Brain Imaging Flashcards
Electroencephalography (EEG)
- measures electrical activity of brain
- number of electrodes applied to scalp
- main diagnostic tests for epilepsy
EEG Strengths
- good temporal resolution (discriminates v brief events in time)
- cheap + safe
- portable + possible to record EEG while people are moving around
EEG Limitations
- poor spatial resolution (hard to determine precisely from which area of the underlying brain the signal has come)
- hard to detect activity from central regions (mainly on surface)
Electrophysiology - single neurons
- Hodgkin & Huxley recorded action potentials in the giant axon of Atlantic squid
- much of sensory + motor cortex mapped this way
- Hubel & Wiesel mapped development + organization of visual system
Electrophysiology Strengths
records directly from individual neurons
so is best way to know what the neurons are doing
Electrophysiology Limitations
- high risk of infection as it is invasive (penetrates brain)
- neurons DONT work in isolation but rather as large networks
- so network activity of multiple neurons impossible to investigate with this method
MRI (correlates brain activity w behaviour)
- magnetic field passes through person’s head, causing H atoms to align with magnetic field
- radio frequency waves temporarily disrupt this alignment causing a signal that can be detected by machine
- because dif areas of brain tissue have dif amounts of WATER, they emit dif signals
- analysis software converts detected signals into v detailed images of dif structures of brain
MRI (Diffusion tensor imaging)
uses the same MRI equipment to detect the large axon tracts (white matter) that flows through the brain & connect dif regions of cortex
fMRI
- cognitive processes use energy
- as brain region uses energy there is INCREASE in blood flow to that region (for more O2 to be used up)
- bc fMRI images the INCREASE IN BLOOD FLOW caused by the USE of oxygen, not the use of oxygen in the first place – there is delay in signal associated w/ brain activity
What is BOLD in fMRI studies
blood-oxygen-level-dependent effect
- oxygenated blood: doesn’t distort surrounding magnetic field, but deoxygenated blood does
- can be detected by scanner
MRI Strengths
- v high spatial resolution (exact locations)
- identifies specifical anatomical / structural / functional properties of dif brain regions
MRI Limitations
- large equipment so requires specialist facility w/ multiple rooms (machine must be kept in magnetically shielded room) + specialist staff
- safety risks
- expensive
PET
uses radioactive substances known as TRACERS to visualize GLUCOSE METABOLISM or the neurotransmitter / receptor function
- can also use radioactive tracers to bind selectively to proteins of interest
- used as diagnostic tool for Alzheimer’s disease (detecting buildup of Amaloid protein)
PET Strengths
- can detect dif chemical in brain associated with METABOLISM or functional properties (e.g. neurotransmitter levels or proteins)
PET Limitations
- expensive + specialist facilities + staff
- low SPATIAL RES compared to MRI
- risks associated w radioactive tracers being injected into participant’s blood
Brain modification
- medical treatment (abnormal brain functions - brain regions removed OR drugs used)
- enhancement (improvement of healthy function to ABOVE or BETTER than normal)
- scientific research (using brain imaging techniques like fMRI only give us correlational info - BRAIN MODULATION otoh gives info abt causation & whether a given brain region is NECESSARY for particular task)
Ablations
deliberate lesions allow a relatively high degree of precision
e.g. frontal leucotomy
Describe frontal leucotomy
- a leucotome was inserted into one of several holes drilled in skill
- a core of tissue removed
- or cutting implement inserted above eyelid & pushed through to frontal loves
CONSEQUENCES: profound personality consequences (e.g. apathy, emotional unresponsiveness, inability to plan)
Who popularised frontal leucotomy?
Walter Freeman in the 40s & 50s - 40,000 operations in USA
Electrical brain stimulation (old fashioned)
- reveals precise localisation of cortical function
- electrically stimulated part of the frontal
cortex in dogs; induced contractions of specific
muscles on the opposite side of the body
Non-invasive electrical brain stimulation - ECT
- electroconvulsive therapy
- seizure-inducing agents were used to treat psychiatric conditions as early as 1500s
- now used to treat severe depression
- electrical stimulation of frontal lobes needs to be strong enough to cause a seizure
transcranial direct current stimulation
(tDCS)
used to treat many psychological disorders
due to the lasting modulation of cortical excitability
a coil carrying an electrical
current generates a brief, focal magnetic pulse which activates a small
region of cortex (approximately 10 – 15 mm, depending on the size of the
coil) underlying the coil.
§ The activation acts like a ‘virtual lesion’, temporarily disrupting the tissue for
a few hundred milliseconds. The technique is painless (unless it triggers
muscle contraction)
