Chapter 4 Flashcards
Advantages of MRI over CT
(1) Does not use ionizing radiation, (2) better spatial resolution (e.g. folds of individual gyri, (3) better discrimination between white matter and gray matter, (4) adapted for use in functional imaging.
Basics of MRI
The signal emitted by relaxing protons depends upon: tissue (hence contrast in structural imaging) and hemodynamic parameters such as blood volume and oxygenation.
Voxel-based morphometry
Measure white and gray matter density in each voxel. Can look for differences between groups or correlations with cognitive measures.
Diffusion tensor imaging
Measures white matter organization (connectivity). Based on limitation diffusion of water molecules in axons: fractional anisotropy.
Functional imaging
Neural activity consumes oxygen as well as generating electrical signals. In order to compensate for increased oxygen consumption, more blood is pumped into the active region. The time taken for this response is slow (several seconds) and so functional imaging has a poor temporal resolution, but good spatial resolution. This is the complementary profile to EEG.
Positron emission tomography
Measures local blood flow (rCBF). Radioactive tracer injected into blood stream. Tracer takes up to 30 seconds to peak: very slow temporal resolution. Effective spatial resolution around 1 cm.
fMRI
Does not use radioactivity, but signal is affected by concentration of deoxyhemoglobin in the blood. This is called the BOLD response. The change in BOLD response over time is called the hemodynamic response function (HRF) and it has a number of distinct phases. The HRF peaks in 6-8 seconds, this limits the temporal resolution of fMRI. Has a high spatial resolution for a noninvasive method (a few millimeters).
Brain region is active?
The brain region has a constant supply of blood and oxygen; if it did not it would die. This means we cannot literally stick someone in a scanner and read their thoughts (because the whole brain would look active). In order to infer functional specialization one needs to compare relative differences in brain activity between two or more conditions: cognitive subtraction. This involves selecting a baseline or comparison condition. A region is active if it shows greater response in one condition relative to another. If the experimenter chooses inappropriate conditions the regions of activity will be meaningless.
Cognitive stages in reading written words aloud and producing spoken semantic associate
Pure insertion
The assumption that adding a different component to a task does not change the operation of other components.
Efference copy
A motor signal used to predict sensory consequences of an action.
Functional integration
The way in which different regions communicate with each other.
Resting state paradigm
A technique for measuring functional connectivity in which correlations between several regions are assessed while the participant is not performing any task.
Default mode network
A set of brain regions that is more hemodynamically active during rest than during tasks.
Excitation
An increase of the activity of a brain region triggered by activity in another region.
