2: Methods in Cognitive Neuroscience I

Lecture 2 let's freaking gooooo

EEG

electroencephalography

MEG

magnetoencephalography

PET

positron emission tomography

MRI

magnetic resonance imaging

fMRI

functional MRI

DTI

diffusion tensor imaging

fNIRS

functional near-infrared spectroscopy

iEEG/ECoG

intracranial electroencephalogtaphy

TMS

transcranial magnetic stimulation

tES (tDCS & tACS)

transcranial electrical stimulation

event (in ERP)

a presented stimulus; a sound or word that triggers a small electrical change in the brain

structural imaging

different types of tissue (skull, gray matter, white matter, CSF fluid) have different physical properties, and so are used to create static maps (CT using x-rays to scan brain and structural MRI

functional imaging

temporary changes in brain physiology associated with cognitive processing (PET & fMRI)

cognitive subtraction

activity in a control task is subtracted from the activity in an experimental task to determine which area in the brain is active in a certain task; experimental - baseline = task activity difference

what is cognitive subtraction a basic method for?

fMRI

BOLD response

blood oxygen level dependent contrast

what different methods are used in cognitive psychology?

experimental method, cognitive neuroscience/neuropsychology, animal models, self-report data, naturalistic observation, case studies, computer modelling

animal models

damaging or genetically modifying brains to see how their behaviour changes

what did cognitive psychology provide?

experimental paradigms and a theoretical framework

cognitive neuroscience approach

providing a brain-based account of cognitive processing (thinking, perceiving, remembering etc.)

the best method for high spatial resolution in humans

fMRI as it shows where in the brain something occurs minutes after it occurs

brain lesions and temporal resolution

ideal, but the temporal resolution is less accurate as days will go past post-lesion before the operation can be studied

how does single cell recording work?

a very small electrode is implanted into the axon (intracellular) or the outside axon membrane (Extracellular) and records the neural activity from a population of neurons by measuring the electrical potential of nearby neurons that are in proximity to the electrode

what is EEG?

the measurement of electrical activity of the brain (when they are acting in synchrony to form an electrical field) by recording from electrodes placed on the scalp

how does EEG work?

each electrode measures the difference in electrical charge (voltage) between the recording and a reference electrode— these differences reflect brain activity

how are the weak electrical signals from the brain accounted for?

they are amplified thousands of times and filtered to remove noise eye blinks and muscle movements

according to data collected from the EEG, how long does it take to react to your name being said?

about 400-600ms

ERP

event-related potential

what are ERPs?

voltage fluctuations that are associated in time with a particular event (visual, auditory, olfactory stimuli)

ERP components

peaks and troughs in the voltage fluctuations

when comparing ERPs from patients with alzheimer’s and those from control subjects..

a markedly reduced P300 is seen for the dementia patients at each electrode site

what does EEG spectral analysis examine and how?

the frequency and power of brain waves by decomposing EEG signals into different bands using mathematical techniques (FFT)

waveforms associated with deep sleep

delta: 0.3-4hz

waveforms associated with deep meditation

theta: 4-8 hz

waveforms associated with eyes closed, thinking activity

alpha: 8-13hz

waveforms associated with eyes open, thinking activity

beta: 13-30hz

waveforms associated with unifying consciousness

gamma: 30+hz

how do waveforms become associated with cognitive functions?

areas of the brain are analysed for frequencies during activity— if oscillating in similar way then the areas are connected in that fucntion

rousselet et al. (2004)

using erp to study face recognition

what erp peak is associated with the perceptual coding of the face?

N170— affected by perceptual changes to image

expand on the N170 erp peak

relatively specialised for faces according to recordings from the right posterior superior temporal sulcus; where we discern something is a face and not a house

what erp peak is associated with face recognition (identity processing)?

N250— unaffected by view changes, affected by familiarity

what erp peak is associated with person recognition (faces and names)?

P400-600— affected by both faces and names

expand on P300 erp peak

relatively specialised for famous and familiar faces

what is MEG?

an imaging technique used to measure the magnetic fields produced by electrical activity in the brain

what technology is used to record MEG measurements?

