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Definition
| A low-resolution, poor-contrast X-ray that was neither painless nor harmless |
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| The resolution of a technique indicates how far away, in space or time, two events have to be before they can be distinguished as two separate events. |
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Definition
| EEG and MEG, which measure the electrical (EEG) or magnetic (MEG) fields generated by the activity of groups of neurons, have very good temporal resolution (in the order of milliseconds). |
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| PET and fMRI are sensitive to the brain’s metabolism (of oxygen in particular) and can be used to localize active tissue with great precision |
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| Electroencephalogram (EEG) |
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Definition
| The electrical activity of nerve cells (neurons) in the brain produces currents spreading through the head. These currents reach the scalp surface and the resulting voltage differences on the scalp can be recorded as the electroencephalogram (EEG). |
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| Magnetoencephalogram (MEG) |
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Definition
| The currents inside the head produce magnetic fields, which can also be measured above the scalp surface as the magnetoencephalogram (MEG). |
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Term
| Continuous or spontaneous EEG |
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Definition
| Voltage fluctuations as a function of time, which can be viewed directly for each electrode during the recording |
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| The time difference between sampling points |
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| The number of samples per second, |
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| To compare results across different studies, electrode locations must be standardized and locations need to be easily determined for individual subjects. Common is the so-called extended 10/20 system, where electrode locations are defined with respect to fractions of the distance between nasion and inion |
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| The little bump on the back of your head |
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| The little skin lobules in front of your ear canals, |
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Definition
| This electrode would be affected by global voltage changes (e.g. slow voltage shifts due to perspiration) in the same manner as all the other electrodes, such that brain non-specific activity is subtracted out by the referencing. |
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Term
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Definition
| To subtract the average across all electrodes from each electrode for each time point |
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Term
| Superconducting quantum interference devices (SQUIDs). |
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Definition
| The strength of the magnetic fields produced by the human brain is less than a millionth of the earth’s magnetic field. Such small field strengths can be measured by so-called superconducting quantum interference devices (SQUIDs) |
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| MEG is particularly insensitive to radial dipoles, those directed towards or away from the scalp (like at the top of a gyrus) |
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Definition
| MEG is particularly insensitive to radial dipoles, those directed towards or away from the scalp (like at the top of a gyrus) |
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Definition
| MEG mainly ‘sees’ tangential dipoles, which are parallel to the scalp |
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Term
| Event-related potential (ERP) |
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Definition
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Term
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Definition
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| Until approximately 20 ms after stimulus onset |
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Definition
| 20–50 ms approx. after onset |
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Definition
| Later than 50 ms approx. after onset |
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Definition
| Filtering is applied to a data set to remove any frequencies that are not part of the signal of interest, i.e. the ERP/ERF components under investigation. |
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Term
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Definition
| In most ERP and ERF studies, a ‘baseline interval’ is defined as several tens or hundreds of milliseconds before stimulus onset. For each recording channel, the mean signal over this interval is computed, and subtracted from the signal at all time points. This procedure is usually referred to as baseline correction. |
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Definition
| The ‘problem’ of trying to identify the cause of the EEG or MEG signal from its effect, and that unfortunately many different causes might lead to the same effect. |
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Term
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Definition
| If the number of activated brain regions that generate the recorded signal is assumed to be known, but their locations are still to be determined, so-called dipole models can be applied. The locations and orientations of dipoles are systematically varied until the best fit between the predicted and the measured potential is achieved. |
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Term
| Distributed source models |
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Definition
| A large number of dipoles (hundreds or thousands) is distributed across the brain volume or the brain surface. Their strengths are then estimated such that their summed activity explains the measured signal and an additional criterion is fulfilled, |
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Term
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Definition
| The oddball paradigm, in which subjects are exposed to a stream of ‘standard’ stimuli occasionally interrupted by ‘deviant’ stimuli, is one of the most widely used paradigms in cognitive electrophysiology. |
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Definition
| The amount of oxygen in the blood. |
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Term
| Positron emission tomography (PET) |
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Definition
| Positron emission tomography (PET), when used by psychologists, measures how the pattern of blood flow in the brain changes (how it is increased in some areas and decreased in others) as a function of cognitive state. |
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Term
| Functional magnetic resonance imaging (fMRI) |
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Definition
| Functional magnetic resonance imaging (fMRI) is used by psychologists in a very similar way, to index regional changes in blood oxygenation as a function of cognitive state. |
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Term
| Regional cerebral blood flow (rCBF) |
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Definition
| Index by which relative changes in brain activity are measured in the case of PET |
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Term
| Hierarchical subtractive design |
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Definition
| In studies of this type, conditions are successively ‘built up’ from cognitive or perceptual components which are assumed to be additive. |
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Term
| Correlational/parametric designs |
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Definition
| Correlational/parametric designs are used to examine the relationship between a stimulus parameter (such as word presentation rate), or a behavioural response (such as reaction time), and brain activity. |
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Definition
| Compounds used by the brain |
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Definition
| Positrons are positively charged electrons that are emitted from the nucleus of unstable (radioactive) isotopes, such as oxygen-15 |
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Term
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Definition
| An emitted positron combines with an ordinary electron of a nearby atom in an annihilation reaction, turning the mass of the two particles into gamma rays that are emitted at 180 degrees from each other. |
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Definition
| High-energy light particles |
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Definition
| PET is often used to study resting metabolism, that is to say, brain activity in the absence of any particular cognitive task |
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Definition
| When psychologists use PET, however, they are generally interested in knowing where activity increases in the brain as a function of a particular cognitive task. These are called activation PET studies. |
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Definition
| Chemicals that act as messengers between cells in the brain |
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Term
| Blood oxygen level dependent (BOLD) contrast |
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Definition
| Changes in concentration of deoxyhaemoglobin and oxyhaemoglobin, which have different magnetic properties, alters the magnetic field strength in active regions, which is reflected in the patterns of energy released by protons during scanning. |
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Definition
| The change in BOLD that occurs with a change in neural activity in a particular brain region |
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Definition
| The haemodynamic response takes 4–5 seconds to develop after neural activity increases, and takes as long again to decay after neural activity stops |
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Definition
| Maps of blood vessel distribution |
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Definition
| Fluctuations in fMRI field strength |
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Definition
| The time required to take one scan |
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Definition
| The time required to take one scan |
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Definition
| The raw data that is collected in PET, SPECT and fMRI does not resemble an image. A mathematical transform is required to turn the data into a set of images |
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Definition
| Images need to be realigned, correcting for head movement |
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Definition
| A standard space provides a way to locate positions in the head (in three dimensions) and allows comparison among different brains |
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Term
| The standardized stereotaxic coordinate system, sometimes called the Talairach coordinate system |
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Definition
| Coordinate system by which any location in the brain can be indexed by the values of three coordinates, using units that are approximately a millimetre |
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Definition
| The process by which brain volumes are changed in shape, size, and orientation so they approximately match that of a template, in standard space |
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Definition
| Reference atlases are in the same space as the template, and give, for each set of x, y, z coordinates, detailed information about the brain at that point |
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Term
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Definition
| Activation at each point is replaced by a weighted average of the activation measured not just at that point, but at all neighbouring points |
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Definition
| Noise in fMRI data can be removed by filtering. For example, it is common to use a high-pass filter in order to remove slowly varying components in the time series |
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