C BBB - Chapter 4: The Imaged Brain

Functional magnetic resonance imaging (fMRI)

A non-invasive brain scanning technique that measures brain activity by tracking changes in blood flow.

Positron emission tomography (PET)

A non-invasive imaging technique that visualizes and measures metabolic and cellular activity in the body.

Computed tomography (CT)

An imaging technique that combines X-rays and computer processing to produce detailed cross-sectional images of the body.

Structural imaging

Measures the spatial configuration of different types of tissue in the brain.

Functional imaging

Measures temporary changes in brain physiology associated with cognitive processing.

Voxel-based morphometry (VBM)

A technique for measuring differences in grey matter and white matter concentration.

Diffusion tensor imaging (DTI)

An MRI-based technique used to measure white matter connectivity between brain regions.

Voxel

A three-dimensional unit of measurement used in brain imaging.

Fractional anisotropy (FA)

A measure of the extent to which diffusion occurs more in some directions than others.

BOLD

Blood oxygen-level-dependent contrast; the fMRI signal related to the concentration of deoxyhemoglobin in the blood.

Hemodynamic response function (HRF)

The pattern of changes in the BOLD signal over time.

Cognitive subtraction

An experimental design in which activity from a control task is subtracted from activity in an experimental task.

Pure insertion (or pure deletion)

The assumption that adding a cognitive component to a task does not alter the operation of other components.

Efference copy

A motor signal used to predict the sensory consequences of an action.

Functional integration

The way different brain regions communicate with one another.

Resting state paradigm

A method for measuring functional connectivity while participants are not performing a task.

Default mode network

A set of brain regions that is more active during rest than during task performance.

Stereotactic normalization

The mapping of individual brain anatomy onto a standard template.

Smoothing

Redistributing activity across neighbouring voxels to improve the signal-to-noise ratio.

Talairach coordinates

Brain locations defined relative to the Talairach and Tournoux atlas.

Family Wise Error (FWE)

A correction method for multiple statistical comparisons based on the number of tests performed.

False Discovery Rate (FDR)

A correction method for multiple statistical comparisons based on the proportion of positive findings.

Inhibition

A reduction in the activity of a brain region or cognitive process caused by another region or process.

Excitation

An increase in the activity of a brain region or cognitive process caused by another region or process.

Activation

An increase in physiological processing in one condition relative to another condition.

Deactivation

A decrease in physiological processing in one condition relative to another condition.

Semantic dementia

A progressive loss of information from semantic memory.

Semantic memory

Knowledge about the world, including concepts, objects, people, places and word meanings.

Multi-voxel pattern analysis (MVPA)

An fMRI analysis method that links distributed patterns of brain activity to cognitive processes.

Vegetative state

A disorder of consciousness characterized by partial arousal following severe brain damage.

X-ray absorption

The principle behind CT in which different tissues absorb different amounts of X-rays.

Ionizing radiation

Radiation used in CT but not MRI.

Blood flow

The movement of blood to active brain regions.

Blood oxygenation

The oxygen level in the blood measured indirectly through the BOLD signal.

Radioactive tracer

A radioactive substance injected into the bloodstream in PET to measure metabolism or blood flow.

Baseline condition

A comparison condition used to determine whether brain activity is higher or lower during a task.

Comparison condition

A condition against which activity in an experimental condition is compared.

Hemodynamic methods

Methods that measure brain activity indirectly through changes in blood flow or blood oxygenation.

Deoxyhemoglobin

Hemoglobin without oxygen; changes in its concentration form the basis of the BOLD signal.

Excitatory neurons

Neurons that increase the likelihood of activity in other neurons.

Inhibitory neurons

Neurons that decrease the likelihood of activity in other neurons.

Modulatory neurons

Neurons that regulate or influence the activity of other neurons.

Relative activity

Brain activity measured in comparison with another condition rather than as an absolute level.

Poor temporal resolution

A limitation of hemodynamic methods because blood-based responses occur slowly compared with neural activity.

Good spatial resolution

A strength of imaging methods such as fMRI because they can localize activity precisely.

Functional near-infrared spectroscopy (fNIRS)

A functional imaging method that measures blood oxygenation using near-infrared light.

Cognitive conjunction

An imaging design that identifies brain regions activated across multiple task comparisons.

Factorial design

An experimental design that examines the effects and interactions of two or more factors.

Parametric design

An imaging design in which brain activity is related to continuous changes in a variable.

Functional connectivity

The correlation of activity between different brain regions.

Effective connectivity

The inferred influence that one brain region exerts on another during a task.

Brain reading

The attempt to infer mental states or cognitive processes from patterns of brain activity.

1st phase of the hemodynamic response function

Initial dip

2nd phase of the hemodynamic response function

Overcompensation

3rt phase of the hemodynamic response function

Undershoot

hemodynamic response function: Undershoot

In response to the increased consumption of oxygen, the blood flow to the region increases. The increase in blood flow is greater than the increased consumption, which means that the BOLD signal increases significantly. This is the component that is normally measured in fMRI, and the size of this peak is taken as indicative of the extent to which this region is active in the task.

hemodynamic response function: Overcompensation

the blood flow and oxygen consumption dip before returning to their original levels. This may reflect a relaxation of the venous system, causing a temporary increase in deoxyhemoglobin again.

hemodynamic response function: initial dip

As neurons consume oxygen there is a small rise in the amount of deoxyhemoglobin, which results in a reduction of the BOLD signal

Predicted fMRI

Timing of stimulus presentation x Hemodynamic response function