<h3><b>How is an EEG Recorded?</b></h3><p><img src="@@PLUGINFILE@@/Differential%20amplification%20.png" alt="Differential amplification" width="450" height="128" role="presentation" class="atto_image_button_middle"></p><p><span style="font-size: 1rem;">An EEG is recorded using a differential amplifier and electrodes. A differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs. In EEG recordings and neurofeedback, a differential amplifier is used to produce each channel or trace of activity.&nbsp;</span></p><p><span style="font-size: 1rem;">There are many different brands of amplifiers in the fields of neurotherapy and neurology and the decision of what kind of amplifier to buy is one of the first major decisions a newcomer to these fields usually makes. Some amplifiers are noisier than others, some are higher quality than others, like any other product. Some amplifiers are used for neurofeedback alone and some amplifiers are used for EEG and QEEG collection, as well as neurofeedback. It behooves the new neurotherapy practitioner to do research to determine which amplifier and neurotherapy modality best suits them. This course is intended to help clarify that process.&nbsp;</span></p><p></p><h3><b>Differential Amplifiers</b></h3><span style="font-size: 1rem;"><img src="@@PLUGINFILE@@/Neurofield%20Amp.png" alt="Neurofield Amp" width="350" height="207" class="img-responsive atto_image_button_left"></span><p></p><p><span style="font-size: 1rem;"><img src="@@PLUGINFILE@@/Brainmaster%20amp.png" alt="Brainmaster amp" width="300" height="207" class="img-responsive atto_image_button_middle"></span></p><p></p><p><b><span><u><img src="@@PLUGINFILE@@/Deymed%20amp.png" alt="Deymed amp" width="300" height="332" class="img-responsive atto_image_button_middle"><br></u></span></b></p><p><b style="font-size: 1rem;"><span><b style="font-size: 1rem;"><span><b style="font-size: 1rem;"><span><b style="font-size: 1rem;"><span>Differential amplifiers&nbsp;</span></b><span style="font-size: 1rem; font-weight: 400;">measure the voltage difference between the two signals at each of its inputs--that of the active vs. the reference electrode, each pair using a third electrode, the <b>ground electrode</b> to literally "ground" the recording.&nbsp;Basically, a grounding electrode establishes a direct connection to the earth or ground.</span><span style="font-size: 1rem; font-weight: 400;">&nbsp;</span></span></b></span></b></span></b><br></p><p>EEG amplifiers&nbsp;<span style="font-size: 1rem;">amplify separately (1) the voltage between one recording site and the ground electrode and (2) the voltage between a second recording site and the ground electrode. Then the electronics subtract one from the other (producing the difference between the two). The subtraction process removes other signals (e.g., 60 Hz line noise) that is present and common to both recording electrodes. The subtraction process itself is imperfect, so that some of the contaminants remain. Removing the contaminants and the EEG activity common to both channels is called<u>&nbsp;</u></span><span xmlns:xlink="http://www.w3.org/1999/xlink" style="font-size: 1rem;"><b style=""><u>common mode rejection</u>&nbsp; or common mode rejection ratio (CMRR)</b></span><span style="font-size: 1rem;">. The higher the CMRR, the better the amplifier system excludes unwanted contaminants. A high quality amplifier will display a very clean EEG signal without the use of filtering.&nbsp;</span><span style="font-size: 1rem;">The resulting signal is amplified and then displayed as a channel of EEG activity.</span></p><p></p><p>The amplifier records the difference between a scalp electrode and a reference electrode. Different montages have different reference electrodes. The image below is an example of a bi-polar montage, such as the type used in many forms of single channel neurofeedback (methods like the Othmer method). in EEG assessment, bi-polar montages are very god for spotting local differences between neighboring channels.</p><p><img src="@@PLUGINFILE@@/Differential%20amplification%202.png" alt="Differential amplification 2" width="500" height="465" class="img-responsive atto_image_button_middle"></p><h3><span style="font-size: 1.715rem;">Analog vs. Digital</span><br></h3><p>Up until the 1990s, many people in neurology practices were using analog EEG recording systems. These were large systems that were the mainstay of neurological practices that cranked out EEG on paper, with ink pens scrawling back and forth.&nbsp; Paper record-keeping for patients' EEGs was the big downside to analog EEG; modernity has led the fields of neurotherapy and neurology to digital EEG processing.</p><p><img src="@@PLUGINFILE@@/analog%20EEG.jfif" alt="Analog EEG" width="350" height="283" class="img-responsive atto_image_button_left">Digital EEG came about in the 90s and has many advantages over analog EEG.&nbsp;Digital recording takes advantage of modern microprocessor costs and general flexibility. Digital copies of EEG are easy to save and share. More specifically, digital EEG allows review with different montages, filters, vertical scaling (gain or sensitivity), and horizontal scaling (e.g., time resolution or compression). It has replaced warehousing or microfilming paper records and allows for electronic exchange of EEGs. Finally, digital EEG makes possible the routine application of a variety of complex signal-processing tasks, such as frequency analysis, automated seizure detection, statistical quantitative analysis, and dipole source localization. Because of digital EEG, we have quantitative EEG and the amazing field of computational neuroscience.