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Evoked potential and psychophysical correlates of changes in stimulus colour and intensity
VMWI
Rut.
Vol.
IO.
pp.
163-118.
Pcrgamon Press 1970. Printed in Great Britain.
EVOKED POTENTIAL AND PSYCHOPHYSICAL CORRELATES OF CHANGES IN STIMULUS COLOUR AND INTENSITY D. REGAN Department of Communication. University of Keele. Keele. Staffordshire
(Received
29. April
1969: in revi.sed fbrm 21 Julx 1969)
INTRODUCTION
Transient and steady-state evoked potentials
Transient evoked potentials (EP’s) to slowly repeated stimuli are commonly analysed into components of different latencies. As the stimulus repetition rate is increased, the responses to successive stimuli superpose to an increasing extent until eventually a ‘dynamic steady-state’ regime may occur in which no individual response can be related to a particular stimulus cycle. The waveforms of such steady-state responses may be analysed into harmonic components. Classfjication @‘steady-state evoked potentials into di&rent jbequency regions
There is growing evidence that steady-state evoked potentials (EP’s) can be classified into at least three different frequency regions, centred near 10 c/s (low frequency), 16 c/s (medium frequency) and 50 c/s (high frequency) respectively (Fig. 1) and that the EP’s IO -
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FIG. I. Three classes of steady-state evoked potentials (EP’s), as shown by plots of amplitude versus stimulus modulation frequency. Dotted lines: IO Q’S (low frequency) class of EPsfundamental and second harmonic components. Chain line: I6 c/s (medium frequency) class of EP’s-fundamental component. -Full lines-53 c/s (high frequency) class of EP’s-fundamental and second harmonic components. Note: The relative amplitudes of these classes of EP vary considerably with subject. electrode position. stimulus intensity, field size and colour. Components other than the fundamental and second harmonic are omitted for clarity.
in these three regions have different relationships with such stimulus parameters as intensity and colour. In many subjects there is a particular stimulus frequency near IO c/s for which the amplitude of the EP is larger than at neighbouring frequencies. It is at this same frequency that spontaneous a activity is most prominent. Although yellow stimuli have been 163
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reported to evoke smaller responses than either red or blue stimuh for modulation frequencies close to 10 cis (REGAN.I966b). colour does not in genera1 exert a strong influence on the 10 c/s responses. The stimulus intensity or field size seem also to have little effect on the shape and peaking frequency of the amplitude verse frequency curve for the EP near 10 c/s. The ‘high frequency’ EP’s, whose maximum response amplitudes occur at roughly SO-55 c/s form the second class (VAN DER TWEEL and LUNEL, 1965; SPEKREIJSE,1966: REGAN, 1968b). For the high frequency EP’s, as for the 10 cis responses, the amplitude IWXUSfrequency curve seems to be similar in shape to the corresponding power spectrum of spontaneous EEG activity (SPEKREIJSE,1966). High frequency EP’s may be differentfy distributed over the scalp and ,be less dependent on stimulus intensity than EP’s of the medium frequency ctass (REGAN, 1968a). The situation is quite different for the third class of steady-state EP components, whose amplitudes are maximal for medium stimulus frequencies in the region of 16 c/s. The shape of the amplitude wsus frequency curve for EP’s in this frequency region can be strongly influenced by chromatic adaptation and by field size (REGAN, 1968a). Further contrasts to the ‘low’ and ‘high frequency’ responses are that the medium frequency class of EP’s may have little relation to the power spectrum of spontaneous EEG activity and do not give strong second harmonic responses. The relative amplitudes of the above classes of EP vary considerably with subject, electrode position, stimulus intensity, field size and cofour.
This paper presents experimental evidence on the question whether EP amplitude correlates with the point of minimum subjective flicker in a heterochromatic photometry situation. This work forms part of an investigation to find whether EP’s in the three different frequency regions described above reflect different information processing systems, and in particular whether steady-state EP’s can provide objective evidence of the different ways in which the brain handles intensity and colour information. SIEGFRIEDet al. (1965) reported that the amplitude of the evoked potential fell to a minimum near the point of minimum subjective flicker when they carried out one version of the standard technique of heterochromatic flicker photometry. In the present investigation, the first two subjects used in the main experiment (Experiment 2) gave results which were in conflict with this finding. On the other hand, the stimulus parameters in the two investigations were quite different, so that it was clearly necessary to carry out a subsidiary investigation (Experiment 1). in’ which the methods used by Siegfried et al. were imitated as closely as possible.. APPARATUS Visual Stimulator Light from a 500 W Xenon arc lamp was split into two beams, and after passing through electromechanical light modulators MI and M2. the two beams were recombined at a beamsplitter B2 (Fig. 3). Monochromatic beams were obtained by interference filters whose half-widths were roughly 10 nm. Side bands were negligible. Neutral density filters were used to change stimulus intensity. The electromechanical light modulators Ml and M2 were developed from a device described by van der Mark (SCHIPPERHEYN.1965). The principle is illustrated in Fig. 4. This device can modulate intensity up to a depth of IO0 per centt. The frequency response is from d.c. to 70 cs ( -3 db).
1The m~ulation depth of a sin~oidally modulated ljght is defined as half the peak-to-peak intensity expressed as a percentage of the mean intensity.
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FIG. 2; Peak-to-peak amplitude of the averaged evoked potential serrus the luminance of one of two alternating beams. The zero of luminance is at the point of minimum subjective flicker. (A): Two red stimuli abruptly alternated at I6 c/s. Minimum amplitude of EP is close to the point of minimum subjective tlicker. (B) and (C): Red stimulus abruptly alternated with a white stimulus at 16 c/s for two subjects. There is no significant minimum in the EP amplitude except for dotted line (J.S.) and even then the minimum in EP amplitude is significantly displaced from the point of minimum subjective flicker (at’O.0 relative luminance). Red stimulus. 640 nm. half width roughly IO nm. 3.6 field with 30 white surround. 2880 counts. Electrode (I). I cm above inion. electrode (2) 5 cm in front of electrode (I). Monopolar reference behind right ear: electrode behind left ear grounded. Full line (circles) monopolar records from electrode (I) and (2). Fine dotted line (squares) monopolar records from electrode (2). Coarse dotted line (triangles) bipolar records between electrodes (I) and (2).
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The beam from the arc lamp was brought to a focus F (Fig. 4) at a lightweight vane driven by a Pye-Ling vibration generator. Movements of the vane controlled the light flux passing to the subject’s eye. Critical positioning of lens L2 ensured that the image formed in the first focal plane of the subject’s eye lens was free of any perceptible irregularity caused by the edge of the vane and that the whole field changed brightness uniformly during modulation (tested at a low frequency of about 0.1 c,s). This adjustment was made easier by placing a diffuser over the hole H. The system was linearized by negative feedback from a photocell which received a fraction of the modulated beam. The two light modulators were driven by separate alternators. One alternator was linked to the other through a differential gear (Fig. 3) which enabled the phase relation between the two light modulators to be continuously varied through 360 When the subject was required to control the relative phases of the beams. it was more convenient to drive the light modulators by a two-phase oscillator. The relative modulation depths
Evoked Potentiaf and Psychophysical Correlates of Changes in Stimulus Cotour and inrcnsity
167
of the two beams were controlled by the double potentiometer P (Fig. 3). which could be operand by the subject. Rotation of the potentiometer finearly increased the modulation depth of one beam. whilst linearly reducing the modulation depth of the other beam. A calibration of the resultant modulation depth for the ttio superposed beams WFSWthe reading of the potentiometer is shown in Fig. 5A. Tbe’critical settings of both the relative phases and relative modulation depths of the beams which wsre required to obtain minimum subjettive flicker. could be made by the subject himself.
RELATIVE
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Fro. 5. The amphtudes and phas& of the fundamental component of the steady-state evoked potential are plottad in (A), (3). (C) and (I?). (A) Two beams of tlte same cotour (red. 640 nm) were rn~u~at~~n antiphase and their relative mod&tioa de@ varied:Dattcd iire is output of pkotac&. Circks are EP amplitudes. The arrow indicates &c mean point of MO subjective Aickct, and the horizontal bfack Line gives the range of four settings.
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Fro. SB, Similar experiment to (A) but now different cotours (yehow (589 nm) and red) were alternated in antiphase. Dotted line is p~ot~el~ output and full line is EP, Maximum yeliow and zero red modulation at 30 on abscissa, maximum red and zero yellow modulation at 330. There is no minimum in EP amplitude which coincides with the point of minimum subjective %&et (arrow).
EEG Analyds The EEG was analysed in two ways (Fig. 3): (a) an averaging computer (Nuclear Chicago) was triggered synchronously with the stimulus modulatioo, and displayed cveraged evoked potentials, (b) a special purpose on-line Fourier Analyser extracted the steady-state EP from the EEG, split up the EP into harmonic components, and then displsyed the amplitude and phases of these harmonic components in the form of running averages. The Fourier Analyser has been de&bed previousiy(REOI\N, I966a). Sine and cosine waveforms at frequencies which were harmonically related to the stimulus modulation frequency (Fig. 3) were generated by maehanically linked alternators. tiogue devices were used to multiply the EEG by the sine and cosine of the fundamental, and second barmouic and any other requind harmonic or subharmonic frequencies. Low-pass filtering fotfowed the rnu~~p~~~o~ stage, then running averages of the sine and cosine products were displayed on pan recorders. These dispiays gave the amplitudes and phases of the harmonic components of the steady-state EP’s.
