Editing Evoked potential (section)

==Steady-state evoked potential==
An evoked potential is the electrical response of the brain to a sensory stimulus. Regan constructed an analogue Fourier series analyzer to record harmonics of the evoked potential of flickering (sinusoidally modulated) light. Rather than integrating the sine and cosine products, Regan fed the signals to a two-pen recorder via lowpass filters.<ref name="Neurophysiology"/>  This allowed him to demonstrate that the brain attained a steady-state regime in which the amplitude and phase of the harmonics (frequency components) of the response were approximately constant over time. By analogy with the steady-state response of a resonant circuit that follows the initial transient response he defined an idealized steady-state evoked potential (SSEP) as a form of response to repetitive sensory stimulation in which the constituent frequency components of the response remain constant with time in both amplitude and phase.<ref name="Neurophysiology">{{cite journal | author = Regan D | year = 1966 | title = Some characteristics of average steady–state and transient responses evoked by modulated light | journal = Electroencephalography and Clinical Neurophysiology | volume = 20 | issue = 3| pages = 238–48 | doi = 10.1016/0013-4694(66)90088-5 | pmid = 4160391 }}</ref><ref name="Electrical">{{cite journal | author = Regan D | year = 1979 | title = Electrical responses evoked from the human brain | journal = Scientific American | volume = 241 | issue = 6| pages = 134–46 | doi = 10.1038/scientificamerican1279-134 | pmid = 504980 | bibcode = 1979SciAm.241f.134R }}</ref>  Although this definition implies a series of identical temporal waveforms, it is more helpful to define the SSEP in terms of the frequency components that are an alternative description of the time-domain waveform, because different frequency components can have quite different properties.<ref name="Electrical"/><ref name="electrophysiology">Regan, D. (1989). Human brain electrophysiology: Evoked potentials and evoked magnetic fields in science and medicine. New York: Elsevier, 672 pp.</ref>  For example, the properties of the high-frequency flicker SSEP (whose peak amplitude is near 40–50&nbsp;Hz) correspond to the properties of the subsequently discovered magnocellular neurons in the retina of the macaque monkey, while the properties of the medium-frequency flicker SSEP ( whose amplitude peak is near 15–20&nbsp;Hz) correspond to the properties of parvocellular neurons.<ref>{{cite journal |author1=Regan D. |author2=Lee B.B. | year = 1993 | title = A comparison of the human 40 Hz response with the properties of macaque ganglion cells | journal = Visual Neuroscience | volume = 10 | issue = 3| pages = 439–445 | doi = 10.1017/S0952523800004661 | pmid = 8494797 |s2cid=3132361 }}</ref> Since a SSEP can be completely described in terms of the amplitude and phase of each frequency component it can be quantified more unequivocally than an averaged transient evoked potential.

It is sometimes said that SSEPs are elicited only by stimuli of high repetition frequency, but this is not generally correct. In principle, a sinusoidally modulated stimulus can elicit a SSEP even when its repetition frequency is low. Because of the high-frequency [[rolloff]] of the SSEP, high frequency stimulation can produce a near-sinusoidal SSEP waveform, but this is not germane to the definition of a SSEP.
By using zoom-FFT to record SSEPs at the theoretical limit of spectral resolution ΔF (where  ΔF in Hz is the reciprocal of the recording duration in seconds) Regan and Regan discovered that the amplitude and phase variability of the SSEP can be sufficiently small that the bandwidth of the SSEP's constituent frequency components can be at the theoretical limit of spectral resolution up to at least a 500-second recording duration (0.002&nbsp;Hz in this case).<ref>{{cite journal |author1=Regan M.P. |author2=Regan D. | year = 1988 | title = A frequency domain technique for characterizing nonlinearities in biological systems | journal = Journal of Theoretical Biology | volume = 133 | issue = 3| pages = 293–317 | doi = 10.1016/S0022-5193(88)80323-0 |bibcode=1988JThBi.133..293R }}</ref>
Repetitive sensory stimulation elicits a steady-state magnetic brain response that can be analysed in the same way as the SSEP.<ref name="electrophysiology"/>

