Terms & Definitions

Amplitude: the size of a wave cycle which is strongly influenced by the number of synchronously firing neurons. Approximately 50% of the EEG amplitude arises from neurons directly beneath the electrode and 95% arises from a 6 cm distance.

Attention: the most common cognitive disturbance in neurocognitive patients involves the domain of attention. It is the capacity of harnessing one’s own neural resources on the processing of one item of information while excluding all others. Attention is comprised of 5 general categories (Fuster, 2009):

  • Alertness: People’s level of consciousness, or general wakefulness, is defined as how quickly and appropriately people respond to stimuli.  The person with prefrontal damage or dysregulation is generally less alert than someone without such damage, especially when the damage or dysregulation is widespread. Generally, the less alert individual is also compromised in spontaneity and general awareness of environmental events than normal.
  • Set: Set is the ability to devote enough neural resources for what will happen, or expected events. It is the prediction based on current context of “now.” Set depends on long-term memory, recent memory, perception and the ability to adjust one’s actions based on the match or mismatch of the expectation.
  • Spatial attention: The basic ability to gauge the spatial orientation of one’s own body in the environment. In a brain-injured patient (or a patient with prefrontal dysregulation), spatial neglect (impaired spatial attention) is a more specific type of spatial attention whereby the patient lacks full awareness of his or her body. It often presents on one side only. Spatial attention allows the person to be able to adjust actions based on their own predictions of environmental changes. When impaired, it produces an inability to anticipate and correct for changes in the world around the person.
  • Sustained attention: The ability to maintain concentration. Patients with prefrontal damage or dysregulation reach their limit quickly in sustaining their own attention. The more complex the activity, the more the attention is taxed quickly and the more the deficits are evident. Since working memory is a form of sustained attention, people with prefrontal lesions or dysregulation are almost uniformly deficient in working memory.
  • Interference control (resistance to distraction): This is what it sounds like. The person with prefrontal lesions or dysregulation will demonstrate deficits in resisting environmental events interfering with tasks at hand. This person would be unable to resist paying attention to irrelevant information, including information from their own body states.

Brain dysregulation: occurs when the brain is out of balance, often due to problems with ionic conductances and excitability of neurons in clusters and groups. Different expressions of dysregulation (deviation from normal) can occur such as in elevated or diminished activity in any frequency band within local brain regions or happening globally throughout the brain.
The location of the brain dysregulation is evidenced by patient symptoms and complaints.

Brain Oscillations: synchronized rhythmic or repetitive firing patterns of large groups of neurons known as cell assemblies or ensembles. Oscillatory patterns occur when neurons fire together, then become suppressed, then fire together again, and so forth. In other words, brain oscillations are thought to be a natural timing mechanism for dynamic processing of cognitive functions occurring at multiple levels (e.g. dynamic processing of attention, perception, memory, motor control, etc.).

Brodmann Areas:  defined in 1909, by Korbinian Brodmann based upon differences in the cytoarchitectural organization of neurons he observed in the cerebral cortex. These areas were later validated in the 1990s with the advent of new imaging technologies such as fMRI and PET. There are 88 Brodmann areas forming the basis for our understanding of brain function and a major contribution in neuroscience.

Coherence: Coherence is amplitude independent and it’s equal to the number of the connections times the strength of the connections. It is a connectivity measure of stability or strength of phase or phase/amplitude coupling in a time series between two electrode sites or two brain regions at a given frequency, expressed as a squared correlation co-efficient (from 0 to 1). Higher coherence values that are closer to 1 show the phase differences have less variability and higher consistency with greater redundancy.

Complex networks: networks which exhibit properties of intrinsic organization with different hierarchical clusters of nodes, hubs, and modules. Examples of complex networks from real-life include social networks, the internet, airline traffic routes, protein networks, and brain networks.

Current Source Density (CSD): can be derived from non-invasive electric potential differences recorded on the scalp (from the EEG). An inverse method such as eLORETA is used to accurately estimate a 3-D distribution of CSD within a solution space (3-D brain volume) consisting of thousands of discrete voxel units. The inverse problem comes from physics and mathematics. It is a way of “working backwards” from the data to identify model parameters. In statistics, we usually solve forward problems, which relate model parameters to the data.

Encephalitis: inflammation of the brain (regardless of cause).

  • Acute encephalitis syndrome: inflammation of the brain, most often caused by viruses. The inflammation, which is essentially what encephalitis is, can be part of another disease or medical problem such as meningitis (inflammation of the covering of the brain (the meninges)). Seven out of ten people with encephalitis will die within a few days. This is a very severe disease.
  • Chronic encephalitis syndrome: is essentially the same as the acute form (see above) but with a longer time course with less severe symptoms. The patients are generally less debilitated over time.

