Spectral analysis of the visually evoked occipitogram in man

Vision Rer. Vol. 8, pp. 4op-431. Psfgnmoa Rres~ 1968. Ritcd

SPECTRAL

in Cbat Britain.

ANALYSIS OF THE VISUALLY OCCIPITOGRAM IN MAN’

EVOKED

(Received 23 Octobet 1967; in ruvisedform 20 Ducemk 1967)

INTRODUCTION htrrv~ous work has shown tbat the wavefo~ of the normal visually evoked autos (VEOC) is complex& dependent upon the wavelength of the stimulus light (SwfpLBy, JONB and FRY, 1%5). We have called this a “coloxoding”. Speci&ahy, these first observations pertained to two trained observers viewing a red (6gOmr&41tHz) and a yellow (575nm421tHz) light at 24 log units or more above threshold, and a blue (4OOnm749tHz) light at O-5fog units or more above threshold-3 Aspects of this coding have a?so been found to persist over severaJ log units of intensity (SHIPLEY, JUNB and FRY, 1966). However, only four colors were used in this early work and intensities were mat&e4 only relative to a threshold (as in the decibel method) and not directly. The present study (Experiment 1) extends the eon of this coding by the use of sixteen sps&al points and of heterochromatieally matched intensities (at a level about 2-5 logs above threshold). Secondly, the method is simplified so that untrained observers could be employe& Some color-normal observers were found who did not display a color code, and possible reasons for this are examined (Experiment 2). Finally, the VEOGS in several color-d&eient observers are reported in the hope that this might help clarify the basis for the coding. No ‘coding was found in any of them. Moreover, the deutans gave significautly reduced amplitudes (Experiment 3). Apparatus The apparatus has been described previously iu detail (SIBKEY,Joand FRY, 1966), except for the transmission charae&istics of the interference filters (Thin Film, Inc., Cambridge, Muss.), which we present in Fig. 1. The widths at half maximum transmission are generally about l&m. The peak ~s~~ons are generally between 35 and 65 per cent. The other essentials are shown in Fig. 2. The collimated light from a 500 W Xenon lamp (Xe) is passed through a chopper (c), a shutter (s), some neutral density step fB.ers 1 Supported by CmWact #DA-M-193~MD-2344 from the O&c of the U.S. Army Surgeon Genial. 2 Current address: X3egx&xncnt of PsycboBogy,Emory Univcmity, At&w Georlgia. 3 C. T; White (at tlx September 4, 1966, A.P.A. Sympdum on Serxery EvaIredPo&&alsj hrrs contirmedthefindingofadiffcmw:~thendandthebhie~~~ ~~~~~ conference abstract, by M. Clyncsand M. K&n t’Specific tvspona of the brain to color stimuli” Pruc. 17th Ann. Co& Et&n.in Med. and Biol. November, 1964, Ckvcland, Ohio, p. 38),whichdeals with tbi~ same problem. They used the method, which we have not yet tried, of stimulating not from black but from one color to anotber, and t&y report that “wen more marked {d&omatic waveform) di@erences are obtained.” C$ also Pans and NAGAYA (1965). 409

T. SHIPLEY,R. WAYNEJam

410 BJO

750

mo

650

FREOtJENCY(tHz) SC0 350

AND AMELLA FRY 500

.M

(ND), a selected interference filter (1) and a balanced neutral density wedge (FV), and is then brought to a focus at the anterior surface of the lens of the observer’s left eye (0) in a Maxwellian view of about 11”. The observer lies supine on an examining table and views the light along the upward directed primary line of sight. His head is held finmly

FIG. 2.

L

As S P P

A schematicdiagram of the apparatus in Experiment 1:

-*m = soowt.Xenonlamp

- aperture stop =&Utter = photocell = glassplate = fiberlightpipe !a, d, 12,ml, tq = optiu needed to bring secondary Xe beam to Max~nellianview at 0 M = 112sheredmirror ND - n&al densitystep filters I = intcrkrmcc filters (Fig. 1) W = neutral density we&-and balana FS = field stop The observer’s view is shown in the insert at the arrow.