Evoked Potential and Psychophysical Correlates of Changes in Stimulus Colour and Intensity
6b
Ii0 RELATIVE
2io PHASES
300 (dogrear)
I69
360
FIG. SC. Amplitudes and phases of the fundamental component of the EP versus the relative phases of the red and yellow beams. The relative modulation depths of the beams were held constant at the setting for minimum subjective flicker (Fig. 5B). Dotted line is output of photocell, full line and circles show f~dam~tal component of EP. Arrow and horizontal bar indicate mean and range of 14 settings on point of minimum subjective flicker. Relative phase for minimum EP is roughly 80” from the relative phase for minimum subjective flicker. Note: 180 on scale means beams in antiphase. The subject sat in a soundproof, screened chamber which minimised any distraction or contamination of the EP by auditory stimulation. He fixated the stimulus centrally with his right eye in Maxwellian view. The left eye was covered. Steady fixation was assisted by the use of a bite bar. The colour vision of all subjects used was found to be normal by the Ishihara test. EXPERIMENT
I
The subjects fixated centrally on a 3-6’ stimulus field in which a standard white light alternated abruptly at 16 c/s with a comparison red or green light. This central patch was surrounded by a white geld which subtended 31” and had the same luminance as the standard white component of the Rickering light. While the coloured and
D. REGAS
170
white lights alternated. the intensity of the coloured beam was varied until the subject reported that he saw minimum flicker. It is at this point that the luminan~ (subjective brightness) of the two beams are defined as equal. Averaged evoked potentials were recorded from four sites: (I) bipolar. between the inion and an electrode 5 cm in front along the midiine. (II) monopolar. between the inion and a reference electrode behind the left ear. (Ill) monopolar. between the electrode placed 5 cm ahead of the inion and the left ear reference. and (iv) bipolar between the inion and an electrode placed 5 cm to the right of the inion along a line perpendicular to the midline. An electrode placed behind the right ear was earthed.
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Frc. 5D. Experiment (55) repeated. but now with the relative phases of treams not in antiphase as in 55. but set to give minimum amplitude for the fundamental EP (see Fig. 5C). A minimum now occurs in EP ampfiiude. whereas in Fig. 59 there was no minimum. Arrow and horizontal bar @es mean and range of 4 settings on point of minimum subjective flicker: the point of minimum EP amplitude is displaced from the point of minimum subjective flicker. in addition. an abrupt phase change of roughly 180’ occurs near the point of minimum EP amplitude (contrast with Fig. 58). Dotted line gives photocell output for Fig. 5B: for easier comparison the photocell curve for the phase has been shifted along the abscissa by an amount corresponding to the displacement of the EP’s minimum amplitude from the point at which the photocell has a minimum output. Note: Modulation depth scale. i.e. ~
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(I) and (2) respectively. Stimulus modula:&fr!&ency
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= 16 cis. The maximum modulation depth ofeither beam is 30 per cent (at 30 or 330 on abscissa). IO’ field of 1800 td with 30” white surround of 540 td. Electrode (1) cm in front of inion. electrode (2) 7 cm in front of electrode (I), electrode (3) to right of midline and 7 cm from electrodes (I) and (2). Electrode behind right ear grounded. Noise level roughly 0.1-0.2 pV.
Evoked Potential and PsychophysicalCorrelates
of Changes in Stimulus
Colour
and Intensity
171
Results Peak-to-peak EP’s to red-white flicker were measured for eight subjects, but for three of these the signal to noise ratio was so unfavourable that no conclusions could be drawn. Green-white flicker was also used with three of the five subjects who gave clear EP’s. All eight subjects were able to make consistent settings of relative intensity at a point where subjective flicker was barely perceptible. Four subjects showed no significant minimum in the EP in the neighbourhood of this point for any electrode position (Fig. 2). On the other hand, a control experiment confirmed that when two beams of thesame colour were alternated, these four subjects all gave a clear minimum in the EP, and that this minimum coincided with the point of minimum subjective flicker (Fig. 2). When different colours were alternated, one subject only (F.M.S.) gave a minimum in the EP which coincided with the point of minimum subjective flicker, and this was found for only one of the electrode positions investigated (IV). For the five subjects who gave measurable EP’s, curves of the type shown in Figs. 2B and 2C were found to have different shapes for different electrode positions. For these five subjects the amplitude of the EP’s to predominantly white flicker was larger than for predominantly red flicker when measured at one electrode position (e.g..Fig. 2B. filled circles), but was smaller or equal at another electrode position (e.g. Fig. 2B, triangles). Discussion The methods used in the present experiment were identical with those used by Siegfried et nl., except that the square wave stimuli were modulated to a depth of 50 per cent instead of 100 per cent in order to reduce the possibility of swamping any minimum in the EP by saturation of the response (VAN DER TWEEL and LUNEL~ 1965; SPEKREIJSE,1966; RECAS. 1968~). In some experiments, 100 per cent modulation was also used for comparison. On the assumption that the subject’s pupil diameter was 2 mm in Siegfried et al.‘s experiments, their retinal illuminations were within O-2log units of that used in the present investigation. Siegfried et al.- described the EP’s recorded from.site (III) only, since these recordings were ‘the largest and most sensitive’. Responses were presented as plots of the peak-topeak amplitude of the averaged EP ver.rm the relative luminance of the two beams. Results from only two subjects were discussed. The present experiments offer only limited support to the finding of SIEGFRIEDet al. (1965) that in a heterochromatic flicker photometry situation a minimum in the amplitude of the EP can be found in close proximity to the point of minimum subjective flicker. In fact such a correlation was found for only one subject (out of eight), and then in only one electrode derivation (out .of four) which suggests that the correlation is uncommon in these experimental conditions. This lack of correlation between the point of minimum subjective flicker and the amplitude of the EP could be explained by the experimental findings described below that flicker perception and the medium frequency class of EP’s involve different phase shifts and different spectral sensitivities. A serious drawback to the popular usage of peak-to-peak EP amplitude in colour studies became evident during the present investigation. This is that peak-to-peak measurements take no account of waveshape, so that fundamental, and second harmonic and other components are lumped together. It can easily occur that a stimulus frequency is such that the fundamental and second harmonic components of the response fall into different classes of steady-state evoked potential (Fig. 1). For example, the fundamental component
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might fall into the medium frequency class of EP’s. whereas the second harmonic component. recorded at the same time from the same site, could fall into the high frequency class of EP’s. Since the behaviour of these two classes of steady-state EP turns out to be totally different in response to alternating colour stimuli, the peak-to-peak measure would therefore present a misleading picture (see below, and REGAN, 1970). EXPERIMENT II
introduction If any phase shifts which occur between the retina and the mechanism of flicker perception are independent of colour, then subjective flicker would be a minimum when the stimulus beams were modulated exactly 180’ out of phase. This is not the case however; departures from the 180” condition have been measured psychophysically (DE LANGE, 19.57; WAZRAVEK and LEEBECK;, 1964). Now if the phase difference (cp) between the stimulus beams which produced minimum subjective flicker were markedly different from the phase diffeience (Q) which produced a minimum EP, then the finding that in some cases there is no minimum at all in the EP curve would be explained (Fig. 2C). Whether cpis markedly different from 8 could be tested by delaying one stimulus behind the other so as to cancel this phase difference. However, the result of this manOeuvre may not be simple when square wave inputs are used as in Experiment I, since the relative phase shifts may be different for the various response components evoked by the harmonicallycomplex waveform. The use of sinusoidally modulated stimuli leads to more satisfactory attempts to balance out phase shifts; such experiments are reported below. The stimulus requirements outlined above were satisfied in a similar way to that adopted by DE LASGE(1957). who superposed a patch ofsinusoidally modulated light ofonecolour on a second patch ofsinusoidally modulated light of another colour. By adjustment of both the relative phases and modulation depths of the two beams, changes in luminance could be cancelled. so that subjectivefy only colour alternations remained. At this point there was zero subjective brightness Bicker. in this present experiment the subject could cont..ruously vary the relative phases and modulation depths of beams I and 2 (Fig. 3). A unique value of the relative modulation depths of the beams,gave minimum or zero flicker. At this point the modulation depth for either beam alone was IS per cent in most experiments: for maximum flicker ofeither beam the depth of modulation for one beam was 30 per cent and was zero for the other. These “small-signal” conditions ensured that the EP was mostly in the pre-~turation region (VAX D&RTWEELand LUTEL. 1963: SPEKREUSE, 1966; REGAS. 1968~). The experimental procedure was as follows: with the beams inantiphase, the subject turned potentiometer P (Fig. 3) until he found a setting of the relative modulation depths of the beams which gave a minimum sensation of flicker. This measurement was repeated four times. With. P set to give minimum subjective flicker, the subject now varied the relative phases of the beams so as to make four further settings on the point of minimum subjective flicker. (The value of the relative phases which gave a minimum sensation of flicker were never found to be signiticantly affected by small changes in the relative modulation depths of the beams, and vice versa.) Evoked potentials for a range of relative modulation depths and relative phases were recorded immediately afterwards. On some occasions these stimulus parameters were changed progressively, and at other times they were changed pseudo-randomly: the experimental conclusions are the same for either.procedure. An experimental session lasted typically three hr. The psychophysical settings were usually repeated at the end of the session. Bipolar recordings were made (I) between electrodes placed I cm and 7 cm above the inion along the midline. and (2) between the electrode placed 7 cm above the in/on and a third electrode to the right of the midline and 7 cm from the other two electrode$. An electrode behind the right ear was grounded. Four subjects whose colour vision was normat by the lshihara test w&e used. Results
Figure 5A shows the results of a control experiment in which two beams of the same colour were modulated in antiphase. The field size was lo’, the retinal illumination 1800 td
Evoked Potentiat and Psyc~ophysi~l
Correlates of Changes in Stimulus Colour
and intensity
