Neuroscience Techniques: Imaging,Electrophysiology

• Lecture 1 includes Structural and Functional Imaging
• Lecture 2 includes Electrophysiology & Brain Stimulation

Background to Neuroscientific Techniques

• Neuroscientific techniques answer questions behavioural testing cannot
• Behavioural testing limits ability to examine implicit processing
• Key brain regions are identified without lesion studies or animal models
• Treatment efficacy is evaluated
• Biomarkers are identified
• Each technique has pros and cons
• The research question along with practical constraints drives the choice of technique

Magnetic Resonance Imaging (MRI)

• Protons in hydrogen molecules orient along the magnetic field when a material is placed in an MRI scanner
• A radio wave (RF Pulse) shifts the alignment of the protons
• Protons emit energy in the form of radio waves once the radio waves are turned off, and the protons return to their original orientation
• A coil of wire placed around the head detects radio waves, which are then sent to a computer
• The density of hydrogen atoms differs in grey matter, white matter, and cerebrospinal fluid, allowing for the differentiation of these regions
• T1 measures how quickly protons realign with the main magnetic field
  • Fat has a high signal and appears bright on T1-weighted anatomical images
  • CSF has a low signal and appears dark on T1-weighted anatomical images
• T2 measures how quickly protons give off energy as they recover to equilibrium
  • Fat has a low signal and appears dark on T2-weighted anatomical images
  • CSF has a high signal and appears bright on T2-weighted anatomical images

MRI Safety

• The MRI machine contains a large magnet which poses danger to ferromagnetic objects
• Participants must remove jewellery, watches, piercings, coins, wallets and glasses if ferromagnetic, when near the magnet
• Those with metal inside their bodies are prohibited
  • eg. pacemakers, aneurysm clips, metal implants (e.g., cochlear implants), IUDs, some dental work, some tattoos
• Ear plugs should be provided due to loud noise
• Claustrophobia and anxiety are a concern

Applications of Structural MRI

• Patient studies can identify areas of damage related to symptoms
• Volumetric changes in relation to behaviour
  • There may not be a clear cut relationship

MRI Pros and Cons

• Advantages:
  • Good spatial resolution
  • Clinical and experimental utility
  • Relatively non-invasive
• Disadvantages:
  • Purely structural
  • Not everyone can be scanned
  • Inability to keep still
  • Expensive

Diffusion Tensor Imaging (DTI)

• DTI measures the diffusion of water molecules in the brain
• A structural measure of white matter and can also be measured using an MRI machine
• Mean Diffusivity (MD) measures overall water diffusion in a voxel and is generally insensitive to the orientation of fibers
  • High MD indicates poorer white matter integrity
• Fractional Anisotropy (FA) maps show orientation
  • Red = left-right
  • Green = anterior-posterior
  • Blue = superior-inferior
• Microscopic and macroscopic factors contribute to anisotropy
  • Axon diameter, axon packing density, myelin thickness and axonal alignment are microscopic cellular contributors
  • Microscopic axonal crossing and partial volume effect are macroscopic architecture

Applications of DTI

• Used in patient studies to examine white matter structural integrity in different disorders, and FA differences of specific structures
• Tracks FA changes in relation to experience

Van der Auwera et al (2021) Research on Dyslexia

• Performed a three-time point longitudinal investigation of the arcuate fasciculus throughout reading acquisition in children developing dyslexia
• DTI was used to measure fractional anisotropy (FA) of the arcuate fasciculus (AF) between children with and without dyslexia
• Conducted study in pre-reading stage (5–6 years old), early reading stage (7–8 years old), advanced reading stage (9–10 years old)
• Differences in FA between children with and without dyslexia are present before the age of reading instruction
• Predictive relationship between AF FA and reading skills throughout development
• There is evidence supporting the idea of an AF deficit being core to poor reading ability

DTI Pros and Cons

• Advantages:
  • Good spatial resolution for white matter tracts
  • Clinical and experimental utility
  • Relatively non-invasive
• Disadvantages:
  • Purely structural
  • Not everyone can be scanned
  • Inability to keep still
  • Expensive

Functional Magnetic Resonance Imaging (fMRI)

• A form of MRI
• T2 weighted to get the functional part (EPI)
• Usually overlaid on a T1 scan taken in the same session
• BOLD (Blood Oxygen Level Dependent) technique used
• Activation in specific brain regions is inferred, based on changes in the level of oxygenated blood in those regions

Applications of fMRI

• Relates structural with functional information, unlike the previous measures
• Study of the functional significance of brain areas
• Allows us to see where compensation is occurring when a function is compensated following injury

Cummine et al (2015) Research on Reading

• States that rapid automatized naming (RAN) predicts reading ability and examines if the brain regions for reading and RAN overlap
• 4 tasks performed (during fMRI):
  • RAN letters
  • RAN numbers
  • Rapid word reading
  • Rapid nonword reading
• RAN and reading activate common brain regions
• There is a correlation between RAN and reading in motor brain regions
• Differences exist between RAN and reading in non-motor regions

fMRI Pros and Cons

• Advantages:
  • Good spatial resolution
  • Functional measure
  • 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 exist depending on statistical thresholds/analyses

Functional Transcranial Doppler Sonography (fTCDS)

• An ultrasound technique based on the Doppler effect
  • A motorbike sounds higher pitched moving towards you than away from you
  • Applies to ultrasound waves depending if blood flows towards or away from the probe
• Measures bilateral blood flow to the middle cerebral artery (MCA) territory

Applications of fTCDS

• Measures cerebral dominance, more accurately than behavioural tests
• Good for examining individual differences in cerebral asymmetries, and also group level differences between patients and controls
• Lateralised functions include:
  • Language
  • Face processing
  • Object recognition

COLA Consortium Research on Language

• Investigates language laterality using behavioural and cerebral blood flow
• Language laterality is not a unitary trait

fTCDS Pros and Cons

• Advantages:
  • Validated measure of language dominance
  • Easy to administer & non-invasive
  • People with cochlear implants, metal, and claustrophobia can participate
  • Resistant to movement
• Disadvantages:
  • Relatively poor spatial resolution
  • Poor temporal resolution
  • Individual differences in MCA architecture / skull density mean clean signal not always possible