{"id":3604,"date":"2021-07-24T14:42:00","date_gmt":"2021-07-24T14:42:00","guid":{"rendered":"https:\/\/speechneurolab.ca\/?p=3604"},"modified":"2023-07-25T21:41:56","modified_gmt":"2023-07-25T21:41:56","slug":"electroencephalography-eeg","status":"publish","type":"post","link":"https:\/\/speechneurolab.ca\/en\/electroencephalography-eeg\/","title":{"rendered":"Electroencephalography (EEG)"},"content":{"rendered":"\t\t
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Electroencephalography is a technique that measures the\u00a0electrical activity\u00a0generated by large populations of neurons through electrodes placed on the scalp.<\/strong><\/p>

This electrical activity is transcribed as a trace called an electroencephalogram (see Figure 1), which illustrates the variations in the electrical activity of the brain over time.\u00a0EEG is not only used to study brain function in healthy people, but also to diagnose certain diseases that alter brain electrical activity (e.g., epilepsy, migraines, sleep disorders).<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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Figure 1.<\/strong>\u00a0Example of an EEG diagram, where each horizontal line represents the activity measured by one electrode.<\/p>

History<\/strong><\/p>

The first EEG recordings date back to the late 19th century.\u00a0In 1875, British physician Richard Caton was the first to suggest, based on his work with animals, that the electrical activity generated by the brain reflects mental activity. His studies showed that patterns of electrical activity vary not only according to the state of consciousness of the animal (e.g., awakening, sleep, anesthesia, death), but also according to external stimulations (Collura, 1993).\u00a0<\/p>

It was only forty-nine years later that the first EEG recording was made with humans.\u00a0In 1924, the German neuropsychiatrist Hans Berger recorded the brain electrical activity of a young patient who underwent trepanation to remove a cervical tumor. Berger was the first to describe the relationship between mental activity and variations in the electrical signal in certain frequency bands in humans. In 1929, Berger published the results of his observations. In this publication, he describes two brain rhythms: alpha and beta (both described below in the section on brain rhythms). Berger\u2019s work marked the beginning of the use of EEG in the clinical and research fields.<\/p>

The source of the EEG signal<\/strong><\/p>

Most of the EEG-recorded electrical activity comes from pyramidal neurons located in layers III, V and VI of the\u00a0cortex\u00a0(see Figure 2).\u00a0The signal originates from the synchronization of about 100,000 neurons.\u00a0In fact, it is not possible to record the electrical activity of individual cells of the human brain. This type of research is only possible in\u00a0animals.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t

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Figure 2.<\/strong>\u00a0The six layers of the human cortex (adapted from\u00a0http:\/\/www.maxicours.com\/se\/fiche\/0\/2\/142702.html<\/a>).<\/p>

The electrical potentials generated by neurons are captured by small sensors, called electrodes, which are usually silver chloride electrodes (AgCl).\u00a0The electrodes can be attached to the scalp by means of paste or with a conductive gel and a cap.\u00a0EEG helmets often have 32 or 64 electrodes, but some can have up to 256!\u00a0 EEG systems thus allow a continuous recording of a huge number of signals.\u00a0Since the EEG signals are of low amplitude (i.e. at the microvolt-range [\u03bcV];\u00a0typically between 0.5 and 100 \u03bcV), they must be amplified thousands of times (10,000 to 50,000) using an amplification device (Teplan, 2002).\u00a0Paste or gel reduces impedance (i.e. resistance to current passage), measured in ohms, which facilitates recordings.<\/p>

In addition to measuring the electrical activity of neurons, EEG also captures the electrical activity generated by body movements, eye blinks, and surrounding devices such as computers and cellphones!\u00a0Therefore, people undertaking an EEG exam are usually asked to keep their face as relaxed as possible, and to minimize eye blinks and body movements.\u00a0Furthermore, laboratory experiments often take place inside a Faraday cage, a special room which blocks of electromagnetic noise generated by electronic devices located outside of the room.\u00a0Minimizing movements and using faraday cages limit the amount noise in the EEG signal and facilitate its analysis.<\/p>

Types of EEG signals<\/strong><\/p>

EEG can be used to measure the spontaneous electrical activity of the resting brain, for example when an awake person does not do a specific activity, which makes it possible to quantify different brain rhythms.\u00a0It is also possible to use EEG to measure electrical activity associated with speech and language processes (e.g.,\u00a0Tremblay et al., 2021<\/a>;\u00a0Pinto et al.,\u00a02019<\/a>;\u00a0Tremblay et al., 2008<\/a>) or cognitive processes such as memory, for example, which allows measuring evoked potentials. Brain rhythms and evoked potentials are defined in the following sections.<\/p>

Brain rythms<\/em><\/p>

Brain rhythms are spontaneous signals that can be measured even in the absence of external stimulation.\u00a0These rhythms are often used in cognitive neuroscience to classify sleep patterns and to identify patterns of atypical neuronal activity associated with pathologies (e.g., epilepsy, cervical tumours), states of consciousness and alertness.\u00a0The frequency of the brain oscillations (i.e. the number of cycles per second) is measured in Hertz (Hz) (see Figure 3).\u00a0Here are the main brain rhythms:<\/p>

