Neuroscience: Imaging, Electrophysiology & Brain Stimulation

• Techniques covered include structural and functional imaging
• The lectures also cover electrophysiology and brain stimulation

Background

• Neuroscientific techniques help answer questions that behavioral testing limits
• Key brain regions is identified with these techniques without relying on lesion studies or animal models
• Treatment efficacy can be evaluated at a mechanistic level
• Biomarkers can potentially be identified
• Each technique has its own pros and cons
• The research question and practical constraints should drive the choice of technique

Magnetic Resonance Imaging (MRI)

• Protons in hydrogen molecules orient with the magnetic field when a person is placed in an MRI scanner
• Radio waves shift the alignment of these protons
• Once the radio waves are turned off, protons emit energy as radio waves when returning to their original orientations
• A coil of wire that surrounds the head detects these radio waves
• Then they are sent to a computer
• The density of hydrogen atoms in gray matter, white matter, and cerebrospinal fluid differs, which leads to emission of different signals
• T1 measures how quickly protons realign with the main magnetic field
• T2 measures how quickly protons give off energy to recover to equilibrium

MRI Safety

• The MRI machine is a large powerful magnet
• Ferromagnetic objects are very dangerous near the magnet
• Those near the magnet must remove jewelry, watches, piercings, coins, wallets, and glasses if ferromagnetic
• Metal in the body is prohibited
• Pacemakers, aneurysm clips, metal and cochlear implants, IUDs, certain dental work, and some tattoos are prohibited
• The machines are loud so participants should have ear plugs
• Claustrophobia and anxiety are also a concern

Structural MRI

• Useful for identifying damaged areas and their related symptoms
• It is also useful for identifying volumetric changes with behavior in patient studies
• There is not always an obvious relationship between the two

MRI: Pros and Cons

• Advantages: Good spatial resolution, non-invasive, and has clinical and experimental utility
• Disadvantages: purely structural, expensive, not everyone is able to enter, may cause claustrophobia, and participants must also stay still

Diffusion Tensor Imaging (DTI)

• It measures the diffusion of water molecules
• It is a structural measure of white matter that can also be measured using an MRI machine
• Mean Diffusivity (MD) measures overall water diffusion in a voxel
• MD is insensitive to the orientation of fibers and often used clinically
• A high MD indicates poorer white matter integrity
• Fractional anisotropy maps show orientation but not direction
• Red indicates left-right
• Green indicates anterior-posterior
• Blue indicates superior-inferior
• Sources of FA are microscopic and macroscopic

DTI: What is it good for?

• Patient studies can assess white matter structural integrity in different disorders, like Schizophrenia
• Also helpful in identifying FA differences of specific structures in dyslexia

Van der Auwera et al (2021)

• A three-time point longitudinal investigation of the arcuate fasciculus throughout reading acquisition in children developing dyslexia was conducted
• DTI measured fractional anisotropy of the arcuate fasciculus between children with and without dyslexia across a longitudinal sample
• Including pre-reading stages, early reading stages and advanced reading stages
• Fractional anisotropy differences emerged among children with and without dyslexia before reading instruction
• There is a predictive relationship between the arcuate fasciculus and reading skills
• This supports the idea of an arcuate fasciculus deficit being core to poor reading ability

DTI: Pros and Cons

• Advantages: good spatial resolution for white matter tracts, non-invasive, clinical and experimental utility
• Disadvantages: purely structural regarding white matter, expensive, those with metal implants or claustrophobia may be limited, participants also need to stay very still

Functional Magnetic Resonance Imaging (fMRI)

• Type of MRI
• T2 weighted to get the functional part (EPI: echo-planer imaging)
• Usually overlayed on a T1 scan taken in the same session
• The BOLD (Blood Oxygen Level Dependent) technique is used to infer activation in specific brain regions by measuring the changes in oxygenated blood

fMRI: What is it good for?

• Relates structural and functional information
• Good for studying the functional significance of specific brain areas
• Allows one to see where compensation is occurring when a function is compensated after an injury

Cummine et al (2015)

• Rapid automatized naming (RAN) is known to predict reading ability. This research looked at whether the reading and RAN overlap when it comes to the regions of the brain it affects
• Methods: 4 tasks during fMRI
• RAN letters, numbers, rapid word reading, rapid nonword reading
• Results: RAN and reading activates common brain regions, correlation between RAN and reading in motor brain regions, and differences between RAN and reading in non-motor regions

fMRI: Pros and Cons

• Advantages: Good spatial resolution, functional measure (where specific activity is occurring), allows identification/corroboration of key areas involved in different processes
• Disadvantages: Poor temporal resolution, BOLD response takes ~4 seconds, indirect measure of activity, issues of interpretation depending on statistical thresholds/analyses

Functional Transcranial Doppler Sonography (fTCDS)

• Form of ultrasound technique based on the Doppler effect
• For example: A motorbike sounds different pitched towards you than away from you, same can be said about frequency of ultrasounds waves depending if blood flows towards or away from the probe
• Measures bilateral blood flow to the middle cerebral artery (MCA) territory

fTCDS: What is it good for?

• Measures cerebral dominance with more accuracy than behavioral tests
• Good for looking at individual differences in cerebral asymmetries, and group level differences between patients and controls
• Lateralised functions include:
• Language
• Face processing
• Object recognition

COLA Consortium (Parker et al., 2022)

• Looks at inconsistent language lateralisation and tests the dissociable language laterality hypothesis using both behavior and cerebral blood flow.

fTCDS: Pros and Cons

• Advantages: Validated measure of language dominance, easy to administer & non-invasive, participants with cochlear implants, metal, or claustrophobia are able to participate, resistant to movement
• Disadvantages: Relatively poor spatial resolution (can only determine more left or right), temporal resolution is poor, individual differences in MCA architecture/skull density mean that getting a clean signal is not always possible