===The "simultaneous stimulation" technique===
This technique allows several (e.g., four) SSEPs to be recorded simultaneously from any given location on the scalp.<ref name="Psychiatry">{{cite journal |author1=Regan D. |author2=Heron J.R. | year = 1969 | title = Clinical investigation of lesions of the visual pathway: a new objective technique | journal = Journal of Neurology, Neurosurgery, and Psychiatry | volume = 32 | issue = 5| pages = 479–83 | doi = 10.1136/jnnp.32.5.479 |pmid=5360055 | pmc = 496563 }}</ref> Different sites of stimulation or different stimuli can be tagged with slightly different frequencies that are virtually identical to the brain, but easily separated by Fourier series analyzers.<ref name="Psychiatry"/> For example, when two unpatterned lights are modulated at slightly different frequencies (F1 and F2) and superimposed, multiple nonlinear cross-modulation components of frequency (mF1 ±  nF2) are created in the SSEP, where m and n are integers.<ref name="electrophysiology"/> These components allow nonlinear processing in the brain to be investigated. By frequency-tagging two superimposed gratings, spatial frequency and orientation tuning properties of the brain mechanisms that process spatial form can be isolated and studied.<ref>{{cite journal |author1=Regan D. |author2=Regan M.P. | year = 1988 | title = Objective evidence for phase–independent spatial frequency analysis in the human visual pathway | doi = 10.1016/S0042-6989(88)80018-X| journal = Vision Research | volume = 28 | issue = 1| pages = 187–191 | pmid = 3413995 |s2cid=21369518 }}</ref><ref>{{cite journal |author1=Regan D. |author2=Regan M.P. | year = 1987 | title = Nonlinearity in human visual responses to two–dimensional patterns and a limitation of Fourier methods | journal = Vision Research | volume = 27 | issue = 12| pages = 2181–3 | doi = 10.1016/0042-6989(87)90132-5 | pmid = 3447366 |s2cid=3175111 }}</ref> Stimuli of different sensory modalities can also be tagged. For example, a visual stimulus was flickered at Fv Hz and a simultaneously presented auditory tone was amplitude modulated at Fa Hz. The existence of a (2Fv + 2Fa) component in the evoked magnetic brain response demonstrated an audio-visual convergence area in the human brain, and the distribution of this response over the head allowed this brain area to be localized.<ref>{{cite journal |author1=Regan M.P. |author2=He P. |author3=Regan D. | year = 1995 | title = An audio–visual convergence area in human brain | journal = Experimental Brain Research | volume = 106 | issue =3 | pages = 485–7 |pmid=8983992 | doi=10.1007/bf00231071|s2cid=27044876 }}</ref> More recently, frequency tagging has been extended from studies of sensory processing to studies of selective attention<ref name="Selective">{{cite journal |author1=Morgan S. T. |author2=Hansen J. C. |author3=Hillyard S. A. | year = 1996 | title = Selective attention to stimulus location modulates the steady-state evoked potential | journal = Proceedings of the National Academy of Sciences USA | volume = 93 | issue = 10| pages = 4770–4774 | doi = 10.1073/pnas.93.10.4770 |pmid=8643478 | pmc= 39354 |doi-access=free }}</ref> and of consciousness.<ref name="Srinivasan">{{cite journal |vauthors=Srinivasan R, Russell DP, Edelman GM, Tononi G | year = 1999 | title = Increased synchronization of neuromagnetic responses during conscious perception | journal = Journal of Neuroscience | volume = 19 | issue = 13| pages = 5435–48 |pmid=10377353| doi = 10.1523/JNEUROSCI.19-13-05435.1999 | pmc = 6782339 | doi-access = free }}</ref>