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and there was a 30” white surround of 550 td. The ampfitude of the f~damentaf component of the steady-state EP feif to a ~nimum at the same point at which the subject was unable to see any flicker, and at which a photocell gave a minimum output (Fig. 5). This minimum amplitude was roughly O-2 py, which was near the noise level of the display. Fig. SB shows a similar experiment, except that in this case two d@zrent cofours were modulated in antiphase and then superposed. Here also the subject (D.R.) reported that flicker was almost imperceptibie at the same point that a photocell (with C.I.E. spectral sensitivity) in place of the eye had previously recorded zero flicker. In contrast with the control experiment, the amplitude of the fundamental EP now showed no ~~~i~u~ at the point of minimum subjective flicker (in seven cases out of eight, see below). For subjects D.R. and P.C.‘there was no minimum at all in the EP curve (compare Fig. 5, A and B). This held both for the fundamental component of the steady-state EP, and for the peak-to-peak amplitude of the EP simuf~neousfy recorded with.an averaging computer. The amplitude of the fundamental EP recorded from site I for subject J.S. had a shallow minimum but this was considerably displaced from the point of minimum subjective flicker. The curve recorded from site 2 was more nearly -flat, although the responses were smaller and more variable. For subject F.M.S. the curve recorded from site 2 was.flat. In contrast with the above findings, the curve recorded from site I for subject F.M.S. fell to a minimum at the point of minimum subjective flicker, and so correlated with subjective fficker. The relative ampfitudes of the responses evoked by red and yeffow modulations varied with the subject and electrode site. The following ratios were obtained with red and yellow stimuli which were adjusted to have equal fuminances and modulation depths (30 per cent), so that their Ricker would cancel subjectively. For subject D.R. the amplitudes of the red and yellow responses were roughly equal for site 1, but the red response was three to five times larger than the yellow response for ,site 2. For subjects J.S. and P.C. the red response was one and a half to two times larger than the yellow response for site 1, but roughly equal for site 2. For subject F.M.S. the red response was three times larger than the yellow response for site 1 (similar to Fig. 5B), but roughly equal for site 2. An absence of any minimum at all in the steady-state EP (e.g. Fig. 5B) could arise if the phase difference between the stimuli which produced minimum flicker sensation were very different to the phase difference which produced a minimum EP. An attempt was therefore made to cancel phase shifts by recording steady-state EP’s when the modufat~on of one stimulus beam was delayed behind the other. Results are shown in Fig. 5C. TAX amplitudes of the EP’s for subjects J.S. and D.R. showed a minimum when the yellow stimulus beam was modulated at 16 c/s and 60”-90’ in advance of the red beam. For subject P.C. the minimum occurred when the yellow beam was modulated 30”-60’ in advance of the red beam. For subject F.M.S. the minimum occurred when the yellow beam was modulated roughly 120” in advance of the red beam for site 2, although as mentioned above the minimum for site 1 occurred when the beams were near antiphase. These phase relations were (in 7 cases out of 8) cfearfy different to the near-antiphase condition which gave a minimum sensation of flicker. For subject D.R. the relative phases of the stimulus modulations was then set to give the minimum EP (90” lead for the yellow moduIation), and the expetiment of Fig. 5B was repeated. The result, shown in Fig. SD was that a clear minimum now occurred, where
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previously there had been no minimum. The new minimum usually reached the noise level. This suggested that the absence of a minimum in Fig. 5B could indeed be described in terms of relative shifts of phasing between the signals which lead to the perception of flicker, and the signals which lead to the generation of the fundamental component of the steady-state EP. Similar results were obtained for subjects J.S. and P.C. The relative phases of the two beams which .gave a minimum amplitude for the funidmental component were also measured for a range of other pairs of colours throughout the spectrum. The phase differences were found to vary widely with colour, and could reach 180;. The red bea? only was modulated, and the amplitude of the fundamental component of the EP plotted YerSmmodulation depth. This was repeated for modulation of the yellow beam only (Fig. 6). The curves were found to be of quite different shapes. 7 .
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FIG. 6. Plot of amplitude of the fundamental component of the steady-state EP in microvolts YPTSW stimulus modulation depth for red and yellow modulations showing that ‘the fundamental EP to yellow modulation saturates, but the fundimental EP to red modulation does not saturate. Stimulus field of 15’ subtense and 1800 td conskting of a red patch (640 nm) superposed on a yellow patch (589 nm) of equal lumitiance. White surround of 540 td and 30” subtense. Modulation frequency 18.2 c/s. Full line-r&modulation. Chain line-yellow modulation. Bipolar recording between an electrode 8 cm in front of inion along the midline and an electrode to right of midline and 7 cm both from midline electrode and from inion. ,Right earlobe grounded. Subject J.S.
Evoked Potential and Psychophysical Correlates of Changes in Stimulus Colour and Inrcnwy
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Discussion
The intensities of two superposed beams of different colours were sinusoidally modulated so as to generate an alter~dtin~-~raveiengthsstimulus. the relative modu~dtion depths and phases of the beams were then varied. and the conditions for minimum subjective flicker compared with the conditions for a minimum amplitude of the fundamental component of the steady-state EP. Experiment 2 was restricted to the medium frequency class of EP’s, and therefore these conclusions are also limited to this class of EP. The amplitude of the fundamental component was found to fall to a minimum near the point of minimum subjective flicker for only one subject (F.M.S.). and then for only one out of two electrode derivations. However. in this single case the correlation held for all the modulation frequencies investigated (16 c:s and 24.4 c;s) under the two different conditions of (a) varying the relative modulation depths of the two stimulus beams while their relative phases were held constant. and (b) varying the relative phases of the two modulated beams at constant modulation depths. In contrast. recordings of EP’s from the other electrode site (If for subject F.M.S. and for both sites for subjects D.R.. P.C. and J.S. led to findings quite different to those described above for F.M.S. The amplitudes of the fundamental components of the EP’s did nor have a minimum which coincided with the point of minimum subjective flicker when either the relative modulation depths or the relative phases of the beams were varied. so that whether the subject could see flicker had no correlation with the amplitude of the fundamental component of the EP (Fig. 5). For subjects J.S., P.C. and D.R. there was either no minimum at all. or only a shallow minimum. in the plots of ET‘ amplitude VCI’SUS relative modulatitin depths for antiphase stimuli. This could be explained if the relative phases of the red and yellow stimuli which produce a minimum flicker sensation were markedly different from the relative phases which give a minimum EP. In fact the phnse d~~~reflce betaeetr rite red afld yellow s(i~~~uI~~.~ beo~~t.~~s~~~c~ prodtlce~i ~li~li~~~i~l~ subjec~i~~eflicker was Czdeed fbund fo be corlsid~~rilb~~ d#erent fi-omrlre phase d~~~irettc~~ which gave a minimum EP nmplitude. A minimum sensation of flicker resulted when the phase of the yellow modulation lagged 165 -170. behind the phase of the red modulation (J.S., P.C. and. D.R.). For the EP.measure on the other hand. the amplitude of the EP was a minimum when the modulation of the yellow beam lagged 60’-90 behind the modulation of the red beam (electrode site 2 for D.R.. site I for J.S.). 30 -60 (site 2 for P.C.). or 1001-130’ (site 2 for F.M.S.). Phase differences of 180 can be expected between different electrode sites, or due to different spatial orientations of EP sources, but such differences are quite distinct from those described here. In most ,cases. even when phase was corrected. the EP minimum still did not coin‘cide with the point of minimum subjective flicker. Such a disparity could arise if stimuli which, although cancelling in their efTect.for flicker perception. nevertheless evoked steady-state fundamental EP’s of different amplitudes. This possibility was investigated in the following way. The plots of the amplitude of the fundamental EP component versus stimulus modulation depth for individual red and yellow modulations (Fig. 6) showed that for yellow modulation the EP amplitude increased as modulation depth increased, up to a depth of roughly 20 per cent and thereafter either remained constant or fell (for subject J.S.) at higher modulation depths. For red modulation the curve was quite different in that the amplitude of the fundamental EP component increased steadily with increasing modulation depth up to 100 per cent modulation depth (Fig. 6). The pairs of modulation depths which gave minimum flicker sensation could be calculated. since over the range
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of stimulus conditions used the modulation depths tthich _eave a minimum sensation of flicker obeyed the quite different relation m Y Iy = mR lR (mu, m, = yellow and red modulation depths; I,. I, = mean retinal illuminations of the yellow and red beams. Three predictions were made (a) the reiative modulation depths for a minimum fundamental EP component could be calculated from no more data than curves such as those of Fig. 6, (b) the disparity bstween the modulation depths for minimum flicker sensation and minimum fundamental EP would grow smaller at lower modulation depths. (c) the fundamental EP component to red modulation of greater than a critical podulation depth (for example the depth indicated by the upper dotted line in Fig. 6) could not be cancelled by any value of yellow modufation depth, while below this critical red modulation depth canceliation could occur (for example for the .pair of modulation depths cut by the lower dotted line in Fig. 6). These predictions were tested for modulation depths of 100, 55. 25 and I5 per cent and found to hold. Thus the points of minimum flicker sensation and minimum fundamental EP are completkiy dissociated and the difference between them can be manipulated by appropriate choices of modulation depths and intensities. For all four subjects the relative amplitudes of the fundamental component of the steady-state potentials evoked by red and yellow modulation were different for different electrode posiiions. This is consistent with a previous suggestion (REGAN, 1968a) that, for the medium frequency class of steady-state EP’s, different stimulus colours evoked potentials from differently distributed or orienated generators. Experiment 1 also gave similar findings. It should be emphasized that these characteristics appiy specifically to the medium frequency cIass of steady-state EP generated by unpatterned field stimulation, and that in other frequency regions or for patterned fields the behaviour of the steady-state EP can be comptetety different. For example, in contrast to the medium frequency EP’s. the amplitude of high frequency EP’s correlated with the point of minimum flicker for stimulus colours throughout the spectruti. The amplitudes of these high frequency EP components seem to be very similar when the differently coloured stimuli are of equal photopic luminances and equal modulation depths. In further contrast to the medium frequency class of EP’s3 the high frequency EP’s have a minimum amplitudes when the two stimulus beams are in antiphase. This suggests that the high,fiequency components of the steadystate EP’s might appropriately be used to provide a specific measure of changes in the luminance of the stimulus and give an objective assessment of photopic spectral sensitivity (REGAN, 1970). ~~~~o~~~e~ge~e~t~-i am gratefut to the Medical Research Council for their support. I am indebted to ROBERTF. CARTWRIGHT for constructing much of the equipment and for valuable contributions to its design. Thanks are due to Mr. H. WXRDELLand staff for invaluable technical assistance, and to JEFFREY SEDGELY, FRANCISSHAW, PETERCLARKE.and all others who acted as subjects.
