Methods

What kind of imaging is this?

fMRI

What kind of imaging is this?

(anatomical) MRI

What kind of imaging is this?

PET

What kind of imaging is this?

MEG

Methods to study brain function

1. Electrophysiological recordings

2. Functional imaging

3. Lesion studies

4. Electrical stimulation of the brain

5. The Wada Test

6. The Split Brain

Electrophysiological recordings

• Electroencephalogram (EEG)

  • Cap with 20 electrodes put at precise locations

    • One electrode used behind the ears as a reference to check the potential (voltage), similarly to how an ECG uses the ankle

  • Average EEG looks different depending on the state of the brain (awake, drowsy, etc.)

  • Is used to see if one is struggling with one of the senses to see if it is a brain issue, and where that would be.

  • Great temporal resolution

    • Determined by the number of electrodes?

Types of waves in EEG

• Delta waves

  • Up to 3 Hz

  • High amplitude waves

  • Seen in

    • adults during slow wave sleep

      • frontally

    • awake babies

      • posteriorly

  • May occur focally with subcortical lesions

• Theta waves

  • 4-7 Hz

  • Seen in (location unrelated to the task at hand)

    • Young children

    • Drowsy adults

    • During meditation

• Alpha waves

  • 8-12 Hz

  • Basic rhythm in posterior regions, higher amplitudes on dominant side

  • Found at central sites during rest

  • Strongly correlated with visual input

    • Emerges with eye closure and relaxation

  • Prominent indicator of a coma

• Beta waves

  • 12-30 Hz

  • Normal with low amplitude

  • Usually seen in symmetrical distribution and most prominent frontally

  • Associated with active thinking and concentration

  • Changes in beta waves indicates drug use

  • Lack of beta waves indicates cortical damage

• Gamma waves

  • 30-100 Hz

  • Thought to represent binding of different populations of neurons together into a network for carrying out a certain cognitive or motor function

  • Typical in cross-modal sensory processing

  • Decrease in gamma waves indicates cognitive decline

EEGs are described in terms of

• Rhythmic activity

  • These are subdivided by frequency bands

• Transient activity

  • Event related potentials (ERPs)

Event related potentials (ERPs)

Stimulus-correlated in the EEG

• (Transient activity?)

• Usually averages 9 lines into 1 line

• (Picture):

  • Information about e.g. a click sound. Takes ~250ms to perceive the sound, and at around ~400ms the brain tries to recognize the sound

  • P3 becomes flatter as we habituate/adapt/get used to the sound

  • During adaptation, the entire signal becomes flatter

    

Epilepsy

• Reoccurring abnormal brain activity

• Can be traced in the brain to see where it originates

  • Very prominent changes suggest where the epilepsy may originate

• Not heritable

• May be caused by e.g. damage in the brain, drug use, lesions, brain tumours causing pressure —> neurons fire impulses.

EEG pros/cons

• Great temporal resolution

  • Can spot it in real time (within ms)

  • Restricted by number of electrodes

• May not see signals deep within the brain (microvolts are not always detectable through the skull) without drilling a hole in the skull to record deeper areas in the brain

Functional imaging methods

• fMRI

  • functional Magnetic Resonance Imaging

• MRI

  • anatomical Magnetic Resonance Imaging

  • Pick up ALL protons in the brain (not just in the blood) to get an anatomical imagine of the brain more accurate than x-rays

• PET

  • Positron Emission Tomography

• MEG

  • Magnetoencephalography

How does fMRI work?

• Neural activity is correlated with blood flow

  • Differences in the blood flow is visualised by the big machine

  • Head is fixated to stay still

  • Machine applies alternating magnetic fields to test the hemoglobin

    • distinguishes it from oxygenated (diamagnetic) and deoxygenated (paramagnetic)

      • Diamagnetic blood is repulsed by the magnetic field

      • Paramagnetic blood is attracted by the magnetic field

        • Proton has a spin and acts as a compass needle

          • When the magnetic field turns on, the proton points north

          • When the magnetic field is turned off, the proton returns to its original position

          • This is recorded by the machine

          • As a result, we can reconstruct the regions of the brain in 3D

            • Shows us which part of the brain is active when exposed to different things

              

How does PET work?

• Blood flow is seen by a radioactive labelled glucose inserted in your blood

  • Accumulates in the area that the brain is active

    • Beta decay: Radioactive tracer emits positrons

      • When an electron and positron meet, they annihilate and produce 2 photons that fly away in opposite directions

        • If e.g. there is more blood flow on the left side of the brain, the left side of the machine receives the photon signal a little bit faster than the right side, and we can produce a picture of the brain

  • Half life of the isotope is 4 minutes, but don’t go around doing this often in your life (maybe once or twice at most)

How does MEG work?

• Records tiny magnetic fields in the brain

  • With a “HUGE HAIRDRYER” / mask from hell

• High temporal resolution

• Neurons are basically cables and also produce magnetic fields

• PROBLEMS WITH MEG:

  • Very low (microvolts) so produce very small magnetic fields

    • To pick up on these very small magnetic fields, the hospital must be very isolated form other magnetic fields (Faraday’s cage)

  • Range only 1-2 mm (cannot see deeper tissues)

Lesion studies

• Right/Left hemisphere damage does different things

• (E.g.)

  • strokes

  • meningioma

    • tumour of meninges

      • leads to loss of speech

  • tumours

    • leads to loss of touch, temperature etc

  • cysts

    • membrane-like filled with liquid due to uncontrolled growth, puts pressure on surrounding tissue

      • leads to issues with problem-solving, personality change etc

What is a stroke?

• Death of nerves due to poor blood supply

• Strokes can be

  • Ischemic strokes

    • Lack of blood flow due to clotted arteries

  • Hemorrhagic strokes

    • Lack of blood flow due to bursting of an artery (and thus due to bleeding)

Electrical stimulation of the brain

• Instead of passively listening to the brain (EEG), we add electricity

• TMS

  • Transcranial Magnetic Stimulation

  • Apparatus produces strong magnetic pulse

    • Can be directed at a certain brain area

    • Induces electrical activity in the neurons

      • e.g. a magnetic pulse in the visual centre causes a bright flash

How does electrical stimulation of the brain work?

• Open the skull (about 1 cm) and place electrodes on the surface of the brain and apply tiny amounts of electricity and see how the patient responds

  • Patient is awake and aware during the procedure

    • Does not hurt as the brain lacks pain receptors

The Wada Test

• Takes advantage of the fact that the right and left hemisphere have separate blood supplies

• Inject anaesthesia into one brain half to put the hemisphere at sleep

  • Lasts 3-4 minutes before it wakes up again

• E.g. gives the person a spoon in the left hand, the right hemisphere recognizes the touch of it but does not know the word as that is the left hemisphere’s job

  • When the patient is asked when the left hemisphere is awake again, they do not know the name of the object, but the left hand can point at the correct option when given choices.

The Split Brain

• Cut the corpus callosum so the hemispheres cannot communicate

  • This process is called callosotomy

• Historically done to alleviate life-threatening epilepsy that did not respond to medication

• When seeing things with the right eye, the patient knows the word for the object (left hemisphere sees it and knows it)

• When seeing things with the left eye, the patient does not see anything, but can pick it out with their right hand from different options

Corpus callosum

• Bunch of neurons that cross information over between the hemispheres

Left brain / Right brain abilities

• Left brain

  • Sequential

  • Analytical

  • Verbal

  • Logical

  • Grammar

  • Words

• Right brain

  • Simultaneous

  • Holistic

  • Imaginistic

  • Intuitive

  • Intonation (languague)

  • Context