Lecture 4: Methods for Studying Functional Specialization in Brain

Single cell recording

Microelectrode is placed close to an individual neuron to record the discharge of action potentials in cell; has been used to identify neurons that respond to particular stimuli

Electroencephalogram (EEG)

Electrodes are attached to skull and wired up to computer; technique provides amplified recording of waves of electrical activity that sweep across brain’s surface; most widespread and least expensive technique for studying electrical activity of large population of neurons

Gamma

• Very high frequency waves of 26-42+ Hz

• Thought to signal active exchange info between cortical and other areas of brain

• May be seen in conscious state or during REM

Beta

• Irregular, low-amplitude waves of 12 to 25 Hz

• Most evident in frontal and usually seen on both sides of brain in symmetrical distribution

• Associated with state of arousal or alertness

Alpha

• Synchronous waves of 7.5 to 13 Hz

• Associated with relaxed awake state

Theta

• Synchronous waves of 3.5 to 7.5 Hz

• Transition between sleep and wakefulness

• Associated with deep meditation, hynagogic state, creativity, and memory retrieval

Delta

• Low frequency, high-amplitude waves of less than 4 Hz

• Occurs during deepest stages of sleep and loss of consciousness

Quantitative Electroencephalography (qEEG)

New variant of EEG that processes recorded EEG using computer algs; digital data is analyzed; processed EEG commonly converted into color maps of brain functioning.

Magnetoencephalography (MEG)

Large numbers of magnetically sensitive sensors are placed on scalp, measures magnetic fields created by brain’s electrical activity; allows finer spatial resolution & less susceptible to distortions from skull than EEGs

CT (computed tomography) or CAT scan

Series of x-ray photos taken from different angles and combined by computer into a composite representation of a slice through brain or body

PET (positron emission tomography) scan

Visual display of brain activity that shows where radioactive form of glucose goes while brain performs a given task

MRI (magnetic resonance imaging)

Technique that uses magnetic fields and radio waves to produce computer-generated images that distinguish among different types of soft tissue; allows us to see structures within the brain

fMRI (functional magnetic resonance imaging)

Variant of MRI that measures level of activity in different parts of brain; superseded PET in many domains

VBM (voxel based morphometry)

Used structural MRI scans to reveal differences between groups’ regional brain volume and tissue concentration; each brain is superimposed on a template then smoothed so each voxel represents average of itself and its neighbors, image volume is then compared across brains at every voxel

One of first well-known VBM studies

Showed that posterior hippocampi of London taxicab drivers were significantly larger than that of controls

Diffusion Tensor Imaging (DTI) & fractional anisotropy (FA)

Special type of MRI; reflects anatomical features white matter, such as axon caliber, fiber density, and myelination

f/ALFF (fractional amplitude of low frequency fluctuations)

Neuroimaging methods used to measure spontaneous fluctuations in BOLD-fMRI signal intensity for given region in resting brain

fNIRS (functional near-infrared spectroscopy)

Noninvasive optical imaging technique that measures changes in hemoglobin concentrations within the brain; higher temporal resolution than fMRI but inferior spatial resolution and penetration depth

PET & fMRI better at

Providing info about brain regions involved in cognitive activity

EEG better at

Providing info about precise sequence of events as info is processed

MEG can

Measure both time course and underlying neural substrate directly

rTMS (Repetitive transcranial magnetic stimulation)

An intense pulse of magnetic energy sent through coil placed on surface of skull, resulting in electrical firing of neurons beneath the scalp; can briefly enhance or disrupt neural activity

rTMS and depression

When applied to left prefrontal cortex, reduces symptoms of depression without any side effects in 30-40% of patients

Penfield studies

Electrical stimulation of association areas of brain during open brain surgey while patient is fully conscious

Electroconvulsive therapy (ECT)

Treatment for major depression; person is anesthetized and given a drug that paralyzes muscles, electrodes are placed on patients’ scalp, jolt of electricity applied triggering brief seizure

ECT advantages

• Works for 70% of people w/ depression who don’t respond to other treatments

• Improvement in symptoms much faster

• Credited with saving many from suicide

ECT disadvantages

• Prolonged & excessive use causes brain damage

• High relapse rate

Transcranial direct current stimulation (tDCS)

Weak current applied to scalp on left side of dorsolateral prefrontal; safer than ECT

Deep brain stimulation

Neurosurgical procedure involving placement of neurostimulator, which sends electrical impulses through implanted electrodes, to specific targets in the brain

Optogenetics

Involves inserting opsin genes into neurons; causes neuron to manufacture light-sensitive opsin proteins and incorporate them into membrane which become part of ion channels, can then be activated by particular wavelength of light

Marshel, Kim, & Machado (2019)

Mice were shown pics of vertical & horizontal stripes on a monitor, trained to lick pipe only if they saw vertical stripe; using light, researchers identified neurons in visual cortex that switched on in response to different stripes; then turned off monitor & used light to switch on neurons for vertical stripes

Liu, Ramirez, & Tonegawa (2014)

Neuroscientists tagged neurons associated w certain memory, then used light to artificially induce those neurons to fire to make new associations between events and environments w no ties to reality — implanting false memories in mice using optogenetics

Problems w/ neuroimaging data

Can be noise in system, everyone’s brain is slightly different