Spectral Analysis of the VisuallyEvokedOccipitogmmin Man

411

but co~o~bly in the Mowers view by a hospital res~~g pillow. The electrical contact to the observer is made by placing one electrode on the scalp in the midline at or just above the oc&pital pole and another directly above this (Scm for observer R.W.J. and 3cm for A.F. and TX); a ground electrode is placed on the right ear. The on-going EEG is monitored on an oscilloscope. The evoked response is recorded on a Neural computer.4 Since it was essential in this study to match the brightnesses of the lights which were to be used, a special attachment was added to the basic apparatus. The primary beam of the interference mon~~omator passes undisturbed through the instrument up to the mirror (m$ which then blocks out half of the beam. The remaining half-beam passes through in the usual fashion and, after reflection from (M) is focused at the obser&s eye (0). It is seen as a half disc of homo~neo~ light. The other half of the field is formed as follows. The glass plate @) diverts a small peroentage of the Xe light which is then focused onto the end of a fiber-optics light pipe {/p). This carries the Xe white Iigbt to the neutral density filter (_Q, which a~enuat~ it to a level about equal to the auction introduced by the interference alters (J) in the primary beam. Lens &I) then focuses the light from the end of the fiber-bundle onto the difising m(d). Lens (Q focuses the image of the screen in Maxwellian view at the observer’s eye (0), after mfleetions from the front surface mirrors (ml, mz) and the half-silvered mirror (M>. The observer is thus presented with an 11” split field in Magi view+ as shown in the insert at the arrow. The upper half of this field is filled with mon~~o~tic light exactIy as it will be presented for the VEOCi recordings, the lower half of this field is fBled with a fixed intensity of blue-white Xe light. This system allows one to make the heterochromatic brightness matches nnder exactly the same stimuhrs sequence as used to evoke the VEOGs. The only &an@ between the brightness matching pods and the procedure used for the VEOG recordings is the removal of mirror (nr2)- This allowed the VEDGs to be taken with the fuIt 11” se@ as in our previous work. The fact that a split fidd was used for the brightness matching while a full field was used for the VRCX3sdoes not, in our opinion, affect the transformability of the matches from one situation to another. In both cases, the observer attempted to 6xate the center of the field and in both cases macular and perimacular regions were involved*

The neutral filter u) was chosen and fixed for afl brightness matihes so that the brightness level was the maximum possible for the instrument. With a few exceptions, this fell between 2.0 and 490log units above ~hold for all observers and all colors. It measured 1927log cd/n@ (with an S.E.I. visual exposure meter). As in our previous work on the VEOG, the observer was light adapted for exactly 30 set to l-27 cd/m2 of tungsten white (27” field). This particular intensity was chosen to

4 informationan 66s v&r is avaSt& f&m Mr. PBl?ip Boo&g R~MSU&Center, AmGtiaa ~~~y,P~~~~. ~s~t~a~a~. ~t~~~~ to didtal conversion which is exactly proportional to the input anaIooue voltage. It sums t&e vrrfuer,in the memory and displays them in both analogue and digital form at the cad of each run. A technial description of the principles used in the Neurac is given in W. A, C!LMUC’S (1962) description of the ARC computer {in: “Processing neuroektric data” Cam kms of the ~~~~ Chip, Res.Lob. Efecf., MET Press, 2nd edition, Cambridge, Mass.).

