(seeg). the signal recorded from ecog is on a different scale of activity than the brain activity recorded from scalp eeg. low voltage, high frequency components that cannot be seen easily (or at all) in scalp eeg can be seen clearly in ecog. further, smaller electrodes (which cover a smaller parcel of brain surface) allow for better spatial resolution to narrow down the areas critical for seizure onset and propagation. some clinical sites record from penetrating microelectrodes. eeg is not indicated for diagnosing headache. recurring headache is a common pain problem, and this procedure is sometimes used in a search for a diagnosis, but it has no advantage over routine clinical evaluation. home ambulatory eeg sometimes it is more convenient or clinically necessary to perform ambulatory eeg recordings in the home of the patient. these studies typically have a duration of 24–72 hours. research use eeg, and the related study of erps are used extensively in neuroscience, cognitive science, cognitive psychology, neurolinguistics and psychophysiological research, but also to study human functions such as swallowing. any eeg techniques used in research are not standardised sufficiently for clinical use, and many erp studies fail to report all of the necessary processing steps for data collection and reduction, limiting the reproducibility and replicability of many studies. but research on mental disabilities, such as auditory processing disorder (apd), add, or adhd, is becoming more widely known and eegs are used as research and treatment. advantages several other methods to study brain function exist, including functional magnetic resonance imaging (fmri), positron emission tomography (pet), magnetoencephalography (meg), nuclear magnetic resonance spectroscopy (nmr or mrs), electrocorticography (ecog), single-photon emission computed tomography (spect), near-infrared spectroscopy (nirs), and event-related optical signal (eros). despite the relatively poor spatial sensitivity of eeg, the "one-dimensional signals from localised peripheral regions on the head make it attractive for its simplistic fidelity and has allowed high clinical and basic research throughput". thus, eeg possesses some advantages over some of those other techniques: hardware costs are significantly lower than those of most other techniques eeg prevents limited availability of technologists to provide immediate care in high traffic hospitals. eeg only requires a quiet room and briefcase-size equipment, whereas fmri, spect, pet, mrs, or meg require bulky and immobile equipment. for example, meg requires equipment consisting of liquid helium-cooled detectors that can be used only in magnetically shielded rooms, altogether costing upwards of several million dollars; and fmri requires the use of a 1-ton magnet in, again, a shielded room. eeg can readily have a high temporal resolution, (although sub-millisecond resolution generates less meaningful data), because the two to 32 data streams generated by that number of electrodes is easily stored and processed, whereas 3d spatial technologies provide thousands or millions times as many input data streams, and are thus limited by hardware and software. eeg is commonly recorded at sampling rates between 250 and 2000 hz in clinical and research settings. eeg is relatively tolerant of subject movement, unlike most other neuroimaging techniques. there even exist methods for minimizing, and even eliminating movement artifacts in eeg data eeg is silent, which allows for better study of the responses to auditory stimuli. eeg does not aggravate claustrophobia, unlike fmri, pet, mrs, spect, and sometimes meg eeg does not involve exposure to high-intensity (>1 tesla) magnetic fields, as in some of the other techniques, especially mri and mrs. these can cause a variety of undesirable issues with the data, and also prohibit use of these techniques with participants that have metal implants in their body, such as metal-containing pacemakers eeg does not involve exposure to radioligands, unlike positron emission tomography. erp studies can be conducted with relatively simple paradigms, compared with ie block-design fmri studies relatively non-invasive, in contrast to electrocorticography, which requires electrodes to be placed on the actual surface of the brain. eeg also has some characteristics that compare favorably with behavioral testing: eeg can detect covert processing (i.e., processing that does not require a response) eeg can be used in subjects who are incapable of making a motor response some erp components can be detected even when the subject is not attending to the stimuli unlike other means of studying reaction time, erps can elucidate stages of processing (rather than just the result) the simplicity of eeg readily provides for tracking of brain changes during different phases of life. eeg sleep analysis can indicate significant aspects of the timing of brain development, including evaluating adolescent brain maturation. in eeg there is a better understanding of what signal is measured as compared to other research techniques, e.g. the bold response in mri. disadvantages low spatial resolution on the scalp. fmri, for example, can directly display areas of the brain that are active, while eeg requires intense interpretation just to hypothesize what areas are activated by a particular response. depending on the orientation and location of the dipole causing an eeg change, there may be a false localization due to the inverse problem. eeg poorly measures neural activity that occurs below the upper layers of the brain (the cortex). unlike pet and mrs, cannot identify specific locations in the brain at which various neurotransmitters, drugs, etc. can be found. often takes a long time to connect a subject to eeg, as it requires precise placement of dozens of electrodes around the head and the use of various gels, saline solutions, and/or pastes to maintain good conductivity, and a cap is used to keep them in place. while the length of time differs dependent on the specific eeg device used, as a general rule it takes considerably less time to prepare a subject for meg, fmri, mrs, and spect. signal-to-noise ratio is poor, so sophisticated data analysis and relatively large numbers of subjects are needed to extract useful information from eeg. with other neuroimaging techniques simultaneous eeg recordings and fmri scans have been obtained successfully, though recording both at the same time effectively requires that several technical difficulties be overcome, such as the presence of ballistocardiographic artifact, mri pulse artifact and the induction of electrical currents in eeg