Delta rhythms:<\/span>\u00a0These are slow waves.\u00a0They are present during a state of deep meditation or during sleep without dreams.<\/p>

Theta rhythms:<\/span><\/span>\u00a0These are waves present during a deep sleep.\u00a0These waves play a role in learning and consolidation in long-term memory.<\/span><\/p>

Alpha rhythms:<\/span><\/span>\u00a0These waves are present during a state of alertness or light meditation.\u00a0They are associated with the coordination of mental activities and learning.\u00a0They are also associated with semantic processing, which allows us to access knowledge about the meaning of words (Klimesch, 1997;2012).<\/span><\/p>

Beta rhythms:<\/span>\u00a0These waves are present during an awakening state when our attention is engaged by cognitive activities (e.g., making a decision, solve a problem) or by the environment. They would play an important role for the\u00a0integration of audiovisual signals<\/a>\u00a0during speech perception (Romero et al., 2015).<\/p>

Gamma rhythms:<\/span>\u00a0These are the fastest waves.\u00a0They are associated with the processing of information by different regions of the brain (i.e., the synchronization of several regions of the brain).\u00a0They are also present in states requiring a high level of attention or concentration.\u00a0Research works also suggest that these oscillations may be associated with the acoustic treatment of speech sounds (Ou & Law, 2018).<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t

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Figure 3. <\/span><\/strong>Different brain oscillations (adapted from <\/span>http:\/\/tpe-batement-binauraux.webnode.fr\/quest-ce-que-les-battements-binauraux-\/<\/a>)<\/span><\/p>\n

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Evoked potentials<\/strong><\/p>

Unlike cerebral rhythms, evoked potentials represent the response of the central nervous system to an external stimulation (e.g., the appearance of a word on a computer screen) or an internal event (e.g., emotion, thought, memory). There are several types of evoked potentials, depending on the nature of the stimulation:<\/span><\/span><\/p>

\u2022 Visual evoked potentials
\u2022 Auditory evoked potentials
\u2022 Somatosensory evoked potentials (e.g., related to sense of touch)
\u2022 Cognitive evoked potentials
\u2022 Motor evoked potentials<\/span><\/p>

Since the electromagnetic noise captured by the electrodes is generally much higher than the EEG signal of interest, which is of very low amplitude, it is necessary to record several times the EEG signal associated with the same activity or repeated stimulation.\u00a0 Averaging the signal over multiple trials will reveal the pattern of brain response. For example, in our laboratory, to measure the evoked potentials associated with speech perception, we would present a hundred words to participants, then calculate the average of the signal recorded during a few hundred milliseconds after the presentation of each word.\u00a0This would reveal brain responses associated with word perception and distinguish it from those associated with background activity and noise.<\/p>

The evoked potentials are identified according to their polarity (N = negative, P = positive), their time of appearance in milliseconds and their location on the scalp.\u00a0For example, we can see on Figure 4 that the polarity of the evoked potential P1 is positive (the positive values are presented at the bottom of the graph) and that it occurs about 100 milliseconds after the presentation of a stimulus.\u00a0P1 is associated with the processing of visual stimuli; the most reactive electrodes are located at the back of the head where the visual processing centers are located.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t

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Figure 4.<\/strong> Diagram illustrating several evoked potentials (Components of ERP<\/a>) from Choms<\/a>, licensed under CC BY-SA 3.0<\/a>.<\/p>\n

The evoked potentials are useful both for clinical and research applications in the field of cognitive neuroscience. In a clinical setting, sensory evoked potentials (e.g., visual or auditory) can assist in the diagnosis of neurological diseases (e.g., multiple sclerosis, tumours of the auditory system). In research, evoked potentials are very important to study the time course of brain activity related to auditory, speech, language, memory for example (Luck, 2014).<\/p>\n

Advantages and disadvantages of EEG<\/strong><\/p>\n

One of the advantages of EEG technique is its excellent temporal resolution (~1ms), which allows one to explore brain activity in real time, for example during the preparation or production of language, cognitive or motor tasks, or during sensory processing. Another advantage of EEG is that it is a \u201csilent\u201d technique compared to the MRI, which is interesting for researchers interested in hearing process, like us. EEG does not require lying in a narrow area without moving, making it a friendlier, less impressive methods than MRI. EEG acquisition and operating costs are much lower than that of MRI. In addition, EEG can be used with people of all ages, from newborns to the elderly as well as with clinical populations. There are no counterindication to EEG.<\/p>\n

Disadvantages of EEG include its spatial resolution, which is much lower than for MRI, particularly because the electrical signals are attenuated by the structures they pass through (e.g., skull, scalp) that can distort them. This means that it is less straightforward to localise the source of EEG signals. Moreover, EEG, unlike MRI, does not provide a three-dimensional image of the brain.<\/p>\n

However, EEG can be combined with MRI and TMS techniques using MRI and\/or TMS compatible systems. This fall, our laboratory acquired a TMS compatible EEG system! Using this new system, we will explore the effects of TMS on the brain networks involved in speech, and how TMS can affect speech performance!<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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Further readings:<\/p>\n