412

T. SHIPLZY, R. WAYNE JONLS ANDAMIILIA FRY

be a brightness match to the fixed intensity Xenon white of the secondary beam. The observer was then dark adapted for exactly one minute. Then the split field was presented in Maxwellian view two times per second in flashes of 10 msec duration.5 The color to be matched was first presented obviously brighter than the Xe and brought down to it in intensity by the wedge (w); next it was presented obviously dimmer and brought up to it in intensity; finally, it was again presented obviously brighter.6 The match-point was taken as the arithmetic mean of these three settings. The standard deviation of these matches was reduced with practice to about O-05 log units. In general, these flickering heterochromatic brightness matches held true when the chopper was stopped and the split field was presented as steady to the eye; but there were exceptions, especially for a color-de&Sent Observer.' Thus, the brightnesses of all the monochromatic lights obtained by the filters shown in Fig. 1 were matched carefully to that of the Xe white at 1.27 log cd/m? We then de&mined the VEGGs for each of these lights. The observer was light adapted for 30 set, as before, and then dark adapted for one min. The VEOG (as the brightness match) took about 25 set, which, together with a short 5 set rest period, gave us a cyclic procedure of two min per run. The VEGG itself is the sum of only 32 gashes, summed over a period of 5OOmsecafterthefIash. Initially, we divided the spectrum into two sweeps, one running up in 12 monochromatic steps and the other running back down in 13 steps. Each step was separated by approximately 1Onm. The two sweeps in turn were broken into two. This gave us four experimental sessions: three with six colors, and a fourth with seven; or 25 colors in all. Since the VEOG was determined at least three times for each color (thus six min per color), this gave us four experimental sessions of neverless than 36 min. (Because of interruptions, momentary degradations of the Maxwellian view, and the like, these sessions usually ran to about 45 nun.) Consequently, the full spectral run was a composite of four separate experimental sessions. When this experiment was completed, however, it became clear to us that intersession amplitude variability, even for trained observers, was such as to seriously effect

6 Under these conditions one observes the apparent motion known as gamma motion (e.g. BARCLAY, 1963 inter AZ). To our knowledge gamma has never been studied in monochromatic light or in the Maxwellian view, but this technique does provide the most reftned conditions for its elicitation. and it prechrdes any attempt to base gamma upon pwiUary movements (as in BADWIN and Tnw.n~, 1%5). As~choppa~tbsbeam,~~tappearstoexpandortocontractrapidlyoverthe~cld,and the rate at which it does this ssems to be prowrtfonal to its huninosity. T?ds~~uith.h.&~ (1963) that gamma is based upon retinal lwponse endisnts. The he&o&o were aided by the task of equah&g the two gamma pulse rates. It is possible that this criterion is noi identical to the equal brightness criterion, but it is never&less a relatively consistent psychophysiological criterion, which is all that is r&red. 7 Such exceptions do not appear to have been mentioned previously. In addition to a loss of saturation, even the hues of some of the colors changed from the steady to the flickering state. For instance: blueviolet tended to become uncomaminated blue; orange tended towards ted. It occurs to us that this might beanothawayofestablisbinOthasocollsdpurah~:ie.astliosecolors~donotdianOeina~ fromthesteadytotheflickerstate. On~~~forsomecolon,~bosichuebeamctontaminated by a mixture (additive?) with its own a&r-image. In fact, a@r a very few gashes, especially in the bright blues. the orighral color itself may entirely disappear and the after-image appear in its stead. This was true for some of the VEOGs previously reported by us (m, JONES and ERY,1966). and will always be a source of impurity in VHKis obtained with lights presented at rates which do not allow for the complete disappearanceof the after-image.

Spectral Analysis of the Visually Evoked Occipitogram in Man

413

TIME (msec) Fk3.3. In&radoo uqita #alift in the cauponatsofthcvEoo~vdolmafor rhgbmng c&m for one color-normal o(A.F.). the internal consi&ncy of the data. This may be seen in Fig. 3 where we show the results for one observer over a small section of the spectrum. The fhst (54Onm) and third (5OOnm) responses were obtained during one session, the second (53Onm) and fourth (490nm) responses were obtained during another session. Since it would be unlikely for the responses from so closely spaced colors to oscillate up and down in this fashion (see the 2nd and 4th positive wavelets), we can only assume that some inherent variability in the computer, in the electrode contact to the head, or in the state of alertness of the sub% was at fault. This type of variability was apparent for all observers. In order to obtain as meaningful and as leptokurtic a picture of spectml performance as possible, we concluded that the whole spectrum should best be covered in one session rather than by averaging between sessions. To this end, the successive steps between Glters were enhuged to about 2Onm, and only two runs were taken at each of 13 Mers. Given two runs each on 13 colors, at two minutes per run, this required a session of 52 minutes duration. Three additional colors were run at another time to till in at the bhte and to dovetail in at the point of change from the yellow to the red, giving 16 colors in all. Finally, to decream the possibility of contaminating any single response with the response from the proce&ng gash, the flash rate was lowered to l/set. Rudts for Experiment 1 The results for the two color-normal observers are shown in Fig. 4. Those for the deuteranomalous observer (T.S.) are not reported here (but cjI Fig. 16 below) because they exhibit no differentiation in either amplitudes, latencies, or waveforms as a function F