wires that move within the strong magnetic fields of the mri. while challenging, these have been successfully overcome in a number of studies. mri's produce detailed images created by generating strong magnetic fields that may induce potentially harmful displacement force and torque. these fields produce potentially harmful radio frequency heating and create image artifacts rendering images useless. due to these potential risks, only certain medical devices can be used in an mr environment. similarly, simultaneous recordings with meg and eeg have also been conducted, which has several advantages over using either technique alone: eeg requires accurate information about certain aspects of the skull that can only be estimated, such as skull radius, and conductivities of various skull locations. meg does not have this issue, and a simultaneous analysis allows this to be corrected for. meg and eeg both detect activity below the surface of the cortex very poorly, and like eeg, the level of error increases with the depth below the surface of the cortex one attempts to examine. however, the errors are very different between the techniques, and combining them thus allows for correction of some of this noise. meg has access to virtually no sources of brain activity below a few centimetres under the cortex. eeg, on the other hand, can receive signals from greater depth, albeit with a high degree of noise. combining the two makes it easier to determine what in the eeg signal comes from the surface (since meg is very accurate in examining signals from the surface of the brain), and what comes from deeper in the brain, thus allowing for analysis of deeper brain signals than either eeg or meg on its own. recently, a combined eeg/meg (emeg) approach has been investigated for the purpose of source reconstruction in epilepsy diagnosis. eeg has also been combined with positron emission tomography. this provides the advantage of allowing researchers to see what eeg signals are associated with different drug actions in the brain. recent studies using machine learning techniques such as neural networks with statistical temporal features extracted from frontal lobe eeg brainwave data has shown high levels of success in classifying mental states (relaxed, neutral, concentrating), mental emotional states (negative, neutral, positive) and thalamocortical dysrhythmia. mechanisms the brain's electrical charge is maintained by billions of neurons. neurons are electrically charged (or "polarized") by membrane transport proteins that pump ions across their membranes. neurons are constantly exchanging ions with the extracellular milieu, for example to maintain resting potential and to propagate action potentials. ions of similar charge repel each other, and when many ions are pushed out of many neurons at the same time, they can push their neighbours, who push their neighbours, and so on, in a wave. this process is known as volume conduction. when the wave of ions reaches the electrodes on the scalp, they can push or pull electrons on the metal in the electrodes. since metal conducts the push and pull of electrons easily, the difference in push or pull voltages between any two electrodes can be measured by a voltmeter. recording these voltages over time gives us the eeg. the electric potential generated by an individual neuron is far too small to be picked up by eeg or meg. eeg activity therefore always reflects the summation of the synchronous activity of thousands or millions of neurons that have similar spatial orientation. if the cells do not have similar spatial orientation, their ions do not line up and create waves to be detected. pyramidal neurons of the cortex are thought to produce the most eeg signal because they are well-aligned and fire together. because voltage field gradients fall off with the square of distance, activity from deep sources is more difficult to detect than currents near the skull. scalp eeg activity shows oscillations at a variety of frequencies. several of these oscillations have characteristic frequency ranges, spatial distributions and are associated with different states of brain functioning (e.g., waking and the various sleep stages). these oscillations represent synchronized activity over a network of neurons. the neuronal networks underlying some of these oscillations are understood (e.g., the thalamocortical resonance underlying sleep spindles), while many others are not (e.g., the system that generates the posterior basic rhythm). research that measures both eeg and neuron spiking finds the relationship between the two is complex, with a combination of eeg power in the gamma band and phase in the delta band relating most strongly to neuron spike activity. method computer electroencephalograph neurovisor-bmm 40 produced and offered in russia in conventional scalp eeg, the recording is obtained by placing electrodes on the scalp with a conductive gel or paste, usually after preparing the scalp area by light abrasion to reduce impedance due to dead skin cells. many systems typically use electrodes, each of which is attached to an individual wire. some systems use caps or nets into which electrodes are embedded; this is particularly common when high-density arrays of electrodes are needed. electrode locations and names are specified by the international 10–20 system for most clinical and research applications (except when high-density arrays are used). this system ensures that the naming of electrodes is consistent across laboratories. in most clinical applications, 19 recording electrodes (plus ground and system reference) are used. a smaller number of electrodes are typically used when recording eeg from neonates. additional electrodes can be added to the standard set-up when a clinical or research application demands increased spatial resolution for a particular area of the brain. high-density arrays (typically via cap or net) can contain up to 256 electrodes more-or-less evenly spaced around the scalp. each electrode is connected to one input of a differential amplifier (one amplifier per pair of electrodes); a common system reference electrode is connected to the other input of each differential amplifier. these amplifiers amplify the voltage between the active electrode and the reference (typically 1,000–100,000 times, or 60–100 db of voltage gain). in analog eeg, the signal is then filtered (next paragraph), and the eeg signal is output as the deflection of pens as paper passes underneath. most eeg systems these days, however, are digital, and the amplified signal is digitized via an analog-to-digital converter, after being passed through an anti-aliasing filter.