REFERENCES ARMISGTON.J. C. (1966). Spectral sensitivity of simultaneous electroretinograms and occipital responses. In Clinical Electroretinography (1966). Pergamon. 225-233. C~VONIUS.C. H. (1965). Evoked response of the human visual cortex: spectral sensitivity. Psychon. Sci. 2, 185-186.
DE LANGEPZN. H. (1957). Attenuation characteristics and phase-shift characteristics of the human foveacortex systems in relation to flicker-fusion phenomena. Thesis. University of Delft, Netherlands. MACKAY,0. M. {Ed.). Evoked potentials as indicators of sensory information processing. Neurosciences Research Program Bulletin. In Press. REGAS.0. (1966a). Some characteristics of average steady-state and transient responses evoked by modulated light. Ekctroenceph. din. l~e~lrop~?siol. 20, 238-248.
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REGAS. D. (1966b). An effect of stimulus colour on average steady-state potentials evoked in man. iVarurr. I;ond. 210. 10561057. RTCAN. D. (1968a). Chromatic adaptation and steady-state evoked potentials. Vi.vion Res. 8. 149-158. REGAN. D. (1968b). A high frequency mechanism which underlies visual evoked potentials. Elecrroenceph. clin. Neurophysiol.
25.23 1-237.
REGAN, D. (1968~). Evoked potentials and sensation. Percept. and Ps.vchophysic.5 4. 347-350. REGAN, D. (1970). An objective method of measuring the relative spectral luminosity curve in man. 1. opt. Sot. Am. In press. SCHIPPERHEYN.J. J. (1965). Contrast detection in frog’s retina. Acra Ph.niol. fharmacol. Neerl. 13. 23 l-277. SIEGFRIED.J. B.. TEPAS. D. 1.. SPERLING. H. G. and HISS. R. H. ( 1965). Evoked brain notentials correlates of psychophysical responses: heterochromatic flicker photometiy; &i&e 149. 321-323. SPEKREUSE.H. (1966). Analysis of EEG responses in man. Thesis. University of Amsterdam. Netherlands. TWEEL. L. H. VAN DERand LUNEL. H. F. E.. VERDUYN(1965). Human visual responsesto sinusoidally modulated light. Elecrroenceph. clin. Neuroph,wiol. 18. 587-598. WALKAVEN. P. L. and LEEBECK.H. J. (1964). Phase shift of sinusoidally alternating coloured stimuli. J. opt. Sot.
Am. _sj. 78-82.
Abstract-The intensities of two superposed beams of different colours were_sinusoidaliy modulated at frequencies near 16 C/XC so as to generate an alternating-wavelength stimulus. and used in a joint psychophysical and evoked potential (EP) investigation. The relative phases and modulation depths of the two beams were varied and the conditions for minimum EP amplitude compared with the conditions for minimum (or zero) subjective flicker. When the relative modulation depths of the beams were varied. the fundamental EP gave (a) no minimum, of(b) a minimum displaced from the subjective ,minimum of(c) rarely. a minimum coincident with the subjective minimum. This depended on the subject and the electrode position. Cases (a) and (b) could be explained by the findings that (I) the relative phases of the beams for minimum subjective flicker differed considerably (50:-l IO’) from the relative phases for minimum EP. and (2) the relation between the amplitude of the fundamental component of the EP and stimulus modulation depth was different for different colours. Minimum subjective flicker seems to be related to stimulus intensity. and modulation depth in a different way than is minimum amplitude of the fundamental component of the EP. so that whether the subject sees flicker has no correlation with the minimum in the fundamental EP. R&sum&On module sinusoidalement g des frhuences voisines de I6 c/set les intensitis de deux faisceaux superposb de couleurs diffkrentes, et le stimulus P longueur d’onde altemante ainsi engendrk est ttudit P la fois par psychophysique et par les potentiels &oquCs (EP). On varie les phasesrelativeset les profondeurs de modulation des deux faisceaux, et lesconditions de I’a’mplitude minimale de EP sont compar&s B celle du papillotement subjectif minimal (ou nul). Quand on modifie les profondeurs de modulation relative des faisceaux. le EP fondamental prtsente (a) soit aucun minimum, (b) soit un minimum diplaci par rapport au minimum subjectif, (c) soit rarement un minimum qui coincide avec le minimum subjectif. Cela depend du sujet et de la position de I’+ctrode. On peut expliquer les cas (a) et (b) & partir des r&ultats suivants: (1) les phases relatives des faisceaux pour le papillotement subjectif minimal diffkrent considerablement (de 50 g I IO’) des phases relatives du EP minimal. et (2) la relation entre I’amplitude de la composante fondamentale de EP et la profondeur de modulation du stimulus diffkre pour les diverses couleuis. Le papillotement subjectif minimal semble dtpendre de I’intensid du stimulus et de !a profondeur de modulation d’une faqon diffkrente de I’amplitude minimale de la composante fondamentale de EP. si bien que le fait que le sujet percoit un papillotement n’a pas de corrClation avec le minimum du EP fondamental.
178
D. REM>
Zusemtifassung Die IntensitCtsn zueier verschieden Cirblger. einander Cberiagcrten Lichtstrihle wurden sinusmllssig bsi einer Frequenz von ungeCihr 16 Hz moduliert. urn einen Wechselschwellenreiz zu erzeugen: sie wurden in einer gemeinsamen Unterruchung mit psychophysischen Methoden und evozicrten Potentialen (EP beniitzt. Die relative Phase und Modulierung der beiden Strlihle wurden gezndert und die Bedingunp fiir die kleinste EP-Griisse mit dem fiir eine minimale (oder Null-) Flimmerempfindung notwendipen Wert verglichen. Wenn die relative Modulierung der Sttihle verindert wurde. gab da fundamentdle EP (a) keinen Mindestwert. oder (b) einen Lorn subjektiven Minimum verschobcnen Wert. oder (c) eine seltene Ubereinstimmung der beiden Mindeswerte. Dies hing vom Subjekt und der Ableitungselektrodenstellung ab. Eine Erkllrung der Fllle (a) und (b) mug in der Beobachtung zu suchen sein. dergemisr. es erstens grosse Unterschiede zwischen den Phasen der Strlhle fir das minimnle subjektivc Flimmern und des EP pdb und dass zweitens das Verhiltnis zwischen der G&se des Grund-EP-Bestandteiles und der Reizmodulierung von der Farbe abhing. Das minimale subjektive Flimmem scheint von der ReizgrGsse und der ModuiierunpsstBrke anders als der Mindestwert des Grund-EP-Bestandteiles abzuhlngen. Es folpt. dass das Flimmersehen nichts mit dem Mindestucrt des fundamentalen EP zu tun hat. PewMe MHTeHcnBHocTn neyx HanaraeMbtx nylr~oe ceeTa pa3nWvHoro ueeTa 6bt,iW MOKlyItnpOBaHbl CWHyCOWRanbHO, C qaCTOTOR OKOJtO 16 UHKJ-IOB B CeKyHtny, TaKnSl o6pa3oM. vTo6bt reHepWpoBaTb CTMMYJI c W3MeHRK)ueFiCR nnWHOi% BOnHbt. 3TM CTnMyJtbl 6btnn nCtlOEtb30BaHbl B CO’teTaHHblX ItCWXO~W3WOJtOrW’4eCKnX nCCJt&lOBaHnRX n ‘WCCJleLlOBaHnRX Bbl3BaHHblX tlOTeHUnaJtOB (Bt-l). OTHOCWTenbHble @a3bl M my6nHa MOLlyStRUW~ ZlByX Ily’tKOB CBeTa W3MeHAJtnCb W CpaBHnBaJlWCb yCItOBW5l. tlpn KOTOpblX B03HnKaJtn 5lnHWMaJlbHble Bll M MnHnMaJlbHbIe (WJtW Hyneebte) cy6aeKTneHble MeJtbKaHWR. Koraa OTHOCnTfYlbHaR rnL6ma MOllyJlPltlnW tly’iKOB n3MeHRJtaCb. TO: a) He B03HnKaJlO MMHnMaJtbHOrO OCHOBHOrO Bll. 8) MnHn,MaJlbHblfi Bll 6bin CMellleH IlO CpaBHeHWiO C MnHnMaJlbHOh Cy6beKTnBHOh pealctlneii W C) peLlK0, MWHnMiUlbHbte Bl-i COBllailaJlW C MWHWMyMOM MeItbKaHWii. 3TO 3aBnCeJlO OT WHPWBn~yaJtbHblXOCO6eHHOCTe2i WCllblTyeMblx W OT IlOJtO~eHnll 3JleKTpOnOB. Tnnbt peaKUW$i a) n I$) MOryT 6bITb 06arrCHeHbl TeM, ST0 I) OTHOCWTeJlbHble @a3bl LlJlR IlyYKOB CBeTa tlpn Bbi3blBaHnW SInHnMaJtbHblX Cy6beKTnBHblX MeJtbKaHnti 3Ha’iWTeJlbHO pa3JnqaH3TCII (Ha 50-110”) OT OTHOCnTeJibHbIX oa3, Heo6xonWhiblx ilnX B03HnKHOBeHnR MnHnMaJlbHblX Bn, W 2) TCM, ‘tT0 COOTHOUteHne MeNly aMllJtWTy4Oii OCHOBHOrO KOMtlOHeHTa BI-I n rny6nHoR MOLiyJtRUWM CTn.My,Ta 6bmo pa3JlW’iHO IIJIR pa3HblX UBeTOB. MnHWMaJlbHble Cy6beKTnBHble MeJlbKaHWR. tlOBnLtWMOMy. COOTHOCRTCI C MHTeHCnBHOCTbIO crnsfynnunn M rny6nHoR kionynnunn nHblv nyTe,v, 9eM 3To nMeeT Memo npn BO3HWKHOBeHWn MnHnMaJlbHblX aMtlJlnTyLt OCHOBHOrO KOMttOHeHTa Bl-l. CJlenOBaTeJlbHO TO, BWRWT JlW RCtlblTyeMblti MeJlbKaHWR. He CBI3aHO C B03HWKHOBeHWeM MWHnManbHOrO OCHOBHOrO Bll.
Rut.
Vol.
IO.
pp.
163-118.