414

T. SHIPLHY, R. WAYNIZ JomplANDAINXJA FRY

of color, as aheady reported (SHIPLEY,JONESand FRY, 1965). The results from the more complete runs through the spectrum (as in Fig. 3) are entirely compatible with these data. Considering the reliability of these data and the fact that the changes in waveform which we observe in Fig 4 have been obtained in a great number of VEOGs from these

TIME

Fm. 4. Tb

VEOGa

(msec)

._

for 16 dors at mat&d intmmtm For two c&r-~ (RW.J. and A.F.).

.._

obacmal

two observers taken over a period of at least one year (CASHIPLJX,Jonas and FRY, 1%5), we feel justified in making a preliminaq analysis of them by inspection. HXLLand PARR (f 963) and especially WALTRRS(1%3) have noted the dangers inherent in the visual analysis of conventional EEG data, but these strictures need not necemar2y apply to the VEOG. This may partly he inferred from the poor predictability between the EEG and the VEOG as shown by Koor and BACXHI(1964a) and espeoially by RODIN et al.(1965). Prece&ng on this basis, we present in Fig. 5 a best fitting average of the waveform shifts which appear to us to be present. These shifts are demonstrable for both observers, and we summa&e them here. 1. There is a gradual change in waveform from the red to the violet, from which we may extract three patterns: one characteristic of the violet-green-yellow range (38o-48054Onm); one characteristic of the yellow-oranges (59Onm) ; and one characteristic of the reds (64Onm). The VEOGs could readily be sorted into these three groups without knowledge of the stimulus wavelengths, even though the patterns do change gradually one into the other.

Spectral Analysis of the Visually Evoked Occipitogram in Man

TIME

415

(msec)

FIG. 5. Comparison of best-fitting VEOG waveforms to 5 s&ctai wav&agths for two color-normal ~lorading om (R.WJ. and A.F.), as de&d from the data in Fig. 4.

2. The particularly fast change in waveform near 5!Nnm is perhaps the most striking aspect of these data. 3. The culmination time of the major negative trough at 300-350 msec seems virtually unaffected by changes in the color of the stimulus light, and remains relatively stable. 4. Assuming, on the basis of this constant trough time, that the single most prominent positive peak represents the same wavelet in the blue as in the red, then the culmination time for the peak in the red is signiiicantly shorter (175 msec) in comparison to the culmination time in the blue (225 msec), the standard deviation of culmination times being about 7-g msec. 5. The first positive peak (which may be a red peak) loses amplitude in the mid-spectral range but appears to recover somewhat in the responses to violet light, although the first peak here is faster than the large red peak. In order to examine these impressions more rigorously, a Fourier analysis was performed on the data of Fig. 4, and the essential findings are shown in Fig. 6.8 In this figure we plot the amplitude coefIicients of the first eight cosine terms, in a conventional Fourier approximation, against the stimulus wavelength. This type of analysis approximates the waveforms of the VEOGs by succeeding pairs of sine and cosine coefficients, each term representing succeeding frequencies of oscillation. The absolute amplitudes of the ordinates indicate the contribution or power of the given frequency at each wavelength; the change s This was B computer analysis done for us by Dr. Mason Cox, of the American Optical Company, in whose debt we are.