SQUIDs (300)

where are MEG measurements commonly used?

research and clinical settings

MEG and temporal & spatial resolution

excellent

MEG vs EEG

weaker than EEG as magnetic fields are formed from already weak electrical fields

problems with MEG

SQUIDs need to be kept in a very cold environment, equipment is expensive, less widely used so less research using it is published

how does MRI work?

uses different magnetic properties of types of tissue and of blood to produce images of the brain

what does PET do?

measures local blood flow (rCBF)

how does PET work?

a radioactive tracer is injected into the bloodstream and takes up to 30 seconds to peak. when material undergoes radioactive decay, positron is emitted and can be picked up by a scanner detector— areas of high radioactivity are associated with brain activity based on blood volume

why are areas of high radioactivity associated with brain activity based on blood volume?

more blood is needed in the brain while undergoing a task, so the radioactive tracer injected into the blood will flow where that blood flows

what can radioactivity tracers be used for?

to measure different brain properties (FDG— metabolism), dementia (different biomarkers)

what does fMRI do?

directly measures the concentration of deoxyhaemoglobin in the blood

how does fMRI measure concentration of deoxy hb?

deoxy hb has magnetic properties that create a detectable magnetic signal change that the fMRI scanner maps to identify active brain regions

what is the bold response?

the concentration of deoxy hb in blood

haemodynamic response function

change in BOLD response over time

temporal resolution of fMRI

limited as haemodynamic response function peaks in 6-8 seconds

fMRI measure

voxels (volume pixels)

activation maps

produced by fMRI to show which parts of the brain are involved in a particular mental process

use of fMRI

helps to study the correlation between brain activity and stimulus timings

how to infer the functional specialisation of the brain

as the brain has a constant supply of blood and oxygen, one has to compare relative difference in brain activity between two or more conditions, which involves selecting a baseline or comparison condition

when is a brain region observed in fMRI active?

when it shows a greater response in one condition relative to another

problems with inferring functional specialisation

if the experimenter chooses inappropriate conditions the regions of activity will be meaningless

semantic dementia

implicated by left anterior lobe

semantic memory

implicated by inferior frontal gyrus

advantages of fMRI

excellent spatial resolution, whole brain coverage, non-invasive and safe, widely used in cognitive and clinical research

expand on fMRI’s safety

people with metal implants can get fMRI scans

limitations of fMRI

• correlational technique

• BOLD effect is small and thus the sensitivity is limited

• temporal resolution is poor and limited due to nature of haemodynamic response

• reliability is reduced when a participant moves

• blood levels are affected by everyday substances, aging and impaired cerebrovascular supply

• proximity to sinuses are limited to affect the magnitude of the BOLD response

expand on small BOLD effect from fMRIs

fMRI experiments require multiple samplings of brain responses

expand on reduced reliability caused by movement

artificial environment to do a task, low ecological validity

what everyday substances affect blood levels in fMRI?

caffeine, nicotine, glucose or hormones (such as oestrogen)

what is DTI?

an imaging method that reveals bundles of axons in the brain

how does DTI work?

uses MRI to measure white matter connectivity between brain regions

white-matter pathways or tracts

axons start and end in each other’s vicinity, stay together

how does fNIRS work?

emits light on skull, measures reflection of light— reflectance provides information about activity in brain

what is the wavelength emitted in fNIRS?

between 700-900nm

strong absorbers of light for fNIRS

oxyhb and deoxyhb

how is deoxyhb and oxyhb differentiated?

they have a different profile of reflectance

what causes reflectance of light to change?

neural activity changing concentration of hb

fNIRS spatial localisation

no fine spatial localisation due to bone scattering light (both transmitted and reflected)

fNIRS vs fMRI

only measures superficial activity, cheaper, portable and less intrusive

expand on fNIRS superficial activity

no reflectance from within sulci or deeper structures

fNIRS and infants

drawbacks are alleviated due to thick skull and small head

use of iEEG

placed to locate a seizure and map for neurosurgery purposes—- recorded straight from the cortical surface, approx. tens of thousands of neurons

iEEG spatial and temporal resolution

both high due to being recorded directly inside the human brain when people are undergoing neurosurgery