Advantages of fMRI
(1) Nothing has to be injected into the volunteer;
(2) MRI provides both structural and functional information;
(3) spatial resolution is great;
(4) it can be used to produce three-dimensional images of activity over the entire brain.
Disadvantages of fMRI
(1) fMRI are images of the Blood-Oxygen-Level-Dependent (BOLD) signal, and the relation between the BOLD signal and neural activity is complex (see Hillman, 2014).
(2) Furthermore, fMRI technology has poor temporal resolution, much slower than neural responses (e.g., action potential) which occurs in milliseconds.
Interim summary
- When neurons fire or increase their firing rate, they draw on oxygen and various nutrients.
- The circulatory system of the brain reacts by sending the region that just fired more highly-oxygenated blood than is needed.
- This results in an increased blood oxygen level in the activated region. As a result of the magnetic characteristics of hemoglobin, oxygenated blood has a different magnetic signature than de-oxygenated blood.
- Therefore, with the right pulse sequence, an MRI scanner is able to detect this difference in blood oxygen level.
Relational task
Alternating blocks of judgments about relations among features versus feature matching (planning/reasoning)
Anterior Prefrontal Cortex
Gambling task
Gambling decision making task
Striatum; Ventral Medial Prefrontal; Insula; Orbitofrontal
Social task
Animations of social and random interactions
Medial Prefrontal Cortex; Temporal Parietal Junction; Inferior and Superior Temporal Sulcus
Emotion processing
Valence Judgments (negative and neutral pictures) Amygdala; Hippocampus; Insula; Medial Prefrontal
Signal analysis
Biopotentials are a subset of biomedical signals that can be measured electrically using an electrode pair.
Electrodes are sensors or transducers that convert electrical signals into a format amenable to analysis.
These electrical signals may occur spontaneously, such as when we measure brain states using EEG.
Biopotentials
Biopotentials, a type of electrical potential, represent the flow of current in biological tissue (e.g., nervous tissue).
-Scalp (EEG)
-Skin (Skin Conductance)
-Brain (Stimulation and animal behaviour)
The first, and many ways the most device, piece of kit is, however, the first in the chain: the connection (or “coupler”) to the biological process we are measuring.
For EEG/ECG this connection is an electrode, a small electrical conductor connecting to a non-metallic part of a biological circuit. The golden rule is:
Low output impedance – high input impedance
Electroencephalography (EEG)
-EEG is a widely used technique that can study brain activity, at a millisecond resolution
-EEG is a marker of neuronal activity and it is non- invasive.
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-A limitation of EEG is its poor spatial resolution (i.e., we record signals on the scalp, not inside the brain).
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-Consequently, we record mainly cortical activity
Transcranial Magnetic Stimulation
-Single or repetitive magnetic pulse (>20 Hz) can turn ‘on’ or ‘off’ brain regions
Focused ultrasound
- Target very specific areas
- Can do surgery, stimulation and drug delivery
- Doesn’t ‘burn’ cortex like a laser surgery
- But very very new and few machines exists
Skin conductance
It measures electrical conductance of the skin
- It is an autonomic response
- Increased skin conductance from emotional response and stress (which was covered in detail two weeks ago)
- Commonly known as ‘lie detector’ test
Heart Rate Variability: Part 1
-We must be conscious that HRV analyses the intervals between consecutive heart beats, & not the beats per se
- Beat-to-beat variability manifests physiological rhythms coming from extrinsic processes, largely the autonomic nervous system.
- In our rest state, the parasympathetic system dominates and largely determines our inter-beat intervals.
Heart Rate Variability: Part 2
-Low HRV is a strong marker for pathology.
-Lower levels of HRV are found in patients with most types of psychological disorders.
Thayer (2005; 2006) draws an analogy with the inhibitory control of the heart and the inhibitory control of what he calls “sympathoexcitatory subcortical threat circuits”.