Pcrgamon Press 1970. Printed in Great Britain.
EVOKED POTENTIAL AND PSYCHOPHYSICAL CORRELATES OF CHANGES IN STIMULUS COLOUR AND INTENSITY D. REGAN Department of Communication. University of Keele. Keele. Staffordshire
(Received
29. April
1969: in revi.sed fbrm 21 Julx 1969)
INTRODUCTION
Transient and steady-state evoked potentials
Transient evoked potentials (EP’s) to slowly repeated stimuli are commonly analysed into components of different latencies. As the stimulus repetition rate is increased, the responses to successive stimuli superpose to an increasing extent until eventually a ‘dynamic steady-state’ regime may occur in which no individual response can be related to a particular stimulus cycle. The waveforms of such steady-state responses may be analysed into harmonic components. Classfjication @‘steady-state evoked potentials into di&rent jbequency regions
There is growing evidence that steady-state evoked potentials (EP’s) can be classified into at least three different frequency regions, centred near 10 c/s (low frequency), 16 c/s (medium frequency) and 50 c/s (high frequency) respectively (Fig. 1) and that the EP’s IO -
1
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i STIMULUS
1;
MODULATION
io
3b
FREQUENCY
4’0 (c/r
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6’0
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FIG. I. Three classes of steady-state evoked potentials (EP’s), as shown by plots of amplitude versus stimulus modulation frequency. Dotted lines: IO Q’S (low frequency) class of EPsfundamental and second harmonic components. Chain line: I6 c/s (medium frequency) class of EP’s-fundamental component. -Full lines-53 c/s (high frequency) class of EP’s-fundamental and second harmonic components. Note: The relative amplitudes of these classes of EP vary considerably with subject. electrode position. stimulus intensity, field size and colour. Components other than the fundamental and second harmonic are omitted for clarity.
in these three regions have different relationships with such stimulus parameters as intensity and colour. In many subjects there is a particular stimulus frequency near IO c/s for which the amplitude of the EP is larger than at neighbouring frequencies. It is at this same frequency that spontaneous a activity is most prominent. Although yellow stimuli have been 163
164
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REGAS
reported to evoke smaller responses than either red or blue stimuh for modulation frequencies close to 10 cis (REGAN.I966b). colour does not in genera1 exert a strong influence on the 10 c/s responses. The stimulus intensity or field size seem also to have little effect on the shape and peaking frequency of the amplitude verse frequency curve for the EP near 10 c/s. The ‘high frequency’ EP’s, whose maximum response amplitudes occur at roughly SO-55 c/s form the second class (VAN DER TWEEL and LUNEL, 1965; SPEKREIJSE,1966: REGAN, 1968b). For the high frequency EP’s, as for the 10 cis responses, the amplitude IWXUSfrequency curve seems to be similar in shape to the corresponding power spectrum of spontaneous EEG activity (SPEKREIJSE,1966). High frequency EP’s may be differentfy distributed over the scalp and ,be less dependent on stimulus intensity than EP’s of the medium frequency ctass (REGAN, 1968a). The situation is quite different for the third class of steady-state EP components, whose amplitudes are maximal for medium stimulus frequencies in the region of 16 c/s. The shape of the amplitude wsus frequency curve for EP’s in this frequency region can be strongly influenced by chromatic adaptation and by field size (REGAN, 1968a). Further contrasts to the ‘low’ and ‘high frequency’ responses are that the medium frequency class of EP’s may have little relation to the power spectrum of spontaneous EEG activity and do not give strong second harmonic responses. The relative amplitudes of the above classes of EP vary considerably with subject, electrode position, stimulus intensity, field size and cofour.
This paper presents experimental evidence on the question whether EP amplitude correlates with the point of minimum subjective flicker in a heterochromatic photometry situation. This work forms part of an investigation to find whether EP’s in the three different frequency regions described above reflect different information processing systems, and in particular whether steady-state EP’s can provide objective evidence of the different ways in which the brain handles intensity and colour information. SIEGFRIEDet al. (1965) reported that the amplitude of the evoked potential fell to a minimum near the point of minimum subjective flicker when they carried out one version of the standard technique of heterochromatic flicker photometry. In the present investigation, the first two subjects used in the main experiment (Experiment 2) gave results which were in conflict with this finding. On the other hand, the stimulus parameters in the two investigations were quite different, so that it was clearly necessary to carry out a subsidiary investigation (Experiment 1). in’ which the methods used by Siegfried et al. were imitated as closely as possible.. APPARATUS Visual Stimulator Light from a 500 W Xenon arc lamp was split into two beams, and after passing through electromechanical light modulators MI and M2. the two beams were recombined at a beamsplitter B2 (Fig. 3). Monochromatic beams were obtained by interference filters whose half-widths were roughly 10 nm. Side bands were negligible. Neutral density filters were used to change stimulus intensity. The electromechanical light modulators Ml and M2 were developed from a device described by van der Mark (SCHIPPERHEYN.1965). The principle is illustrated in Fig. 4. This device can modulate intensity up to a depth of IO0 per centt. The frequency response is from d.c. to 70 cs ( -3 db).
1The m~ulation depth of a sin~oidally modulated ljght is defined as half the peak-to-peak intensity expressed as a percentage of the mean intensity.
ClIanae
Evoked Potential and Psychophysical Correlates of Changes in Stimulus Colour and Intensity 4-
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FIG. 2; Peak-to-peak amplitude of the averaged evoked potential serrus the luminance of one of two alternating beams. The zero of luminance is at the point of minimum subjective flicker. (A): Two red stimuli abruptly alternated at I6 c/s. Minimum amplitude of EP is close to the point of minimum subjective tlicker. (B) and (C): Red stimulus abruptly alternated with a white stimulus at 16 c/s for two subjects. There is no significant minimum in the EP amplitude except for dotted line (J.S.) and even then the minimum in EP amplitude is significantly displaced from the point of minimum subjective flicker (at’O.0 relative luminance). Red stimulus. 640 nm. half width roughly IO nm. 3.6 field with 30 white surround. 2880 counts. Electrode (I). I cm above inion. electrode (2) 5 cm in front of electrode (I). Monopolar reference behind right ear: electrode behind left ear grounded. Full line (circles) monopolar records from electrode (I) and (2). Fine dotted line (squares) monopolar records from electrode (2). Coarse dotted line (triangles) bipolar records between electrodes (I) and (2).
165
D. REGAL
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The beam from the arc lamp was brought to a focus F (Fig. 4) at a lightweight vane driven by a Pye-Ling vibration generator. Movements of the vane controlled the light flux passing to the subject’s eye. Critical positioning of lens L2 ensured that the image formed in the first focal plane of the subject’s eye lens was free of any perceptible irregularity caused by the edge of the vane and that the whole field changed brightness uniformly during modulation (tested at a low frequency of about 0.1 c,s). This adjustment was made easier by placing a diffuser over the hole H. The system was linearized by negative feedback from a photocell which received a fraction of the modulated beam. The two light modulators were driven by separate alternators. One alternator was linked to the other through a differential gear (Fig. 3) which enabled the phase relation between the two light modulators to be continuously varied through 360 When the subject was required to control the relative phases of the beams. it was more convenient to drive the light modulators by a two-phase oscillator. The relative modulation depths
Evoked Potentiaf and Psychophysical Correlates of Changes in Stimulus Cotour and inrcnsity
167
of the two beams were controlled by the double potentiometer P (Fig. 3). which could be operand by the subject. Rotation of the potentiometer finearly increased the modulation depth of one beam. whilst linearly reducing the modulation depth of the other beam. A calibration of the resultant modulation depth for the ttio superposed beams WFSWthe reading of the potentiometer is shown in Fig. 5A. Tbe’critical settings of both the relative phases and relative modulation depths of the beams which wsre required to obtain minimum subjettive flicker. could be made by the subject himself.
RELATIVE
MOOUCATfON
DEPYHS
Fro. 5. The amphtudes and phas& of the fundamental component of the steady-state evoked potential are plottad in (A), (3). (C) and (I?). (A) Two beams of tlte same cotour (red. 640 nm) were rn~u~at~~n antiphase and their relative mod&tioa de@ varied:Dattcd iire is output of pkotac&. Circks are EP amplitudes. The arrow indicates &c mean point of MO subjective Aickct, and the horizontal bfack Line gives the range of four settings.
D.
168
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Fro. SB, Similar experiment to (A) but now different cotours (yehow (589 nm) and red) were alternated in antiphase. Dotted line is p~ot~el~ output and full line is EP, Maximum yeliow and zero red modulation at 30 on abscissa, maximum red and zero yellow modulation at 330. There is no minimum in EP amplitude which coincides with the point of minimum subjective %&et (arrow).
EEG Analyds The EEG was analysed in two ways (Fig. 3): (a) an averaging computer (Nuclear Chicago) was triggered synchronously with the stimulus modulatioo, and displayed cveraged evoked potentials, (b) a special purpose on-line Fourier Analyser extracted the steady-state EP from the EEG, split up the EP into harmonic components, and then displsyed the amplitude and phases of these harmonic components in the form of running averages. The Fourier Analyser has been de&bed previousiy(REOI\N, I966a). Sine and cosine waveforms at frequencies which were harmonically related to the stimulus modulation frequency (Fig. 3) were generated by maehanically linked alternators. tiogue devices were used to multiply the EEG by the sine and cosine of the fundamental, and second barmouic and any other requind harmonic or subharmonic frequencies. Low-pass filtering fotfowed the rnu~~p~~~o~ stage, then running averages of the sine and cosine products were displayed on pan recorders. These dispiays gave the amplitudes and phases of the harmonic components of the steady-state EP’s.