T. SHIPLE;Y.R. WAYNEJones ANDhmu

416

FRY

_-------_____________

0-e

WAVELENGTHhd FIG. 6. A plot

of the coaina c&lcicmts of a Fourier Pnsayuis of the data in Fig. 4 aa a function of waheth

(see text for cxpleMtioxl).

in amplitude as a function of wavelength is au indication of coding. There is little if any variance in the amplitudes as a function of wavelength from the lOc/s term onwards. Moreover, these terms are not sig&cantly different from zero, which means that there are no components in these evoked occipitograms occurring at these frequencies. Considering that our amplifier band-pass is 6db down at 42c/s, this cut-off cannot be identied as the source of the lack of power at lOc/s. It must, then, we think, be physiological, The most important aspects of these curves are: (1) the trends against wavelength shown by the 4c/s, 6c/s and SC/Sterms; and (2) the large non-zero amplitudes shown in these same terms. This means that almost all of the power and almost all of the color coding occurs within these frequencies. It would have been neater had the observers agreed as well in trend for the 4c/s and 8c/s terms as they do for the 6c/s, but all six curves do agree in suggesting greater non-zero amplitudes in the central spectral regions. This points towards a loss of power (hence a resemblance) in both the red and the violet. Finally, the rapid acceleration shown at 4c/s especially by observer A.F. between 57Onm and 5!JOnm exactly conl%ms point No. 2 of our visual analysis. (The analysis of the coeBcients of the sine terms was somewhat less complete than that of the cosine terms, but the same general picture emerges.) On this basis, then, we now reject the notion which we held at first, that some aspects of this coding (and its power) might be due to differential alpha intrusion, because our observers’ alpha frequencies are most typically in the higher 8-12c/s ranges. Since on-going EEG activity has many inherent frequencies (it can, in fact be shown to somewhat resemble

SpectralAnalysisof the Visually EvokedOtipitom

in Man

417

band-limited white-noise) and since the alpha frequency, even in a given observer, is not strictly constant, it is virtually impossible to find a stimulus frequency which is not a subharmonic (hence a potential resonator) of some on-going brain activity. It is for this reason that the best work in this area uses random stimulus frequencies (as in CHAJXE and ERTL, 1965). Unfortunately, our present technique-both because of our strict adaptation sequence and our use of a mechanical chopper-does not readily lend itself to this type of stimulus presentation. Although the fact that our responses emerge clearly after only 32 tihes does reflect favorably on our technique (c$ SHIPLBY, JONESand FRY, 1966), it does not allow us to assert that we have entirely excluded some resonance effects. However, it does con&m the fact that the on-going EEG is not strictly random (c$ PERRY, 1966). It is obvious that we are obtaining a signal to noise improvement which is greater than the Gaussian prediction of 432 (c$ BENDAT,1964). Finally, if the coding is indeed confined to the 4&/s range, this means that the highfrequency cut-off in the amplifiers can be lowered even further without destroying the coding. This may allow us, in the future, to considerably enhance the signal to noise ratio, although it will not effect the question as to the origin of this coding.9 EXPERIMENT2 This study was undertaken to investigate the generality of these findings to other color-normal observers. To this end, the apparatus was modified to allow for light flashes of various durations and contrasts (Fig. 7A) and to simplify the brightness matching (Fig. 7B). Only five colors were used (66onm, 590, 575, 510, and 470), so as not to put excessive demands upon untrained observers. The apparatus modifications are shown in Fig. 7. A tiny mirror (m), reflecting light to the triggering photocell (P), was mounted on the chopper (C) so as to

A FIG. 7. ApparaW P As m C

B modifiation for Experiment - photocell - ppaturr stop -mirror - chopper

2:

9 One mggcation coma froma subjective observation in which the three authors ccnnplctcly agree. The Mue h&es sexned to “hit” with a jarring sensation, almost causing us to “dodge” or to “duck”. This was not true for the long wavekngth stimuli. Perhaps thi8 refkcts the %ousal” cxnnponent (cf. SHUUY, 1964) which rrrrmyworkersm to 6nd in the VEOG, and is ImcoMwM with color vision per dc. On the other hand, Schroeder (of the RCA Laboratories) has quiteingeniously derived the various VEOG waveforms from a waveguide theory of cone function, together with tiomc minimal assumption8 about nerve signal tranhssion and amp&r charactcristks (private communication). while we naturally favor some such cxplanaGon on principle, we are by no means ozrtain that other conthgcncics have been cxcludcd.