&nbsp;</p><h3><span style="font-size: 1.715rem;"><br></span></h3><h3><span style="font-size: 1.715rem;"><br></span></h3><h3><span style="font-size: 1.715rem;">Impedance</span><br></h3><p>Impedance is a measure of the impediment to the flow of current, measured in ohms or kilo ohms (kohm). Larger numbers mean higher resistance to current flow. The higher the impedance of the electrode, the smaller the amplitude of the EEG signal. In EEG studies, should be at lest 100 ohms or less and no more than 5 kohm. In neurofeedback, often 15kohm is permissible.</p><p>Checking impedance is a fundamental part of working with EEG and you will undoubtedly become familiar with checking impedances in your neurotherapy career. To get good impedance means getting good connectivity, which allows for a clean recording.&nbsp;</p><h3>What is Recorded in EEG?</h3><p>Synchronous brainwaves from the brain are recorded in EEG, which are seen due to volume conduction.&nbsp;<strong>Volume conduction </strong>can be defined as the transmission of electric or magnetic fields from an electric primary current source through biological tissue towards measurement sensors.&nbsp;Volume conductor models are a basis for&nbsp;source analysis&nbsp;in&nbsp;Electroencephalography (EEG)&nbsp;and&nbsp;Magnetoencephalography (MEG).&nbsp;<img src="@@PLUGINFILE@@/Volume%20Conduction.gif" alt="Volume Conduction" width="386" height="302" class="img-responsive atto_image_button_right"><br></p><p><span>The activity that is measured in EEG&nbsp;is the result of movements of ions, the so-called&nbsp;<span style="">impressed currents</span>, within the&nbsp;cortex&nbsp;sheet of the human brain. The&nbsp;<span style="">current dipole</span>&nbsp;is generally used as the&nbsp;''atomic structure''&nbsp;of the impressed current.&nbsp;</span></p><p>The current <b>dipole</b> causes ohmi<span>c&nbsp;<span style="">return currents</span>&nbsp;to </span>flow through the surrounding medium. The EEG&nbsp;measures the potential differences of the return currents at the scalp surface. The reconstruction of the impressed primary current distribution from measured surface fields is called the<span><span style=""><u style=""><b>&nbsp;</b><span style="">inverse problem</span><b>.</b></u></span></span> Its solution requires the simulation of the field distribution for a current dipole in the corresponding volume conductor using the quasi-static Maxwell equations, the so-called&nbsp;<span><u>forward problem</u></span>. All inverse methods (e.g., dipole fit and scan methods as seen in Independent Components Analysis, current density reconstructions such as LORETA) are based on the solutions of the forward problem and are thus influenced by the degree of realism in the modeling of the volume conductor.</p><h3>Alternating Current vs. Direct Current</h3><p>Both AC and DC describe types of current flow in a circuit. In&nbsp;<b><span>direct</span>&nbsp;current (DC)</b>, the electric charge (current) only flows in one direction. Electric charge in alternating current (AC), on the other hand, changes direction periodically. All brainwaves are alternating current brainwaves, the brain does not make DC.<br></p><p>AC can come in a number of forms, as long as the voltage and current are alternating. If we hook up an oscilloscope to a circuit with AC and plot its voltage over time, we might see a number of different waveforms. The most common type of AC is the sine wave. The AC in most homes and offices have an oscillating voltage that produces a sine wave.<br></p><p>Direct current is seen in High Voltage Direct Current (HVDC) transmission lines, these are the power lines that often traverse large stretches of land. The reason for this is there is little energy loss when DC is used, and the electricity can literally move at the speed of <img src="@@PLUGINFILE@@/HVDC.JPG?time=1636045837032" alt="HVDC" width="300" height="206" class="img-responsive atto_image_button_left">light.&nbsp;<span style="font-size: 1rem;">In the field of neurotherapy and neurology, there are DC coupled amplifiers and AC coupled amplifiers.&nbsp;An electronic amplifier takes in a weak signal and makes it stronger. To do this, you need active components, such as vacuum tubes, transistors or integrated circuits, and a power source.&nbsp;A steady DC voltage that gets mixed with an AC signal is called an offset. DC amplifiers amplify the whole signal, both AC and DC parts. This creates noise and other problems for music, so sound engineers usually use AC amplifiers.&nbsp;A DC amplifier accepts all parts of a signal, AC and DC. Engineers call this DC coupling or direct coupling.&nbsp;An amplifier with reactive components in the signal path, such as transformers or capacitors, blocks DC and permits only AC to pass through. Amplifiers with these components are called AC-coupled.</span></p><p>DC amplifiers are used in science, medicine and engineering whenever people deal with DC and slow-moving signals. Sound engineers may use DC amplifiers when they need excellent low-frequency response.&nbsp;Home stereos make up the biggest use of AC amplifiers. Radio and television engineers use very-high-frequency AC amplifiers. The AC coupling conveniently rejects noisy DC signals.<br></p><h3>tDCS and tACS</h3><p><img src="@@PLUGINFILE@@/Stimulation%20Modalities.png" alt="Stimulation Modalities" width="800" height="415" class="atto_image_button_middle"></p><p>Transcranial Direct Current Stimulation and Transcranial Alternating Current Stimulation are two forms of neuromodulation that allow clinicians to stimulate the individuals brain or body. This is a form of neurotherapy and a very effective one, at that. Many neurotherapy practitioners incorporate these stimulation modalities into their practices. The Neurofield Genesis is a stimulation device that allows for all of the above forms of simulation in clinical practice (as well as pink noise and brown noise). This topic will be covered in greater depth in chapter 9. In this image, you see many forms of alternating current (continuous sine wave, triangle wave, square wave, random noise, and pulsed waves. DC is direct current and, thus, does not have a frequency.&nbsp;</p><p></p>