Evoked Potential and Psychophysical Correlates of Changes in Stimulus Colour and Intensity
6b
Ii0 RELATIVE
2io PHASES
300 (dogrear)
I69
360
FIG. SC. Amplitudes and phases of the fundamental component of the EP versus the relative phases of the red and yellow beams. The relative modulation depths of the beams were held constant at the setting for minimum subjective flicker (Fig. 5B). Dotted line is output of photocell, full line and circles show f~dam~tal component of EP. Arrow and horizontal bar indicate mean and range of 14 settings on point of minimum subjective flicker. Relative phase for minimum EP is roughly 80” from the relative phase for minimum subjective flicker. Note: 180 on scale means beams in antiphase. The subject sat in a soundproof, screened chamber which minimised any distraction or contamination of the EP by auditory stimulation. He fixated the stimulus centrally with his right eye in Maxwellian view. The left eye was covered. Steady fixation was assisted by the use of a bite bar. The colour vision of all subjects used was found to be normal by the Ishihara test. EXPERIMENT
I
The subjects fixated centrally on a 3-6’ stimulus field in which a standard white light alternated abruptly at 16 c/s with a comparison red or green light. This central patch was surrounded by a white geld which subtended 31” and had the same luminance as the standard white component of the Rickering light. While the coloured and
D. REGAS
170
white lights alternated. the intensity of the coloured beam was varied until the subject reported that he saw minimum flicker. It is at this point that the luminan~ (subjective brightness) of the two beams are defined as equal. Averaged evoked potentials were recorded from four sites: (I) bipolar. between the inion and an electrode 5 cm in front along the midiine. (II) monopolar. between the inion and a reference electrode behind the left ear. (Ill) monopolar. between the electrode placed 5 cm ahead of the inion and the left ear reference. and (iv) bipolar between the inion and an electrode placed 5 cm to the right of the inion along a line perpendicular to the midline. An electrode placed behind the right ear was earthed.
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Frc. 5D. Experiment (55) repeated. but now with the relative phases of treams not in antiphase as in 55. but set to give minimum amplitude for the fundamental EP (see Fig. 5C). A minimum now occurs in EP ampfiiude. whereas in Fig. 59 there was no minimum. Arrow and horizontal bar @es mean and range of 4 settings on point of minimum subjective flicker: the point of minimum EP amplitude is displaced from the point of minimum subjective flicker. in addition. an abrupt phase change of roughly 180’ occurs near the point of minimum EP amplitude (contrast with Fig. 58). Dotted line gives photocell output for Fig. 5B: for easier comparison the photocell curve for the phase has been shifted along the abscissa by an amount corresponding to the displacement of the EP’s minimum amplitude from the point at which the photocell has a minimum output. Note: Modulation depth scale. i.e. ~
ml
is linear (m,, m, = modulation depths
(I) and (2) respectively. Stimulus modula:&fr!&ency
ofbeams
= 16 cis. The maximum modulation depth ofeither beam is 30 per cent (at 30 or 330 on abscissa). IO’ field of 1800 td with 30” white surround of 540 td. Electrode (1) cm in front of inion. electrode (2) 7 cm in front of electrode (I), electrode (3) to right of midline and 7 cm from electrodes (I) and (2). Electrode behind right ear grounded. Noise level roughly 0.1-0.2 pV.
Evoked Potential and PsychophysicalCorrelates
of Changes in Stimulus
Colour
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Results Peak-to-peak EP’s to red-white flicker were measured for eight subjects, but for three of these the signal to noise ratio was so unfavourable that no conclusions could be drawn. Green-white flicker was also used with three of the five subjects who gave clear EP’s. All eight subjects were able to make consistent settings of relative intensity at a point where subjective flicker was barely perceptible. Four subjects showed no significant minimum in the EP in the neighbourhood of this point for any electrode position (Fig. 2). On the other hand, a control experiment confirmed that when two beams of thesame colour were alternated, these four subjects all gave a clear minimum in the EP, and that this minimum coincided with the point of minimum subjective flicker (Fig. 2). When different colours were alternated, one subject only (F.M.S.) gave a minimum in the EP which coincided with the point of minimum subjective flicker, and this was found for only one of the electrode positions investigated (IV). For the five subjects who gave measurable EP’s, curves of the type shown in Figs. 2B and 2C were found to have different shapes for different electrode positions. For these five subjects the amplitude of the EP’s to predominantly white flicker was larger than for predominantly red flicker when measured at one electrode position (e.g..Fig. 2B. filled circles), but was smaller or equal at another electrode position (e.g. Fig. 2B, triangles). Discussion The methods used in the present experiment were identical with those used by Siegfried et nl., except that the square wave stimuli were modulated to a depth of 50 per cent instead of 100 per cent in order to reduce the possibility of swamping any minimum in the EP by saturation of the response (VAN DER TWEEL and LUNEL~ 1965; SPEKREIJSE,1966; RECAS. 1968~). In some experiments, 100 per cent modulation was also used for comparison. On the assumption that the subject’s pupil diameter was 2 mm in Siegfried et al.‘s experiments, their retinal illuminations were within O-2log units of that used in the present investigation. Siegfried et al.- described the EP’s recorded from.site (III) only, since these recordings were ‘the largest and most sensitive’. Responses were presented as plots of the peak-topeak amplitude of the averaged EP ver.rm the relative luminance of the two beams. Results from only two subjects were discussed. The present experiments offer only limited support to the finding of SIEGFRIEDet al. (1965) that in a heterochromatic flicker photometry situation a minimum in the amplitude of the EP can be found in close proximity to the point of minimum subjective flicker. In fact such a correlation was found for only one subject (out of eight), and then in only one electrode derivation (out .of four) which suggests that the correlation is uncommon in these experimental conditions. This lack of correlation between the point of minimum subjective flicker and the amplitude of the EP could be explained by the experimental findings described below that flicker perception and the medium frequency class of EP’s involve different phase shifts and different spectral sensitivities. A serious drawback to the popular usage of peak-to-peak EP amplitude in colour studies became evident during the present investigation. This is that peak-to-peak measurements take no account of waveshape, so that fundamental, and second harmonic and other components are lumped together. It can easily occur that a stimulus frequency is such that the fundamental and second harmonic components of the response fall into different classes of steady-state evoked potential (Fig. 1). For example, the fundamental component
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might fall into the medium frequency class of EP’s. whereas the second harmonic component. recorded at the same time from the same site, could fall into the high frequency class of EP’s. Since the behaviour of these two classes of steady-state EP turns out to be totally different in response to alternating colour stimuli, the peak-to-peak measure would therefore present a misleading picture (see below, and REGAN, 1970). EXPERIMENT II
introduction If any phase shifts which occur between the retina and the mechanism of flicker perception are independent of colour, then subjective flicker would be a minimum when the stimulus beams were modulated exactly 180’ out of phase. This is not the case however; departures from the 180” condition have been measured psychophysically (DE LANGE, 19.57; WAZRAVEK and LEEBECK;, 1964). Now if the phase difference (cp) between the stimulus beams which produced minimum subjective flicker were markedly different from the phase diffeience (Q) which produced a minimum EP, then the finding that in some cases there is no minimum at all in the EP curve would be explained (Fig. 2C). Whether cpis markedly different from 8 could be tested by delaying one stimulus behind the other so as to cancel this phase difference. However, the result of this manOeuvre may not be simple when square wave inputs are used as in Experiment I, since the relative phase shifts may be different for the various response components evoked by the harmonicallycomplex waveform. The use of sinusoidally modulated stimuli leads to more satisfactory attempts to balance out phase shifts; such experiments are reported below. The stimulus requirements outlined above were satisfied in a similar way to that adopted by DE LASGE(1957). who superposed a patch ofsinusoidally modulated light ofonecolour on a second patch ofsinusoidally modulated light of another colour. By adjustment of both the relative phases and modulation depths of the two beams, changes in luminance could be cancelled. so that subjectivefy only colour alternations remained. At this point there was zero subjective brightness Bicker. in this present experiment the subject could cont..ruously vary the relative phases and modulation depths of beams I and 2 (Fig. 3). A unique value of the relative modulation depths of the beams,gave minimum or zero flicker. At this point the modulation depth for either beam alone was IS per cent in most experiments: for maximum flicker ofeither beam the depth of modulation for one beam was 30 per cent and was zero for the other. These “small-signal” conditions ensured that the EP was mostly in the pre-~turation region (VAX D&RTWEELand LUTEL. 1963: SPEKREUSE, 1966; REGAS. 1968~). The experimental procedure was as follows: with the beams inantiphase, the subject turned potentiometer P (Fig. 3) until he found a setting of the relative modulation depths of the beams which gave a minimum sensation of flicker. This measurement was repeated four times. With. P set to give minimum subjective flicker, the subject now varied the relative phases of the beams so as to make four further settings on the point of minimum subjective flicker. (The value of the relative phases which gave a minimum sensation of flicker were never found to be signiticantly affected by small changes in the relative modulation depths of the beams, and vice versa.) Evoked potentials for a range of relative modulation depths and relative phases were recorded immediately afterwards. On some occasions these stimulus parameters were changed progressively, and at other times they were changed pseudo-randomly: the experimental conclusions are the same for either.procedure. An experimental session lasted typically three hr. The psychophysical settings were usually repeated at the end of the session. Bipolar recordings were made (I) between electrodes placed I cm and 7 cm above the inion along the midline. and (2) between the electrode placed 7 cm above the in/on and a third electrode to the right of the midline and 7 cm from the other two electrode$. An electrode behind the right ear was grounded. Four subjects whose colour vision was normat by the lshihara test w&e used. Results
Figure 5A shows the results of a control experiment in which two beams of the same colour were modulated in antiphase. The field size was lo’, the retinal illumination 1800 td