===The "sweep" technique===
The sweep technique is a hybrid frequency domain/time domain technique.<ref name="Rapid">{{cite journal | author = Regan D | year = 1973 | title = Rapid objective refraction using evoked brain potentials | journal = Investigative Ophthalmology | volume = 12 | issue = 9| pages = 669–79 | pmid = 4742063 }}</ref> A plot of, for example, response amplitude versus the check size of a stimulus checkerboard pattern plot can be obtained in 10 seconds, far faster than when time-domain averaging is used to record an evoked potential for each of several check sizes.<ref name="Rapid"/>
In the original demonstration of the technique the sine and cosine products were fed through lowpass filters (as when recording a SSEP )  while viewing a pattern of fine checks whose black and white squares exchanged place six times per second. Then the size of the squares was progressively increased so as to give a plot of evoked potential amplitude versus check size (hence "sweep"). Subsequent authors have implemented the sweep technique by using computer software to increment the spatial frequency of a grating in a series of small steps and to compute a time-domain average for each discrete spatial frequency.<ref name="Infant">{{cite journal |author1=Norcia A. M. |author2=Tyler C. W. | year = 1985 | title = Infant VEP acuity measurements: Analysis of individual differences and measurement error | journal = Electroencephalography and Clinical Neurophysiology | volume = 61 | issue = 5| pages = 359–369 | doi = 10.1016/0013-4694(85)91026-0 | pmid = 2412787 }}</ref><ref>{{cite journal |author1= Strasburger, H. |author2=Rentschler, I. | year = 1986 | title = A digital fast sweep technique for studying steady-state visual evoked potentials| url = http://webdoc.sub.gwdg.de/pub/med/2007/strasburger-86-digital.pdf | journal = Journal of Electrophysiological Techniques| volume = 13 | issue = 5| pages = 265–278}}</ref>
A single sweep may be adequate or it may be necessary to average the graphs  obtained in several sweeps with the averager triggered by the sweep cycle.<ref name="pattern">{{cite journal | author = Regan D | year = 1975 | title = Colour coding of pattern responses in man investigated by evoked potential feedback and direct plot techniques | journal = Vision Research | volume = 15 | issue = 2| pages = 175–183 | doi = 10.1016/0042-6989(75)90205-9 | pmid = 1129975 | s2cid = 42218073 }}</ref> Averaging 16 sweeps can improve the signal-to-noise ratio of the graph by a factor of four.<ref name="pattern"/>
The sweep technique has proved useful in measuring rapidly adapting visual processes<ref>{{cite journal |author1=Nelson J. I. |author2=Seiple W. H. |author3=Kupersmith M. J. |author4=Carr R. E. | year = 1984 | title = A rapid evoked potential index of cortical adaptation | journal = Investigative Ophthalmology & Visual Science | volume = 59 | issue = 6| pages = 454–464 |pmid=6209112 | doi=10.1016/0168-5597(84)90004-2}}</ref> and also for recording from babies, where recording duration is necessarily short. Norcia and Tyler have used the technique to document the development of visual acuity<ref name="Infant"/><ref name="Spatial">{{cite journal |author1=Norcia A. M. |author2=Tyler C. W. | year = 1985 | title = Spatial frequency sweep VEP: Visual acuity during the first year of life | journal = Vision Research | volume = 25 | issue = 10| pages = 1399–1408 | doi = 10.1016/0042-6989(85)90217-2 | pmid = 4090273 |s2cid=23557430 }}</ref> and contrast sensitivity<ref name="Electrophysiological">{{cite journal |author1=Norcia A. M. |author2=Tyler C. W. |author3=Allen D. | year = 1986 | title = Electrophysiological assessment of contrast sensitivity in human infants | journal = American Journal of Optometry and Physiological Optics | volume = 63 | issue = 1| pages = 12–15 | pmid = 3942183 | doi=10.1097/00006324-198601000-00003|s2cid=19809242 }}</ref> through the first years of life. They have emphasized that, in diagnosing abnormal visual development, the more precise the developmental norms, the more sharply can the abnormal be distinguished from the normal, and to that end have documented normal visual development in a large group of infants.<ref name="Infant"/><ref name="Spatial"/><ref name="Electrophysiological"/> For many years the sweep technique has been used in paediatric ophthalmology ([[electrodiagnosis]]) clinics worldwide.

===Evoked potential feedback===
This technique allows the SSEP to directly control the stimulus that elicits the SSEP without the conscious intervention of the experimental subject.<ref name="Neurophysiology"/><ref name="pattern"/> For example, the running average of the SSEP can be arranged to increase the luminance of a checkerboard stimulus if the amplitude of the SSEP falls below some predetermined value, and to decrease luminance if it rises above this value. The amplitude of the SSEP then hovers about this predetermined value. Now the wavelength (colour) of the stimulus is progressively changed. The resulting plot of stimulus luminance versus wavelength is a plot of the spectral sensitivity of the visual system.<ref name="Electrical"/><ref name="pattern"/>