Cognitive Neuroscience Methods

• Lecture 1: Methods in Cognitive Neuroscience
• Cognitive neuroscience aims to provide a brain-based understanding of cognitive processes such as thinking, perceiving, and remembering
• Technological advancements have made studying the brain safer and more refined compared to older methods like those used by Penfield
• Cognitive psychology supplies the experimental designs and theoretical structures used in cognitive neuroscience

Cognitive Neuroscience approach

• Cognitive neuroscience examines psychological theories via data extracted from local blood oxygen levels and reaction times
• It is important to note that these measurements solely record data, and not necessarily any of the underlying processes
• Reaction times were observed to be faster for the second word when it followed the same word
• There was less activation to the same word compared to a different word in the left fusiform

Methods for Studying Brain Function

• Single unit recording
• Electroencephalography (EEG)
• Magnetoencephalography (MEG)
• Positron Emission Tomography (PET)
• Magnetic Resonance Imaging (MRI)
• Functional MRI – fMRI
• Diffusion Tensor Imaging – DTI
• Functional Near-Infrared Spectroscopy – fNIRS
• Intracranial electroencephalography (iEEG) – ECOG
• Transcranial magnetic stimulation – TMS
• Transcranial electrical stimulation - tES (tDCS & tACS)
• Single-cell recordings are useful for studying electrical brain properties
• EEG/ERPs and MEG are non-invasive recording methods analyzing electrical and magnetic brain activity respectively
• Transcranial Magnetic Stimulation (TMS) employs non-invasive brain stimulation and assesses electromagnetic brain properties
• PET is invasive, recording hemodynamics
• fMRI is non-invasive, and assesses hemodynamics

Single Cell Recording Techniques

• Ideally, single-cell recordings isolate individual neurons
• Very small electrodes implanted into or outside the axon record the action potential of a neuron

Electrophysiological techniques

• Involve implanting a small electrode into or near an axon to measure the action potential of individual neurons
• The electrode can be placed either inside the axon (intracellular) or outside the axon membrane (extracellular)
• Extracellular recordings are more common in mammals due to neuron size
• These techniques record neural activity from a population of neurons
• Electrodes made of thin wires are implanted in specific brain areas to record electrical potential of nearby neurons

EEG

• EEG measures the brain's electrical activity by recording from electrodes on the scalp
• The traces recorded are known as an electroencephalogram (EEG)
• The EEG uses the 10-20 system of electrodes
• It represents an electrical signal from many neurons
• EEG signals reflect the change in potential difference between two scalp electrodes
• EEG readings from multiple trials are averaged together to form an event-related potential (ERP)
• ERP are voltage fluctuations linked in time to a particular event, like visual, auditory, or olfactory stimuli

Using ERP to Study Face Recognition

• Different ERP peaks show different parts of face processing
• N170 is specialized for face recognition and located in the right PSTS
• P300 is related to familiar faces
• Recognizing a face takes around 700–800 ms
• Alzheimer's patients exhibit a markedly reduced P300 at each electrode site

MEG

• Magnetoencephalography (MEG) detects magnetic fields caused by brain electrical activity using SQUIDs
• It is used in research and clinical settings with high temporal and spatial resolution
• EEG records electrical fields, whereas MEG records magnetic fields
• To pick-up a magnetic field, MEG machines use a cold setting

Interim Summary - Recording Techniques

• Neuronal activity generates electrical and magnetic fields, measured invasively (single cell recordings) or non-invasively (EEG, MEG)
• Single cell studies show how neurons code information by measuring response to external stimuli
• Synchronized neuron populations create an electrical field detectable at the scalp (EEG)
• Averaged waves linked to stimulus onset yield an ERP
• An ERP is an electrical indicator of cognitive components which contribute to stimulus processing
• Variations in stimuli(e.g. any face vs. famous face) can cause variations in ERP waveform
• Characteristics of these EEG waveforms can tell us about cognitive processes

Magnetic Resonance Imaging (MRI)

• It creates brain images using the differential magnetic properties of tissue and blood

Structural vs. Functional Imaging

• Structural imaging creates static maps using different tissue properties (skull, gray matter, white matter, CSF fluid), for CT and structural MRI
• Functional imaging uses temporary changes in brain physiology during cognitive processing for PET & fMRI

PET

• PET measures local blood flow (rCBF) via a radioactive tracer injected into the bloodstream
• It takes up to 30 seconds for the tracer to peak
• Positron emission from radioactive material decay is detected
• High radioactivity areas are associated with brain activity and blood volume

fMRI

• fMRI directly quantifies deoxyhemoglobin concentration in blood, labeled as the BOLD response(Blood Oxygen Level Dependent contrast)
• Change in BOLD response over time signifies the hemodynamic response function which peaks in 6–8 seconds and restricts fMRI's temporal resolution

With fMRI

• Correlation between activity and the stimulus is studied
• fMRI generates activation maps displaying brain regions involved
• It measures activity in voxels (volume pixels), the smallest distinguishable part in 3D image

Determining Brain Region Activity

• The constant supply of blood and oxygen in the brain means we cannot directly read thoughts through brain scans
• Active brain regions are identified by comparing relative differences in brain activity across conditions
• Functional specialization is inferred by comparing activity under different conditions
• "Active" means it has a greater response vs other conditions with a baseline or comparison condition
• Bad conditions make regions meaningless

Designing an fMRI Study

• FMRI studies are used for understanding: Recognizing written words, saying words, and retrieving the meaning
• These studies involve visual analysis, written word recognition, word meaning, and word sound, and the study's speech output

Cognitive subtractions

• Cognitive subtraction contrasts activity in a control task with that in an experimental task
• A key problem relates to the difficulty of developing an ideal baseline task

Disagreements Between Imaging and Lesion Studies

• Neuroimaging and lesion studies can sometimes overlap when drawing conclusions
• Imaging data could not be used in the task, but lesion data did suggest that the IFG region supports semantic memory

Comparing imaging methods

• When determining stimulus categorization, the word had to be in the same category as other words
• For the controlled study, they had to say whether or not it was same or different for letters

Neuroimaging (fMRI) Results

• Areas of brain activation in the semantic minus letter category Comparison include the IFG region

Neuroimaging (PET) Results

• Using the exact same protocol, activations are observed in the temporal area, similar to the effects that lesion patients experience

DTI – Diffusion Tensor Imaging

• Modified MRI scanner reveals bundles of axons in the living brain
• Measures white matter organization based on limited diffusion of water molecules in axons
• This technique enables the visualization of brain connections

Functional Near-Infrared Spectroscopy (fNIRS)

• fNIRS measures the same BOLD response as fMRI but differently
• Measures ‘Light' in infrared range which passes through skull and scalp but is scattered differently by oxy- v. deoxyhemoglobin
• The equipment is portable and can tolerate more head movement than fMRI, but it cannot image deep structures

Intracranial electroencephalography (iEEG) or ECOG

• This method records directly from inside the human brain has the highest resolution in space and time
• Electrodes placed on the cortical surface for neurosurgery to identify seizure location for mapping functions
• Useful for recording from tens of thousands of neurons

Intracranial Recordings (in Humans)

• Extracellular activity was recorded from 1177 cells in human medial frontal and temporal cortices
• These recordings were taken whilst the patient executed/observed hand grasping actions, and showed facial emotional expressions (control condition)
• Neurons in the supplementary motor area (SMA) and hippocampus responded to both observation and execution of actions