Neuroimaging - 4 Lectures

LECTURE ONE: TRANSCRANIAL
MAGNETIC STIMULATION
 Neuroanatomy - straining, connectivity, cytoarchitectonics, imaging the live human
brain
 Effects of brain lesions
 Recording and stimulating neural activity – micro and macro electrodes in animal
studies, non-invasive studies in humans: EEG, ERPs, PET, fMRI, TMS, Optical Imaging
(NIRS)

Ideally, spatial resolution – celluar level, Temporal resolution – millisecond scale. Non-
invasive, whole brain – but no such method!

Computerised Tomography: X-ray

Positron Emission Tomography: radioactive 2DG (harmless)

NEUROANATOMY

 After the organisms death, neural tissue preserved by means of a fixative
 Gross anatomy (grey matter v white matter)
 Microscopic anatomy – tissue sliced into fine sections using a microtome (light
microscopy), straining to enhance contrast to identify nuclei.

CYTOARCHITECTONICS

 Broadmann Areas:
- Segment brain based on appearance in microscope
- Appearance reflects type of cells
- Type of cells sometimes correlates with function.
- Broadmann‟s study restricted to a small number of brains.
- Variation across individuals: probabilistic cytoarchitectonics.



TRANSCRANIAL MAGNETIC STIMULATION

 Brief, high current pulse produced in a coil of wire placed above scalp magnetic field:
lines of flux perpendicular to the plane of the coil
 Magnetic field induces electric field perpendicular to magnetic field
 Electric field leads to neuronal excitation.
 Non-invasive, painless, safe, used to study :
- Behaviour during “virtual brain lesions”
- Chronometry
- Functional Connectivity
 TMS coils in different shapes
- Circular coils relatively powerful

, - Figure of eight – more focal, max current at intersection of two round components
 Safety
- Single-pulse TMS considered very safe,
- Repetitive TMS greater effects, but very rarely do seizures occur
- Local pain when stimulation site covered by muscles.
 Depending on stimulation parameters, TMS can either excite cortex (muscle twitches or
phosphenes) or temporally disturb cortical function (“virtual lesion”) inhibition –
(interference with perception or task performance).
 Motor cortex Stimulation
- Activates corticospinal neurons trans-synaptically
 Occipital cortex Stimulation:
- Excitary effects – phosphenes
- Suppression of motor perception and letter identification (inhibition)
 Somatosensory Cortex stimulation
- May elicit tingling, block the direction of peripheral stimuli,
- Can modify somatosensory evoked potentials (SEPs)
 Auditory Cortex Stimulation
- Interpretation of results challenging: loud click.
 Frontal cortex stimulation
- Effects on subject‟s mood? Therapeutic use?

Advantages

- Temporal resolution of millisecond range
- Virtual tension in subject may be better defined than lesion in patient
- Short duration of experiment minimizes risk of plasticity
- Repeated studies in the same subject
- Group studies with standardised experimental set up
- Study double dissociations: stimulate or temporarily disrupt different cortical regions
during one task, one region during different tasks

Disadvantages:

- Spatial undersampling
- Only cortical areas accessible
- Auditory cortex stimulation problematic
- Loud coil click, need “sham stimulation”



Neurochemical Methods:

- Detect chemically defined substances through specific staining
- Immunocytochemistry identifies peptides or proteins as antigens to which antibodies
bind.
- Antibodies linked to fluorescent dyes
- Identification of stained neurons under microscopes