain regions that are critical for function A
Ex. Split brain patient - Severing the fibers of the corpus callosum leads to certain cognitive impairments BUT the corpus callosum does not carry out those
cognitive functions
Split-brain patients and saying what see (and why)
Most people have language centers in left
hemisphere
The visual system is contralaterally organized:
left field of vision is represented in right hemisphere
Shown in L hemisphere, r Hem process, but language in L (cannot say what see) but can pick up (motor control is on R side)
Logic/Issue of lesioning studies
Functions may be unrelated to brain region X itself, but related to keeping brain region X intact
Neuroplasticity, reorganizes and compensates for damage
Double Dissociation
determine if 2 factors are independent
ex. Brochas area - speach production NOT comp
Werinchek - imarirs speach comp but NOT production
Conduction aphasa
damage to arcutate fasciculus (white matter bundle of axions that connect brocha and wernicke;s area)
TMS
most common, figure 8 looking, area stimulating in middle
Use localized magnetic field pulses to alter electrical activity of neurons
- TMS coils can activate or deactivate regions of neurons depending on stimulation frequency
- Can be used to determine causal role of brain region
tDCS
Trancranial direct current stimulaiton
weak electral currnent between two patch electrodes
stimulation lower than TMS so can avoid adverse side effects
Can INC or DEC activity not compleet inhibit or stumulate
not as regonaly specific as TMS
Anodal (tDCS)
Inc activity
Cathodal (tDCS)
dec activity
tACS
A varation of tDCS that also involves an oscillating current
EEG
Measure voltage fluctuations from electrical activity of groups of neurons
NOT action potentals
electral activty through scalp
electroencephalogram
ERPs: brief changes in EEG time-locked to a stimulus event
Individual trials all have lots of background oscillation activity (noise), Take average of many trials to remove noise and get clean ERP signal
Betta Band
higher freq
Time-frequency analysis
amount of activity for a given frequency band
Pro EEG
- Excellent temporal resolution
- ERPs associated with cognitive processes
- Much more affordable than fMRI or MEG
Different frequency bands have been correlated with
different cognitive functions
Con EEG
- Poor spatial localization
- Not entirely solved with source modeling
- Poor access to deeper brain structures (e.g. MTL)
ECoG
Intracranial recording
• Electrode contact grids placed directly on the cortex
• Electrocorticography (ECoG)
- Cleaner signal than EEG, better spatial specificity
- However, invasive technique: recorded in epileptic patients who have grids implanted for medical purposes
MEG
Functional brain imaging
• Records magnetic fields generated by electrical currents using SQUIDs (Superconducting quantum interfering devices)
• Excellent temporal resolution, good spatial resolution
• Magnetic signals less distorted by skull
• Can also see ERP-type responses using MEG
• Event-related (magnetic) fields
• To best use spatial information of MEG, structural information needed
• MRI scan used to localize MEG
• Requires a special magnetically shielded room
• Magnetic fields being measured are very small
• Set-up can be quite expensive
In what ways are EEG and MEG similar?
- Both measure surface/scalp-level activity of groups of neurons
- Data can be analyzed similarly (e.g. with the event-related potential averaging method)
- Both have high temporal resolution for neural responses
what are some considerations for when you’d use EEG over MEG?
- EEG is more portable and not as sensitive to small disruptions in magnetic field, so you can take it for more ‘field studies’
- EEG is also way cheaper, so if you’re strapped for a budget and still want time sensitive data
CT Scan
Computerized Axial Tomography (CAT / CT)
- Allows for localization of brain damage
- Uses X-rays for visualization • Requires exposure to radiation
- Different tissues have different densities
- X-rays pass through fluid easily but are absorbed by higher densities like bone
- Density gradient: CSF < brain tissue < blood < bone
MRI
Magnetic resonance imaging
- Uses a large magnet measured in Tesla (strength of magnetic field) • 1 Tesla (T) = 10 000 Gauss • Earth’s magnetic field = 0.5 Gauss • 3T magnet = 60 000 times Earth’s field
- Combined with radio frequency pulse
How MRI Works
- Many organic elements in the body are magnetic
- Hydrogen is the most abundant in the body • Protons spin around a random axis
- When placed in magnetic field the protons become aligned in parallel
- A radio frequency pulse is used to push protons out of alignment with magnetic field
- The time it takes for protons to revert back to original orientation is measured through head coil
- Protons relax at different rates in different tissues, which makes a gradient that can be reconstructed as an image
DTI
Diffusion tensor imaging (DTI): measurement of connectivity
- Uses slightly different MRI protocol based on what direction water diffuses in nerve fibers
- Based on axis of water diffusion, DTI models connectivity of white matter tracts: tractography maps
MRI vs. fMRI
- MRI: High resolution (1mm), one image
- fMRI: Low resolution (~3mm), many images over time
BOLD
used in fMRI
BOLD: blood oxygen level-dependent • Neurons get oxygen from hemoglobin in red blood cells • When neuronal activity increases, blood flow increases to the area • More oxygenated blood flows to region
• Differences in magnetism of blood depends on oxygenation • Make differences in MR signal • Use differences in signal to infer activity
HR
Hemodynamic response
• Change in regional cerebral blood flow over time
- Slow response compared to the actual neural signal:
- Neural response happens on the scale of milliseconds • HR starts after 2 seconds and peaks around 6-8 seconds after stimulus onset
Block design
for fMRI
Examine extended HR across the same trial type
Event-related design
- HR for different, individual trials
- Can examine trial-specific HR
fMRI Analisis
- Detects change in signal from one condition to another
- Must always contrast two images • Functional images subtracted from each other and superimposed onto anatomical image • Choice of baseline depends on research question
MRI and fMRI: Pros
PROS: • Non-invasive • No radiation • Can do multiple scans on the same person • Excellent spatial resolution, temporal resolution is okay
MRI and fMRI: Cons
CONS: • Very expensive • Correlational measure (in that what we see could be due to several different factors)
PET
- Measures local changes in cerebral blood flow over a few minutes
- Radioactive isotope tracers introduced into body • Isotopes rapidly decay, which is measured to produce signal
PET Pros
- Tracking multiple metabolic processes
- Can label specific neurotransmitters with isotopes that will go to receptor sites
PET Cons
- Invasive: Radioactive isotopes
- Can only be done limited number of times
- Limited temporal resolution • Depends on half-life of isotope • E.g. oxygen-15 gives average of activity over 1.5 minutes
Optical Imaging
- Laser of infrared light
- Sensors detect distortions in light
- Slow signal: measures absorption from blood
- Fast signal: measures scattering of light related to neuronal firing
- High temporal and spatial resolution
- Only cortical activity can be detected • Too much light gets absorbed at deeper levels to be measured
Simultaneous EEG - fMRI
Originally applied to improve localization of sources for epileptic EEG activity (Ives et al., 1993) • Combining the relative strengths of each modality
Disconnection syndrome
general term for a collection of neurological symptoms caused – via lesions to associational or commissural nerve fibres – by damage to the white matter axons of communication pathways in the cerebrum
Density gradient:
CSF < brain tissue < blood < bone