418

T.

%UPLEY,

R.

WAY!% JONB AND AMEUA

FRY

trigger the sweep of the brain at any arbitrary moment with respect to the stimulus presentation. The flash duration was controlled by a variable length arcuate slit in the chopper disc. To control the contrast, the normally opaque disc was replaced by a disc constructed of two superimposed polaroid sheets. A fixed aperture was cut from one Polaroid, thus controlling the stimulus duration. This was set at 10 msec, as in all our previous work. The rotation of the two polaroids with respect to each other controlled the intensity of the inter-gash or background stimulation. We have called this procedure dl, since a h. of approximately 2.0 log units above the background (identical h) was thus presented. We chose this ~fferen~ because our previous work (SHIPLEY,JCINBand FRY, 1966) has shown it to be the best for the evocation of the color code. The background itself was set at approximately 2-O log units over absolute threshold, for all observers, all wavelengths, and for our conditions of adaptation (which were identical to those in Experiment No. 1-q.v.). Specifically, then, we used three test conditions: ON-OFF (exactly as in Experiment 1 with the opaque chopper); dl (which is ON-OFF with the polaroid chopper]; and ON (setting the s~uius duration at 500 msec, the exact duration of the brain sweep). Some additional controls on trained observers were taken, which shall be described below. Unless otherwise stated, all VEOGs represent the sum of 32 events. Finally, the brightness matching was done as follows. The overhead tungsten LA source (Fig 7B) was used as the reference standard, while the h to be matched appeared in Maxwellian view in its center. Thus we employed the he~r~o~tic brightness matching procedure between an annular surround and its center. In this experiment, the matches were made with the lights steady-on. Our usual (30 set L.A. + 60 set D.A.) adaptation sequence preceeded each match. This task seemed easy enough for our untrained observers, although the standard deviations (&- O-2 log) were signi&antly larger than those obtained in Experiment 1. Finally, the subjects were examined by a small battery of subjective color vision tests: the Farnsworth lo&hue test, the A.O.-H.R.R. test, an anomaloscope, neutral-point determinations, four pure color namings, and a lOpoint luminosity curve. These last four tests are embodied in an apparatus described fully elsewhere (SHIPLEY, 1965). The judgements were monocular, using an approximately 2” vertically split field obtained with a combination of interference and W&ten filters, together with app~p~a~ integrating spheres, slits, and optics. The light source is a 6V, 18A tungsten ribbon filament lamp from Philips which has a horizontal filament shaped like a rectangular U. (Here it serves at once as three voltage-matched light sources, and is ideal for optics which can be laid out in the shape of a T.) The essential results with these various tests are included, together with the VEOGs for each observer, in the appropriate figures below.

for experiment 2 A. The absence of a code in some color-normal observers. In Fig. 8 we show the results for 10 msec ON-OFF stimulation for two color-normal observers. The Farnsworth lOO-hue test scores are shown in the usual fashion in the upper right-hand corner. It can be seen that these are both superior normal observers. Observer A.F. shows what we have called a color-coding; observer SC does not. Thus, a color-coding cannot yet be shown in all color-normal observers, Since we can find no change in coding with or without dilated pupils, the pupillary reflex is not a factor. We are reasonably sure that individual refractive properties are not

Resultf

Spectral Analysis of the Visually Evoked Oazipitogram in Man

419

TIME (msec:) FIO. 8. (Zcm&mn of best-fitting VEOGs to 5 sekcted wavckmgths for two color-d obmvm: A.Fsolor coding, SC.-_ color m. ‘I& Fmnwor&-MumelI loo-hue intheuppcrright-hmdcomcr teatacomsfmtheae-uegiven,inrheuswl-, ofthe5gure.