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and there was a 30” white surround of 550 td. The ampfitude of the f~damentaf component of the steady-state EP feif to a ~nimum at the same point at which the subject was unable to see any flicker, and at which a photocell gave a minimum output (Fig. 5). This minimum amplitude was roughly O-2 py, which was near the noise level of the display. Fig. SB shows a similar experiment, except that in this case two d@zrent cofours were modulated in antiphase and then superposed. Here also the subject (D.R.) reported that flicker was almost imperceptibie at the same point that a photocell (with C.I.E. spectral sensitivity) in place of the eye had previously recorded zero flicker. In contrast with the control experiment, the amplitude of the fundamental EP now showed no ~~~i~u~ at the point of minimum subjective flicker (in seven cases out of eight, see below). For subjects D.R. and P.C.‘there was no minimum at all in the EP curve (compare Fig. 5, A and B). This held both for the fundamental component of the steady-state EP, and for the peak-to-peak amplitude of the EP simuf~neousfy recorded with.an averaging computer. The amplitude of the fundamental EP recorded from site I for subject J.S. had a shallow minimum but this was considerably displaced from the point of minimum subjective flicker. The curve recorded from site 2 was more nearly -flat, although the responses were smaller and more variable. For subject F.M.S. the curve recorded from site 2 was.flat. In contrast with the above findings, the curve recorded from site I for subject F.M.S. fell to a minimum at the point of minimum subjective flicker, and so correlated with subjective fficker. The relative ampfitudes of the responses evoked by red and yeffow modulations varied with the subject and electrode site. The following ratios were obtained with red and yellow stimuli which were adjusted to have equal fuminances and modulation depths (30 per cent), so that their Ricker would cancel subjectively. For subject D.R. the amplitudes of the red and yellow responses were roughly equal for site 1, but the red response was three to five times larger than the yellow response for ,site 2. For subjects J.S. and P.C. the red response was one and a half to two times larger than the yellow response for site 1, but roughly equal for site 2. For subject F.M.S. the red response was three times larger than the yellow response for site 1 (similar to Fig. 5B), but roughly equal for site 2. An absence of any minimum at all in the steady-state EP (e.g. Fig. 5B) could arise if the phase difference between the stimuli which produced minimum flicker sensation were very different to the phase difference which produced a minimum EP. An attempt was therefore made to cancel phase shifts by recording steady-state EP’s when the modufat~on of one stimulus beam was delayed behind the other. Results are shown in Fig. 5C. TAX amplitudes of the EP’s for subjects J.S. and D.R. showed a minimum when the yellow stimulus beam was modulated at 16 c/s and 60”-90’ in advance of the red beam. For subject P.C. the minimum occurred when the yellow beam was modulated 30”-60’ in advance of the red beam. For subject F.M.S. the minimum occurred when the yellow beam was modulated roughly 120” in advance of the red beam for site 2, although as mentioned above the minimum for site 1 occurred when the beams were near antiphase. These phase relations were (in 7 cases out of 8) cfearfy different to the near-antiphase condition which gave a minimum sensation of flicker. For subject D.R. the relative phases of the stimulus modulations was then set to give the minimum EP (90” lead for the yellow moduIation), and the expetiment of Fig. 5B was repeated. The result, shown in Fig. SD was that a clear minimum now occurred, where
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previously there had been no minimum. The new minimum usually reached the noise level. This suggested that the absence of a minimum in Fig. 5B could indeed be described in terms of relative shifts of phasing between the signals which lead to the perception of flicker, and the signals which lead to the generation of the fundamental component of the steady-state EP. Similar results were obtained for subjects J.S. and P.C. The relative phases of the two beams which .gave a minimum amplitude for the funidmental component were also measured for a range of other pairs of colours throughout the spectrum. The phase differences were found to vary widely with colour, and could reach 180;. The red bea? only was modulated, and the amplitude of the fundamental component of the EP plotted YerSmmodulation depth. This was repeated for modulation of the yellow beam only (Fig. 6). The curves were found to be of quite different shapes. 7 .
. Red
modubtian
6
0
I
20
I
I
60 40 Stimulus modulation depth.
.
/
I
El0 %
I
loo
FIG. 6. Plot of amplitude of the fundamental component of the steady-state EP in microvolts YPTSW stimulus modulation depth for red and yellow modulations showing that ‘the fundamental EP to yellow modulation saturates, but the fundimental EP to red modulation does not saturate. Stimulus field of 15’ subtense and 1800 td conskting of a red patch (640 nm) superposed on a yellow patch (589 nm) of equal lumitiance. White surround of 540 td and 30” subtense. Modulation frequency 18.2 c/s. Full line-r&modulation. Chain line-yellow modulation. Bipolar recording between an electrode 8 cm in front of inion along the midline and an electrode to right of midline and 7 cm both from midline electrode and from inion. ,Right earlobe grounded. Subject J.S.
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Discussion
The intensities of two superposed beams of different colours were sinusoidally modulated so as to generate an alter~dtin~-~raveiengthsstimulus. the relative modu~dtion depths and phases of the beams were then varied. and the conditions for minimum subjective flicker compared with the conditions for a minimum amplitude of the fundamental component of the steady-state EP. Experiment 2 was restricted to the medium frequency class of EP’s, and therefore these conclusions are also limited to this class of EP. The amplitude of the fundamental component was found to fall to a minimum near the point of minimum subjective flicker for only one subject (F.M.S.). and then for only one out of two electrode derivations. However. in this single case the correlation held for all the modulation frequencies investigated (16 c:s and 24.4 c;s) under the two different conditions of (a) varying the relative modulation depths of the two stimulus beams while their relative phases were held constant. and (b) varying the relative phases of the two modulated beams at constant modulation depths. In contrast. recordings of EP’s from the other electrode site (If for subject F.M.S. and for both sites for subjects D.R.. P.C. and J.S. led to findings quite different to those described above for F.M.S. The amplitudes of the fundamental components of the EP’s did nor have a minimum which coincided with the point of minimum subjective flicker when either the relative modulation depths or the relative phases of the beams were varied. so that whether the subject could see flicker had no correlation with the amplitude of the fundamental component of the EP (Fig. 5). For subjects J.S., P.C. and D.R. there was either no minimum at all. or only a shallow minimum. in the plots of ET‘ amplitude VCI’SUS relative modulatitin depths for antiphase stimuli. This could be explained if the relative phases of the red and yellow stimuli which produce a minimum flicker sensation were markedly different from the relative phases which give a minimum EP. In fact the phnse d~~~reflce betaeetr rite red afld yellow s(i~~~uI~~.~ beo~~t.~~s~~~c~ prodtlce~i ~li~li~~~i~l~ subjec~i~~eflicker was Czdeed fbund fo be corlsid~~rilb~~ d#erent fi-omrlre phase d~~~irettc~~ which gave a minimum EP nmplitude. A minimum sensation of flicker resulted when the phase of the yellow modulation lagged 165 -170. behind the phase of the red modulation (J.S., P.C. and. D.R.). For the EP.measure on the other hand. the amplitude of the EP was a minimum when the modulation of the yellow beam lagged 60’-90 behind the modulation of the red beam (electrode site 2 for D.R.. site I for J.S.). 30 -60 (site 2 for P.C.). or 1001-130’ (site 2 for F.M.S.). Phase differences of 180 can be expected between different electrode sites, or due to different spatial orientations of EP sources, but such differences are quite distinct from those described here. In most ,cases. even when phase was corrected. the EP minimum still did not coin‘cide with the point of minimum subjective flicker. Such a disparity could arise if stimuli which, although cancelling in their efTect.for flicker perception. nevertheless evoked steady-state fundamental EP’s of different amplitudes. This possibility was investigated in the following way. The plots of the amplitude of the fundamental EP component versus stimulus modulation depth for individual red and yellow modulations (Fig. 6) showed that for yellow modulation the EP amplitude increased as modulation depth increased, up to a depth of roughly 20 per cent and thereafter either remained constant or fell (for subject J.S.) at higher modulation depths. For red modulation the curve was quite different in that the amplitude of the fundamental EP component increased steadily with increasing modulation depth up to 100 per cent modulation depth (Fig. 6). The pairs of modulation depths which gave minimum flicker sensation could be calculated. since over the range
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of stimulus conditions used the modulation depths tthich _eave a minimum sensation of flicker obeyed the quite different relation m Y Iy = mR lR (mu, m, = yellow and red modulation depths; I,. I, = mean retinal illuminations of the yellow and red beams. Three predictions were made (a) the reiative modulation depths for a minimum fundamental EP component could be calculated from no more data than curves such as those of Fig. 6, (b) the disparity bstween the modulation depths for minimum flicker sensation and minimum fundamental EP would grow smaller at lower modulation depths. (c) the fundamental EP component to red modulation of greater than a critical podulation depth (for example the depth indicated by the upper dotted line in Fig. 6) could not be cancelled by any value of yellow modufation depth, while below this critical red modulation depth canceliation could occur (for example for the .pair of modulation depths cut by the lower dotted line in Fig. 6). These predictions were tested for modulation depths of 100, 55. 25 and I5 per cent and found to hold. Thus the points of minimum flicker sensation and minimum fundamental EP are completkiy dissociated and the difference between them can be manipulated by appropriate choices of modulation depths and intensities. For all four subjects the relative amplitudes of the fundamental component of the steady-state potentials evoked by red and yellow modulation were different for different electrode posiiions. This is consistent with a previous suggestion (REGAN, 1968a) that, for the medium frequency class of steady-state EP’s, different stimulus colours evoked potentials from differently distributed or orienated generators. Experiment 1 also gave similar findings. It should be emphasized that these characteristics appiy specifically to the medium frequency cIass of steady-state EP generated by unpatterned field stimulation, and that in other frequency regions or for patterned fields the behaviour of the steady-state EP can be comptetety different. For example, in contrast to the medium frequency EP’s. the amplitude of high frequency EP’s correlated with the point of minimum flicker for stimulus colours throughout the spectruti. The amplitudes of these high frequency EP components seem to be very similar when the differently coloured stimuli are of equal photopic luminances and equal modulation depths. In further contrast to the medium frequency class of EP’s3 the high frequency EP’s have a minimum amplitudes when the two stimulus beams are in antiphase. This suggests that the high,fiequency components of the steadystate EP’s might appropriately be used to provide a specific measure of changes in the luminance of the stimulus and give an objective assessment of photopic spectral sensitivity (REGAN, 1970). ~~~~o~~~e~ge~e~t~-i am gratefut to the Medical Research Council for their support. I am indebted to ROBERTF. CARTWRIGHT for constructing much of the equipment and for valuable contributions to its design. Thanks are due to Mr. H. WXRDELLand staff for invaluable technical assistance, and to JEFFREY SEDGELY, FRANCISSHAW, PETERCLARKE.and all others who acted as subjects.