Encephalopathy: Many infections involving the entire body system also affect the brain and spinal cord, which is what encephalopathy is (a brain disease).  In most encephalopathies, systemic symptoms of the particular disease are reported by the patient, such as diabetic encephalopathy, but sometimes the CNS findings are more prominent than the underlying disease process. In some cases, CNS symptoms are the only features. Given that this is such a large and varied group of diseases, the syndromic approach (see syndrome) to diagnosis is less effective. In the setting of an undiagnosed CNS infectious syndrome, it is prudent to consider systemic infection as a possible underlying cause.

Frequency band: Coupling and de-coupling of local assemblies and long distance assemblies of neurons give rise to the oscillatory frequencies of the EEG signal. The frequency band is a designated portion or range of the EEG frequency spectrum which describe the number of oscillations recorded in the raw EEG signal over time. There is a direct mathematical relationship between the time domain and the frequency spectrum. Frequencies are traditionally divided into bandwidths according to an ascending order: delta, theta, alpha, beta, and gamma. Further subdivisions and different bandwidth specifications within the frequency spectrum are increasingly common. Characteristic frequency bands are associated with different states of brain functioning (e.g., arousal, vigilance, drowsiness, fatigue, sleep stages, etc.).

Functional connectivity: the brain is in constant flux and different brain regions function in relation to each other. Functional connectivity refers to study approaches that account for these relationships. The location of the different parts of the brain are extremely important for our understanding of dysfunction. The functional connectivity studies, the temporal correlations of synchronized brain activity are measured during the awake resting-state or when performing a particular cognitive task.

Graph theory: Graph theory is branch of mathematics that deals with the process of modeling the topology of networks to describe a complex system of interactions based upon network elements (nodes) and the connections between nodes (edges). In neuroscience, graph theory can be used to study the intrinsic organization of networks within the brain. This approach looks at network topology to describe functional relationships between brain areas and the interactions give rise to various cognitive processes. Some basic terms used in graph theory analysis include:

  • Node: the principle network unit representing a particular brain region of interest. 
  • Edge: a link or connection pathway between two given nodes in the network. In neuroscience, the edge represents the functional connection between two brain regions. Structurally, the white matter fiber tracks are the links between between brain areas. 
  • Degree: the amount of connectivity a given node has (simply the binary presence or absence of connection). Strength is a variant of the degree measure which sums all the weighted edges connecting to a given node to its nearest neighbor nodes.
  • Modules: a subset of nodes within a given network which perform specialized processing while minimizing long-distance connections. Modules are also referred to as communities.
  • Hubs: a highly centralized region with strong links to other nodes or modules facilitating a high degree of functional influence over a network.
  • Provincial hubs: hub regions which are highly connected within one module.
  • Connector hubs: hub regions that link multiple modules.
  • Scale-free network—distribution of node number that follows a power law.
  • Topology: specific pattern of links or connections which defines the network.

Imaging modalities: In the past several decades, there have numerous imaging techniques developed for measuring all aspects of brain structure and function. Each technique has certain advantages as well as limitations.  The ultimate goal of bringing together these imaging modalities is good clinical outcome. In order to achieve that, it requires putting together all the imaging modalities and understanding where we are and understanding where the EEG is with respect to the other imaging modalities:

  • CT (computed tomography) is a computerized x-ray that takes multiple pictures then produces a detailed image from them. It is used to identify abnormalities in the brain and body.
  • MRI (magnetic resonance imaging) is a widely-used technique for obtaining static high resolution images of the body using powerful superconducting magnets and radio signals. Images can be weighted with more or less magnetization depending on the nature of the anatomy being examined for structural integrity or pathology. T1-weighted images are typically used for examining the cerebral cortex, whereas T2-weighted images are better for examining white matter lesions.
  • BOLD fMRI (blood oxygen level dependent functional magnetic resonance imaging) a functional imaging process which builds on MRI to map BOLD hemodynamic responses, an indirect measurement of neuronal activity.
  • DTI (diffusion tensor imaging) is an MRI platform that can produce a “wiring diagram” of the brain, examining at the integrity of the brain’s neural pathways (white matter fibers) in order to detect various brain pathologies. Although DTI is relatively new it has proven to be an invaluable research tool paving the way for major advancements in neuroscience. Clinical usefulness for DTI remains to be established.
  • PET (positron emission tomography) uses a radioactive isotope emissions to measure physiological function by looking at blood flow, glucose metabolism, neurotransmitter activity, and radioactive drugs.
  • SPECT (single photon emission computed tomography) is similar to PET and relies on a radioactive tracer to measure cerebral blood flow.
  • eLORETA (exact low resolution brain electromagnetic tomography) is an electrical neuroimaging method that takes the EEG signal at the scalp surface and localizes the originating electrical sources. With this enhanced spatial resolution, brain maps can then be generated which reveal a 3-dimensional distribution of electrical sources throughout the cortex. eLORETA also uses the same voxel coordinate system as fMRI, PET, and SPECT to allow multimodal comparison between findings.

