natural lesions
lesion due to stroke, brain tumors, brain injury, or degenerative disorders
goal=to examine behavioral and cognitive deficits and infer from them what that part of the brain is doing -> can allow for inference reg. relationship between brain and behavior
limitations of natural lesions
- assumes a mapping between anatomical and cognitive models
- not possible to control size or location of lesion- damage doesn’t respect modules
- originally not possible to know brain area affected until after death
- assumes intact regions continue to fcn normally
- brains are plastic and can lead to recovery of behavior
- lesions do not allow us to determine temporal aspects of info processing
experimental lesions
create small controlled lesions in animals to study its effects
limitations of experimental lesions
- difficult and time consuming to train animals
- ethical concerns
- avoid naive mapping of complex processes onto discrete brain regions
benefits of experimental lesions
- more control over the location & extent of lesion
- better test hypothesis concerning specific regions and groups of regions
- can do reversible lesions which are less invasive and permanent
Transcranial Direct Current Stimulation (TDCS)
passes electrical current through brain between two scalp electrodes. Placement of electrodes can result in widespread of local inc in cortical exciteability
can be used to disrupt activity in a region to further understand its role
used as a tool to enhance performance and decrease deficits following brain injury and disease
Transcranial Magnetic Stimulation (TMS)
pass a v strong electrical current through a coil that generates a magnetic field in the head -> interacts w neurons
single pulse TMS
transiently influences neurons
brief effect
effective for mapping perceptual and motor areas, evaluating cortical excitability, and understanding temporal relationships
repetitive TMS
results in longer term inc or dec in cortical excitability
useful for understanding how brain areas interact during cognition
can be used as a form of temporary lesion
figure 8 coil
used to improve spatial specificity, results in more focused magnetic & electrical field
advantages of TMS
- noninvasive
- temporary effects
- can infer causality
disadvantages of TMS
- similar disad. as lesions
- can’t reach deep structures
- can indirectly activate large areas of cortex
- don’t always know where you’re stimulating
- has risks associated (seizure)
EEG
measures electrical signals of the brain using electrodes on scalp
record potential (volts)= difference between two electrodes
v high temporal resolution (milliseconds)
neural basis of EEG
affects pyramidal neurons of cortex- extracellular current flow is related to dendritic currents
requires a large number of synchronous neurons
reflects diff states of brain-can be used to determine how changes in brain state are related to behavior
event related potential (ERP)
small signal that requires an averaging of data over multiple trials-> useful for investigating timing/reaction rate of brain
responses named according to polarity (P,N) and latency (100 ms, etc)
inverse problem
you can’t determine a unique set of sources in the brain from an external pattern. activity -> brain area is flawed bc a lot of different brain areas can produce that activity
MEG
relies on SQUIDS to detect v smol magnetic fields
better spatial resolution bc magnetic field is unimpeded by tissue
sensitive only to tangential sources, not to radial sources
suffers from inverse problem but simplifies it
PET
decay of radioactive isotopes in blood that when positrons collide, will emit photons that are detectable by scanner
advantages of PET
- brain-based method of activity
- can image deep structures
- new isotopes are being generated to allow for imaging of diff compounds
- many clinical uses
disadvantages of PET
- secondary measure of neural activity
- invasive
- limits # of exposure
- v poor temporal resolution
- only moderate spatial resolution
fMRI
a measure of oxygenated blood flow
during inc activity, the amt of oxygen being delivered to the local area inc, and amt of oxygen removed from blood dec -> oxygen rich blood is LESS magnetic and results in larger BOLD signal
timescale of BOLD both delayed and extended- abt 11 seconds long
better temporal resolution than PET but still much less than EEG/MRI
