Full highlighted lecture notes from the Neuroscience and Behaviour module (C82NAB). Lectures on Transcranial Magnetic Stimulation, EEG, fMRI and Brain lesions.
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
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