REFERENCES ARMISGTON.J. C. (1966). Spectral sensitivity of simultaneous electroretinograms and occipital responses. In Clinical Electroretinography (1966). Pergamon. 225-233. C~VONIUS.C. H. (1965). Evoked response of the human visual cortex: spectral sensitivity. Psychon. Sci. 2, 185-186.
DE LANGEPZN. H. (1957). Attenuation characteristics and phase-shift characteristics of the human foveacortex systems in relation to flicker-fusion phenomena. Thesis. University of Delft, Netherlands. MACKAY,0. M. {Ed.). Evoked potentials as indicators of sensory information processing. Neurosciences Research Program Bulletin. In Press. REGAS.0. (1966a). Some characteristics of average steady-state and transient responses evoked by modulated light. Ekctroenceph. din. l~e~lrop~?siol. 20, 238-248.
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REGAS. D. (1966b). An effect of stimulus colour on average steady-state potentials evoked in man. iVarurr. I;ond. 210. 10561057. RTCAN. D. (1968a). Chromatic adaptation and steady-state evoked potentials. Vi.vion Res. 8. 149-158. REGAN. D. (1968b). A high frequency mechanism which underlies visual evoked potentials. Elecrroenceph. clin. Neurophysiol.
25.23 1-237.
REGAN, D. (1968~). Evoked potentials and sensation. Percept. and Ps.vchophysic.5 4. 347-350. REGAN, D. (1970). An objective method of measuring the relative spectral luminosity curve in man. 1. opt. Sot. Am. In press. SCHIPPERHEYN.J. J. (1965). Contrast detection in frog’s retina. Acra Ph.niol. fharmacol. Neerl. 13. 23 l-277. SIEGFRIED.J. B.. TEPAS. D. 1.. SPERLING. H. G. and HISS. R. H. ( 1965). Evoked brain notentials correlates of psychophysical responses: heterochromatic flicker photometiy; &i&e 149. 321-323. SPEKREUSE.H. (1966). Analysis of EEG responses in man. Thesis. University of Amsterdam. Netherlands. TWEEL. L. H. VAN DERand LUNEL. H. F. E.. VERDUYN(1965). Human visual responsesto sinusoidally modulated light. Elecrroenceph. clin. Neuroph,wiol. 18. 587-598. WALKAVEN. P. L. and LEEBECK.H. J. (1964). Phase shift of sinusoidally alternating coloured stimuli. J. opt. Sot.
Am. _sj. 78-82.
Abstract-The intensities of two superposed beams of different colours were_sinusoidaliy modulated at frequencies near 16 C/XC so as to generate an alternating-wavelength stimulus. and used in a joint psychophysical and evoked potential (EP) investigation. The relative phases and modulation depths of the two beams were varied and the conditions for minimum EP amplitude compared with the conditions for minimum (or zero) subjective flicker. When the relative modulation depths of the beams were varied. the fundamental EP gave (a) no minimum, of(b) a minimum displaced from the subjective ,minimum of(c) rarely. a minimum coincident with the subjective minimum. This depended on the subject and the electrode position. Cases (a) and (b) could be explained by the findings that (I) the relative phases of the beams for minimum subjective flicker differed considerably (50:-l IO’) from the relative phases for minimum EP. and (2) the relation between the amplitude of the fundamental component of the EP and stimulus modulation depth was different for different colours. Minimum subjective flicker seems to be related to stimulus intensity. and modulation depth in a different way than is minimum amplitude of the fundamental component of the EP. so that whether the subject sees flicker has no correlation with the minimum in the fundamental EP. R&sum&On module sinusoidalement g des frhuences voisines de I6 c/set les intensitis de deux faisceaux superposb de couleurs diffkrentes, et le stimulus P longueur d’onde altemante ainsi engendrk est ttudit P la fois par psychophysique et par les potentiels &oquCs (EP). On varie les phasesrelativeset les profondeurs de modulation des deux faisceaux, et lesconditions de I’a’mplitude minimale de EP sont compar&s B celle du papillotement subjectif minimal (ou nul). Quand on modifie les profondeurs de modulation relative des faisceaux. le EP fondamental prtsente (a) soit aucun minimum, (b) soit un minimum diplaci par rapport au minimum subjectif, (c) soit rarement un minimum qui coincide avec le minimum subjectif. Cela depend du sujet et de la position de I’+ctrode. On peut expliquer les cas (a) et (b) & partir des r&ultats suivants: (1) les phases relatives des faisceaux pour le papillotement subjectif minimal diffkrent considerablement (de 50 g I IO’) des phases relatives du EP minimal. et (2) la relation entre I’amplitude de la composante fondamentale de EP et la profondeur de modulation du stimulus diffkre pour les diverses couleuis. Le papillotement subjectif minimal semble dtpendre de I’intensid du stimulus et de !a profondeur de modulation d’une faqon diffkrente de I’amplitude minimale de la composante fondamentale de EP. si bien que le fait que le sujet percoit un papillotement n’a pas de corrClation avec le minimum du EP fondamental.
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Zusemtifassung Die IntensitCtsn zueier verschieden Cirblger. einander Cberiagcrten Lichtstrihle wurden sinusmllssig bsi einer Frequenz von ungeCihr 16 Hz moduliert. urn einen Wechselschwellenreiz zu erzeugen: sie wurden in einer gemeinsamen Unterruchung mit psychophysischen Methoden und evozicrten Potentialen (EP beniitzt. Die relative Phase und Modulierung der beiden Strlihle wurden gezndert und die Bedingunp fiir die kleinste EP-Griisse mit dem fiir eine minimale (oder Null-) Flimmerempfindung notwendipen Wert verglichen. Wenn die relative Modulierung der Sttihle verindert wurde. gab da fundamentdle EP (a) keinen Mindestwert. oder (b) einen Lorn subjektiven Minimum verschobcnen Wert. oder (c) eine seltene Ubereinstimmung der beiden Mindeswerte. Dies hing vom Subjekt und der Ableitungselektrodenstellung ab. Eine Erkllrung der Fllle (a) und (b) mug in der Beobachtung zu suchen sein. dergemisr. es erstens grosse Unterschiede zwischen den Phasen der Strlhle fir das minimnle subjektivc Flimmern und des EP pdb und dass zweitens das Verhiltnis zwischen der G&se des Grund-EP-Bestandteiles und der Reizmodulierung von der Farbe abhing. Das minimale subjektive Flimmem scheint von der ReizgrGsse und der ModuiierunpsstBrke anders als der Mindestwert des Grund-EP-Bestandteiles abzuhlngen. Es folpt. dass das Flimmersehen nichts mit dem Mindestucrt des fundamentalen EP zu tun hat. PewMe MHTeHcnBHocTn neyx HanaraeMbtx nylr~oe ceeTa pa3nWvHoro ueeTa 6bt,iW MOKlyItnpOBaHbl CWHyCOWRanbHO, C qaCTOTOR OKOJtO 16 UHKJ-IOB B CeKyHtny, TaKnSl o6pa3oM. vTo6bt reHepWpoBaTb CTMMYJI c W3MeHRK)ueFiCR nnWHOi% BOnHbt. 3TM CTnMyJtbl 6btnn nCtlOEtb30BaHbl B CO’teTaHHblX ItCWXO~W3WOJtOrW’4eCKnX nCCJt&lOBaHnRX n ‘WCCJleLlOBaHnRX Bbl3BaHHblX tlOTeHUnaJtOB (Bt-l). OTHOCWTenbHble @a3bl M my6nHa MOLlyStRUW~ ZlByX Ily’tKOB CBeTa W3MeHAJtnCb W CpaBHnBaJlWCb yCItOBW5l. tlpn KOTOpblX B03HnKaJtn 5lnHWMaJlbHble Bll M MnHnMaJlbHbIe (WJtW Hyneebte) cy6aeKTneHble MeJtbKaHWR. Koraa OTHOCnTfYlbHaR rnL6ma MOllyJlPltlnW tly’iKOB n3MeHRJtaCb. TO: a) He B03HnKaJlO MMHnMaJtbHOrO OCHOBHOrO Bll. 8) MnHn,MaJlbHblfi Bll 6bin CMellleH IlO CpaBHeHWiO C MnHnMaJlbHOh Cy6beKTnBHOh pealctlneii W C) peLlK0, MWHnMiUlbHbte Bl-i COBllailaJlW C MWHWMyMOM MeItbKaHWii. 3TO 3aBnCeJlO OT WHPWBn~yaJtbHblXOCO6eHHOCTe2i WCllblTyeMblx W OT IlOJtO~eHnll 3JleKTpOnOB. Tnnbt peaKUW$i a) n I$) MOryT 6bITb 06arrCHeHbl TeM, ST0 I) OTHOCWTeJlbHble @a3bl LlJlR IlyYKOB CBeTa tlpn Bbi3blBaHnW SInHnMaJtbHblX Cy6beKTnBHblX MeJtbKaHnti 3Ha’iWTeJlbHO pa3JnqaH3TCII (Ha 50-110”) OT OTHOCnTeJibHbIX oa3, Heo6xonWhiblx ilnX B03HnKHOBeHnR MnHnMaJlbHblX Bn, W 2) TCM, ‘tT0 COOTHOUteHne MeNly aMllJtWTy4Oii OCHOBHOrO KOMtlOHeHTa BI-I n rny6nHoR MOLiyJtRUWM CTn.My,Ta 6bmo pa3JlW’iHO IIJIR pa3HblX UBeTOB. MnHWMaJlbHble Cy6beKTnBHble MeJlbKaHWR. tlOBnLtWMOMy. COOTHOCRTCI C MHTeHCnBHOCTbIO crnsfynnunn M rny6nHoR kionynnunn nHblv nyTe,v, 9eM 3To nMeeT Memo npn BO3HWKHOBeHWn MnHnMaJlbHblX aMtlJlnTyLt OCHOBHOrO KOMttOHeHTa Bl-l. CJlenOBaTeJlbHO TO, BWRWT JlW RCtlblTyeMblti MeJlbKaHWR. He CBI3aHO C B03HWKHOBeHWeM MWHnManbHOrO OCHOBHOrO Bll.