Steady-state evoked potentialsDavid Regan*

Department of Communication, University of Keele, Staffordshire ST5 5BG, England(Received 21 October 1975)

The advantages of steady-state EP recording include (1) speed in assessing sensory function in normal andsick infants (e.g., in amblyopia) and in sick adults (e.g., in multiple sclerosis); (2) monitoring certainactivities of sensory pathways that do not intrude into conscious perception; (3) rapidly assessing sensoryfunction when a large number of subjects must be tested (e.g., in refraction); (4) objective measurement atvery high suprathreshold levels where psychophysical methods are difficult or ineffective; (5) rapidly assessingsensory function in normal subjects when EP variability and nonstationarity preculde lengthy experiments;and (6) providing a speedy objective equivalent to behavioral test in animals.

INTRODUCTION

When are evoked potentials useful?

If there is a simpler way of doing your experiment, it isfoolishness to use evoked potential recording. Nonethe-less, there are a few situations where evoked potentialscan be useful. Before rushing into using EPs, oneshould be quite sure that the situation calls for it.

Two applications where EP recording can be usefulwill not be mentioned further. The first, by Campbelland by Maffei,"I2 has been called "building a bridge fromhumans to animals. " They have shown that, for con-trast perception, evoked potential amplitude can be usedto estimate psychophysical threshold in humans and (be-haviorally) in cats as well. Again, in goldfish steady-state EP recording provides an objective equivalent ofheterochromatic flicker photometry in measuring theeye's spectral sensitivity. The chief attraction of EPrecording here is that a brief experiment might be sub-stituted for lengthy behavioral measurements. Note,however, that although EP amplitude does correlate withpattern perception (over a limited range of contrasts),and can indicate spectral sensitivity, EP amplitude doesnot correlate with many other features of perception(e.g., flicker threshold).

Another possibility is that some phenomena might bevery obvious when shown by EP recording and not so ob-vious otherwise. An example, perhaps, is the indicationgiven by evoked potentials to stereoscopic movements indepth that there seems to be a neural mechanism con-cerned with movements toward the head and not away,while another neural mechanism is sensitive to move-ments away from the head and not towards. 4 This con-clusion was supported with psychophysics, but only afterthe psychophysical experiment was suggested by the EPwork. 5 Subsequently, single neurons were found in thecortex of monkey6 and cat 7 that have this property too.So by recording EPs it is possible to stumble acrosssome thing you might not otherwise have seen.

Transient and steady-state evoked potentials

Transient and steady-state responses give exactly thesame information about a linear system whether the sys-tem is electrical, mechanical, or physiological. So thetwo types of response are exactly equivalent, and which-ever is used can depend merely on which is more conve-nient. On the other hand, transient and steady-state re-sponses can show quite different things about a nonlinear

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system, and the visual pathway is certainly nonlinear.(For example, in a nonlinear system, steady-state re-sponses are not necessarily equivalent to transient re-sponses that are superposed due to their being generatedin quick succession: superposition may well not be lin-ear. ) It therefore makes sense to record both transientand steady-state EPs whenever possible.

The appropriate ways to record steady-state and tran-sient EPs are quite different. For many stimuli eithertransient or steady-state EPs can be recorded. AsKinney has described, transient EPs are produced bygiving the system a "kick" and then measuring voltageversus time with an averager. The kick can be a flash,an abrupt presentation of a pattern, a sudden movementin depth, etc. With steady-state responses you muststimulate repetitively. You "shake the system gently"and wait some time until the transient response to theonset of shaking has died away. At that time there maybe a steady-state response. On the other hand, theremay not be, i.e., after waiting there may be no responseleft.

If there is a steady-state response, it will be a repeti-tive wave, each cycle like any other, so that the runningaverages of both amplitude and phase are constant withtime. An appropriate recording device is a Fourieranalyzer. 8 Even in 1976 it often makes at least financialsense to construct an analog Fourier analyzer ratherthan use a general-purpose computer. One advantage ofFourier-analyzed steady-state EPs is that they can bevery speedily recorded-up to a hundred times quickerin recording time than using an averager. In a clinicalsituation with a very sick adult or with a healthy infantif you cannot make a measurement quickly, you cannotmake it at all. However, the speed given by a Fourieranalyzer is not free. You have to "buy" it, and you buyit at the expense of information. More information-less speed. You can trade off one against the other. Ifyou know exactly what you want to measure and you de-signyour apparatus so that it sees only what you wish tomeasure, then your method can be as fast as possible.Of course, you are then totally blind to anything youhave not foreseen. (Also note that steady-state EPssometimes are very much smaller in amplitude thantransient EPs, in which case the promised speed advan-tage will not materialize. One example of this disap-pointing situation is auditory EPs.)

Another advantage of steady- state over transient E Ps isthat steady-state EPs do not seem to be much affected by

Copyright 1977 by the Optical Society of America 1475

psychological variables such as attention. Consequently,they are less variable. On the other hand, If you wishto study, e.g., attention or the orienting response, thensteady-state EPs are most likely completely useless, sothat you must use transient EPs. In other words, thereis no general-purpose EP method.

SOME METHODS OF RECORDING STEADY-STATEEVOKED POTENTIALS

The following are some particularly speedy methodsthat are based on Fourier analysis: (i) the running aver-age presention, (ii) averaging graphs, (iii) simulta-neous stimulation method, and (iv) evoked potential feed-back, where the evoked potential controls the stimulusand the brain is in a feedback loop.

Fourier analysis with running average display of data

A Fourier analyzer of the type described amounts toa very narrow-band filter that gives both phase informa-tion (this can be regarded as latency or delay) and am-plitude. The output is restricted to two numbers. Fig-ure 1(a) is a graph of the amplitude of the spontaneousEEG plotted versus frequency. Most activity is at fre-quencies near the alpha peak. The vertical line repre-sents the band of frequencies accepted by the Fourieranalyzer. Figure 1(b) shows the selectivity of the anal-yzer more clearly. This was measured empirically witha recording period of I min. The bandwidth at half-sensitivity was i cycle/s and sensitivity is 100 dBdown in 1A of a cycle. This analyzer was not a compli-cated piece of apparatus. It was made in 1961 verycheaply using multipliers made from phonograph pick-up cartridges. Nowadays, of course, you can make ananalog Fourier analyzer much more conveniently andalmost as cheaply.

The way the device works is as follows. It passesvirtually all the signal, but very little noise. When youstimulate repetitively at f Hz it turns out that all theresponse is located only at certain frequencies, viz.f Hz, 2f, 3f, 4f, etc. The Fourier analyzer's frequen-cy is automatically set to precisely f or 2f or 3f etc.,as you choose. Figure 1(a) shows the analyzer set onthe f Hz stimulus frequency. All the response was inthat narrow band and in the one or two other narrowbands at 21, etc. The bandwidth of these EP frequencycomponents can be less than iZ Hz (Ref. 9, Fig. 5. 12).Suppose that the EP contains a fundamental and a secondharmonic component so that if you stimulate at 9 Hz theresponse will be at exactly 9 and 18 Hz. There will beno signal anywhere else, so it is necessary only to lookat those two points. Noise is spread over a broad bandof frequencies, but not much noise is going to getthrough the narrow filter bandwidth. Therefore, theanalyzer clearly extracts the signal from the noise.That means you can choose to have either very goodselectivity and record small EPs, or you can choose tohave high recording speed with less sensitivity. Or youcan have whatever mixture of the two that you wish.

First of all it is necessary to demonstrate that therepetitive stimulation does produce a steady-stateevoked potential. At the start of stimulation, it is not

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