Local Field Potential (LFP): Single neuron discharge is too small to be detected in the EEG. Instead, the EEG measures the LFP which is the electrical field within a volume of tissue produced by the combined action potentials (electric current) from the collective activity of neurons. Every neuron senses electrical field within its local region and fires during the depolarized phase of the LFP. Neuronal inhibition occurs during hyperpolarized phase of the LFP.

Memory: It is difficult to define memory without understanding brain networks. Brain networks (connectomes) are widely distributed throughout the cortex, associating different clusters of neural assemblies. Memory systems are known as representations, since they contain not only the verbal and visual knowledge as part of the memory, but also the goal-directed action sequences. In this manner, these networks represent past, and future actions. People with prefrontal cortical lesions are normally not markedly amnesic, but they do have trouble with recall and recognition with their primary deficit being in organization and monitoring of verbal material. Recent memory is also impaired due to their issues with perceptual attention and drive.  Paradoxically, they have very little difficulty on neuropsychological tests of declarative or episodic memory.

  • Working memory is the ability to keep relevant information “in mind” when performing a task. It is a combination of the essential cognitive functions of speech, reasoning and goal-directed prospective behavior. Working memory is a necessary condition for prospective action, regardless of whether the to-be-performed action is motor, speech or a mental operation. Working memory problems are consistently seen in persons with lateral prefrontal damage/dysregulation; it appears to be distributed throughout the lateral prefrontal cortex, and due to its characterization as sustained attention to internal stimuli, it is subject to interference and distractability. People with cortical damage may still perform well on some working memory tests. Accurate testing requires a test that introduces interference and distraction.

Neurocognitive Disorder (NCD): This “umbrella” term was introduced in the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5, American Psychiatric Association, 2013). Nearly all major NCDs include the dementias (e.g., Alzheimer’s disease, vascular dementia) with specifiers of mild, moderate, and severe. Major NCDs are to be distinguished from mild NCDs (e.g. mild NCD due to Parkinson’s or some other medical condition), cognitive disturbance that does not meet criteria for dementia. There are six different cognitive domains affected by NCDs described in DSM-5: complex attention, executive function, learning and memory, language, perceptual motor, and social  cognition.

Phase: the time-varying characteristics (e.g. phase angle, amplitude) and the position within a given rhythmic cycle of an oscillating wave pattern, frequently measured in degrees (0-360°). The phase difference defines the time delay or advancement components or offset between two or more signals. 

Planning: Planning is foresight, or “memory for the future.” Failures in plan formation are the opposite of deficits in working memory, in that difficulties in planning involve prospective thinking. Working memory is about the past and to some degree, the present while planning involves the future. Like working memory, planning is vulnerable to problems with distraction and attention. Planning deficits tend to occur alongside working memory problems, as a function of lateral prefrontal cortical dysregulation.

Post-infectious Syndrome: Different CNS syndromes may develop after another type of infection. These usually begin with a common, often rather trivial, viral infection that may go unnoticed. A postinfectious neurologic syndrome then develops. These syndromes can include postinfectious encephalitis, postinfectious encephalomyelitis, or transverse myelitis (inflammation of one section of the spinal cord), often damaging the insulating material covering nerve cell fibers (myelin). Transverse myelitis interrupts the messages that the spinal cord nerves send throughout the body. This can cause pain, muscle weakness, paralysis, sensory problems, or bladder and bowel dysfunction.

. These reactions are presumably mediated by an immunologic response to the etiologic microbe or to antigens revealed as a result of the initial infection. Although rare, these syndromes can be severe or fatal.

Quantitative EEG (qEEG): The “q” in qEEG stands for “quantitative” computer analysis of EEG data for vastly improving reliability, objectivity, and sensitivity. Computers are used to instantaneously transform the oscillations into frequencies to reveal subtle patterns of dysregulation undetectable to the naked eye. In this way, qEEG has advantages over conventional non-quantitative EEG which relies upon subjective “eyeball” examination of EEG tracings and therefore has low inter-rater reliability for non-epilepsy cases.

Resting-state:  refers to the “present” momentary state of brain during wakefulness in the absence of explicit task. Resting-state methods are used in studies which examine time series data containing local information on spontaneous brain network function. Thus, the resting-state is indicative of the stable characteristics of intrinsic brain organization, defined as spontaneous interactions in the absence of a specific task state (Raichle, 2010). In studying baseline followed by an active task, it was found that just measuring the resting activity predicted performance on active tasks without having to measure the brain during the task. That is, the resting-state activity is fairly predictive of brain activity during an active task inside the scanner. As a consequence, today, there are fewer active task studies and more resting-state studies.

Sign (diagnostic sign): An aspect of a disease or possible diagnosis observed by a physician or psychologist.

Symptoms: An aspect of a disease reported by patients.

Syndrome: A combination of symptoms and signs that together represent a disease process. Signs are what the physician ‘sees,’ and symptoms are what the patient reports. Migraine is a syndrome (migraine syndrome).