L8 - Altering Cortical Function

Question 1

What are the 6 different ways cortical activity can be altered? Provide an example of each.

Answer 1

1. Development: growth or senescence
2. Experience: Typical or atypical
3. Tissue damage: Trauma, stroke
4. Changes in metabolism or vegetative state (Hypoxia or hypoglycemia)
5. Reversible and non-reversible invasive intervention (spreading depression, electrocautery)
6. Electroshock (TMS, ECT)

Question 2

What is Transcranial Magnetic Stimulation?

Answer 2

• Magnetic induction of eddy currents: current that looks like a figure 8
• Eddy current produces large magnetic pulse that passes through skin, bone, and CSF and depolarizes cortical tissue
• Single or paired pulse TMS leads to transient modulation of neural function
• Repetitive TMS consists of continuous trains of magnetic pulses that induce trains of APs
• Current kept off scalp in order to prevent burning: closer is better for stronger magnetic field
• Used in combination with MRI to place electrode over exact piece of tissue that you want to examine
• Associated with risk of syncope and seizures
• Can either potentiate (LTP) or depress (LTD) cortical function– effects appear to last for a short time and are not permanent

Question 3

What is used as the “neuropsychologist’s electrode” for TMS? What used to be used?

Answer 3

• Location-specific stimulation
• Used to be selective adaptation: e.g., exposing someone repetitively to image of water falling then look at the object not moving. Object will appear to move.
• If system can be adapted, there must be something that adapted the system. If you can diminish a certain response, system must have been there.

Question 4

How do you determine the amount of magnetic field strength that is needed in TMS?

Answer 4

• Magnetic field strength required to give certain effect reported as the & of the field strength needed to generate a threshold motor response
• Example: Threshold motor response is JND in position of hand. Magnetic pulse is increased until they notice hand moves just a little bit. This amount is then used as a baseline and is increased until it results in the desired value.

Question 5

What is the purpose of measuring EMG activity of the obicularis oris muscle during TMS?

Answer 5

• Muscle on contralateral side near mouth
• Done to ensure that the current pulse drives only the cortex and not muscle
• If it drives muscle, it means that they are hitting additional muscles they are not supposed to be hitting

Question 6

Give an example of how TMS can be used as a research tool.

Answer 6

• Examining role of frontal lobes in speech and singing
• Stimulating left frontal lobe affects speech but not melody: Suggests two functions are subserved by different brain areas
• Broca’s patients can still sing words even if they have trouble speaking them

Question 7

Explain a typical Randomized Clinical Trial Study for treatment of Tinnitus.

Answer 7

• Tinnitus: Neurological condition where you experience sound for which there is no physical reference
• Completely randomized counterbalanced double blind clinical trial: subject and experimenter both don’t know which condition subject is in
• Baseline = week before treatment: Questionnaire + EEG and fMRI
• Treatment = 10 working days: Week 1 consists of questionnaires and an audiogram. Then, half of participants receive single site rTMS and half receive multisite rTMS. Week 2 consists of questionnaires and an audiogram.
• Final visit = first week after treatment. Participants receive questionnaires and EEG + fMRI
• Follow-up 1 = 3 months after treatment. Participants receive questionnaires and EEG.
  Follow-up 2 = 6 months after treatment. Participants complete questionnaires.
• If auditory sensitivity is better at 4000 Hz than it was before rTMS, then it had an effect

Question 8

What are the 4 ways to interfere with a number comparison task? What difference exist between the 4 approaches?

Answer 8

1) individual fMRI-guided TMS neuronavigation
2) individual MRI-guided TMS neuronavigation
3) group functional Tailarach coordinates
4) 10-20 EEG position P4

• Individual fMRI-guided TMS neuronavigation yields strongest effect size while EEG yields smallest effect size
• fMRI took the least amount of patients to guess the correct location

Question 9

What is the difference between transcranial direct current stimulation and transcranial magnetic stimulation?

Answer 9

• tDCS:
• Simple
• Causes increased spontaneous firing
• Effects not as large
• Effects of 10 min. stimulation last up to an hour
• Lower chance of seizure
• No change in polarity
• Side effects: skin irritation, phosphenes, dizziness, headache, itching

Question 10

What is radio surgery?

Answer 10

• Selective destruction of brain tissue / blood vessels using radiative energy transfer into the unopened skull
• Challenge is to destroy target tissue while sparing critical tissue
• Must focus radiation while varying the angle of attack and intensity: Must move source of radiation 180 degrees to target specific area a unit amount and everywhere else 1/180th

Question 11

What is the gamma knife?

Answer 11

• Administration of high-intensity nuclear radiation focused at a target
• Contains 201 cobalt-60 radiation sources in a heavily shielded array.
• Stereotaxic positioning of the head is required to ensure dose is maximal at treatment point
• Radiation dose at treatment point is the sum of all rays
• Only the treatment point gets the sum as each individual beam is a relatively low intensity

Question 12

Explain the Talaraich system of brain stereotaxy.

Answer 12

• 3D localization of points in skull volume based on postpartum sections of 60 year old french woman with smaller than average brain size
• 3 coordinates = x, y, and z
• If someone has larger head, coordinates won’t match up, so must use warping algorithm to expand original coordinates or to shrink new coordinates
• Done with software, but no longer in common use today
• Origin defined by anatomical landmarks that exist in everyone’s head to locate the x axis: Horizontal axis passes through anterior commissure and posterior commissure
• Obtained from mid-saggital view: normalizes coordinate system for axes (e.g., for larger head, shrink coordinates by 6/7 to match original)

Question 13

What is a multimodal atlas? Why is it useful?

Answer 13

• Atlas that integrates different kinds of imaging
• Example: In stroke, want to be able to map arterials to correspond with tissues that they irrigate and the functional metabolic activity associated with those areas.

Question 14

What are the 11 ways that drugs can affect synaptic plasticity?

Answer 14

1. Inhibit synthesis of NT
2. Prevent storage of NT in vesicles
3. Stimulate release of NT
4. Inhibit release of NT
5. Stimulate postsynaptic receptors
6. Block postsynaptic receptors
7. Stimulate autoreceptors
8. Block autoreceptors
9. Block reuptake of NT
10. Inactive acetlcholinesterase
11. Substitute for chemical precursor to NT