Electrophysiology and Brain Stimulation

• The lecture covers electrophysiology and brain stimulation techniques.

Structural & Functional Imaging

• Background to neuroscientific techniques.
• Magnetic Resonance Imaging (MRI).
• Diffusion Tensor Imaging (DTI).
• Functional Magnetic Resonance Imaging (fMRI).
• Functional Transcranial Doppler Sonography (fTCDS).

Electrophysiology & Brain Stimulation

• EEG & ERPs.
• Transcranial Magnetic Stimulation (TMS).

Background - Good Techniques

• Portability, spatial resolution, and temporal resolution are all features of a good technique

Electroencephalography (EEG)

• Electrical fields generated in the brain propagate to the scalp's surface which are measured as a continuous signal.
• EEG reflects ongoing oscillatory activity, which is stimulus-independent.
• EEG measures activity from postsynaptic potentials because they last longer than action potentials, and are contained in dendrites/cell bodies of the neocortex.

Measuring EEG Signals

• EEG signal is measured from the participant's scalp.
• Silver & silver chloride (Ag/AgCl) electrodes placed on the scalp measure electrical potentials.
• Electroconductive gel bridges the electrodes and the scalp.
• Electrode impedance (Z) impacts data quality and statistical power, where Z is resistance and inductance.

Recording EEG

• The EEG signal is very small compared to electrical and environmental noise.
• At least 3 electrodes are required: recording, reference, and ground.
• Ground electrodes account for environmental noise.
• EEG signal is calculated with the formula: (Recording - Ground) – (Reference - Ground) which removes noise as the electrodes are grounded.

EEG 10-20 system

• EEG 10-20 system is used for electrode placement.

Frequency Domain (EEG Oscillations)

• EEG oscillations are characterized by frequency, power, and phase.
• Frequency is how fast the oscillations are, measured in Hz.
• Amplitude/power is how strong the oscillations are, measured in microvolts (㎶).
• Phase is the offset, measured in degrees.
• Gamma waves (< 25 Hz) are associated with high focus.
• Beta waves (12 - 25 Hz) are associated with an active mind.
• Alpha waves (8 - 12 Hz) are associated with a resting state.
• Theta waves (4 - 7 Hz) are associated with drowsiness.
• Delta waves (1 - 4 Hz) are associated with sleep.

Alpha Wave

• Alpha waves oscillate approximately 10 times per second (10 Hz).
• They are prominent over the back of the head.
• Associated with relaxation and sleepiness.

Frequency Analysis

• EEG frequencies are variables of interest in some experiments.
• Fourier analysis is used to identify the individual sine waves.
• A frequency does not equal a specific function.

Fourier Analysis

• A waveform is equivalent to the sum of sine waves with different frequencies, amplitudes, and phases.
• Time domain represents amplitude as a function of time.
• Frequency domain represents amplitude and phase as a function of frequency.

Use cases for EEG

• A direct measure of ongoing neural activity.
• Identifying mechanisms of disorder with patients and control groups
• Potential biomarker.
• Neurofeedback.

EEG: Pros and Cons

• Advantages: very good temporal resolution, direct measure of neural activity, non-invasive, usable on many populations, and relatively inexpensive, and implicit measure.
• Disadvantages: very poor spatial resolution, topography does not equal location of activity, activity mostly from cortical regions, and requires participants to remain still.

Event-Related Potentials (ERPs)

• ERPs are "a subsegment of the ERP whose activity represents a functionally distinct neuronal aggregate" (Donchin, Ritter, & McCallum, 1978).

ERP Components

• Latency: the time after stimulus onset in ms.
• Amplitude: potential in microvolts (㎶).

EEG Naming Conventions

• Based on polarity, ordinal position, latency, and function.

EEG Averaging

• Event-related potentials (ERPs) need to be evoked by an event or stimulus, containing both evoked activity (signal) and noise.
• Signal is neural activity associated with the stimulus of interest.
• Noise is unrelated neural activity and electrical noise.
• Averaging is used to cancel out ongoing activity as signal is consistent and noise is random.
• The signal/noise ratio is a function of the square root of the number of trials.

ERPs Good For

• Investigating specific cognitive functions like splitting visual and semantic processes in reading.
• Identifying mechanisms of disorder by comparing between patients and controls.
• Potential to be used as a biomarker.

N170 Research Spotlight

• Faces at lateral occipital sites have greater negativity, peaking around 170 ms post-stimulus onset.
• A meta-analysis examined the impact of facial emotion on N170 amplitude.
• Greatest N170 amplitudes are to angry, fearful, and happy faces.
• Emotion has a confirmed effect on N170 for task-irrelevant facial emotion.

Tanaka & Curran (2000) Research

• Examined the neural basis for expert object recognition.
• Participants were either dog experts (n = 15) or bird experts (n = 15).
• Stimuli were pictures of birds and dogs.
• There was a greater N170 amplitude when viewing the category you are an expert in
• Expert object recognition is distinct from non-expert recognition early in visual processing.

ERPs - Pros and Cons

• Advantages: Very good temporal resolution, direct measure of neural activity, non-invasive, can be used on many populations & individuals who can't undergo MRI, relatively inexpensive, implicit measure
• Disadvantages: Very poor spatial resolution, topography does not equal location of activity, requires participants to remain very still, Not all cognitive processes have a distinct ERP signature

Transcranial Magnetic Stimulation (TMS)

• Works on Faraday’s law of electromagnetic induction.
• A large brief pulse of current through loops of copper wire leads to a magnetic field perpendicular to the plane of the coil.
• This induces an electric field predominantly parallel to the inner surface of the volume conductor adjacent to the coil.
• The electrical field generates secondary currents leading to neuronal changes.

TMS types and Frequency

• Temporary facilitation or inhibition of specific brain regions.
• Single pulse and repetitive TMS (rTMS) types
• Low frequency (slow rTMS; < 1 Hz) applied for 10-15 mins leads to long-term depression (LTD) – longer-term inhibition of neuronal excitability.
• High frequency (fast rTMS; > 1Hz) leads to long-term potentiation (LTP) – longer-term increase of neuronal excitability.
• Combination of pulse rate, frequency, and coil type impacts results.

TMS Use Cases

• Facilitatory: Potential therapeutic benefit, improve symptom severity if a brain area is implicated in a disorder, may promote neuroplasticity (Parkinson's, depression, PTSD)
• Inhibitory: Assessing the functional role of a brain area, can be used to infer causation (stimulating Broca's area means you lose the ability to speak, Broca's area is necessary for speech)

Rösler et al (2014) Research

• Researched language mapping in healthy volunteers and brain tumor patients with a novel navigated TMS system and evidence of tumor-induced plasticity by RNTS
• rnTMS inhibits language in the left hemisphere in controls and to compensate reorganization occurred in patients

TMS – Pros and Cons

• Advantages: Can give us causal information, very informative about brain function, potentially useful in neuropsychology.
• Disadvantages: Not everyone can undergo TMS, can only be used to investigate the cortical surface, and cannot stimulate too near to certain regions.