involved because the Maxwellian view technique virtually eliminates these, and, in any case, the obsemrs in Fig. 8 are both essentially emmetropic.10 They are both graduate students, Caucasian, and in their early twenties. A.F. is female and S.C. male; but since observer R.W.J. (see Exp. 1) is also male, the coding cannot be related to sex. Observer S.C. is ambidextrous; observer A.F. is strongly dextral.ll The remainder of Experiment 2 examines our technique in an attempt to explain this discrepancy. Thee xamination is phrased as a series of questions. B. Is the coding in some way ajknctbn of the number of@hes? Thus, might we destroy a coding by exteuding our usual run? Figure 9 shows the results for a color-coder for two colors taken with 30,60, 120 and 200 10 msec ON-OFF gashes. The difference between the red and the blue is apparent after 30 gashes and it persists clearly after 200 gashes. Similar extension of the tests on a non-coder do not reveal color specific wavelets. Thus, we can neither evoke nor destroy the code simply by increasing the number of stimuli. commmiation:Effectaofamtourahupnasonvimmllyevokedcortiad 10HARn!auldwmn(prhte potentirrkmainprcplrrtion)hoveshown,forexampk,thntoptical~~an~tllevEoG mvsfonnin~~whicfl.~kutsupaficially,re#mbleourcoding. Hmathispdmtbbynomeans trivial, paltkukly in view oftbe signifiamt chromatic aberration of the eye. 11The important relatiomhip between the VEOG and later&y has been explored by &SON (1967) and by F’ERRY (private communication).

et al.

420

T. SHIPLBY.R. WAYNS JONES ANDA~LIA FRY ON-OFF

TIME

(msec)

Fm.9. TbspcoaLtsDceof~red(66onm)vs.blue(47(km)~~inaoocdarnacmal obmvm (R.W.J.) indepdmtly of the number of .summwd vmG&for10lmec0N-o~ nashaL

C. Is there some non-random evoked response to the termiuation of thejlash in the colorcoders which is dfflerent from that in the non-coders? Figure 10 shows, in an ON-OFF color-coder, that we do not !ind a non-random event (i.e. with our usual 32 flashes) upon the cessation of a 500 msec stimulation by monochromatic ligh& regardks of its wavelength. There perhaps is a slight trough about 250 msec, but this exhibits very poor reliability and was not found in other observers. 12 Thus, no unique off-response has been found which could account for the color code. D. What is the efict offla,vh duration upon the color co& ? Figure 11 shows the monochromatic VEOGs for a color-coder for three wavekngths and four ON-OFF fkshdurations : 10 msec, 100,200 and 400. It can be seen that the coding is displayed most clearly with the 10 msec flash; the 100 msec duration consistently hindered the red response in this observer and, while this red response returns somewhat for still longer durations, the difference between red-yellow-blue is never again quite so distinct. Similar tests on a non-coder show that he cannot thereby be made to code. Thus, color-coding & partklly a function of flash duration.13 11 ham offcffats in the auditorily evoked vcrticogram have recently been reparted by ONmn and DA~C’DllrationandnbetimeoftoIltburstspnd~humenvatca~~~tbs74th~g of the &owtical society of Ammica, Miami, Florida), but there ia no -tic STUDY in the lip.

evminourotherwknon~(cf.ponSandN~~~~~, 1965). Thistcadis-uaaaMplouh-. because for tcchnii masons,both ticld size and fiaab duration bad to be dkut@.

Spectral An&is

of the Visually Evoked Occipitogram in Man

421

OFF

TIME hsec) Fro. 11. The distohm

of waveform and the loss of fx&r-wding in 3Naah monochrom8tic

E. Whut happens to the coding when the pMh duration is extended in length to equal the duration of the brain-scan, i.e. as in the pure ON stimulation ? In Fig. 12 we show the results for a color-coder (RX.) and a non-color-coder (I.M.). It is clear that the coding in R.H. is destroyed (c$ Fig. 13 below) and that no coding in I.M. is evoked. Thus, this con&ms and extends the results in No. D, to a condition where responses to the off-set of the light are excluded.

422

TIME Imsec) F-m.12. VBOGs under ptae ON stimulation showing th, poor color-cod& ina color normal coloxoding observer (RJ-L-& Fig. 131,and the consistent failureto cub in a color niwmainon-ading ottaenm: I.MO

AI

FIG. 13. VEOGs under AI

TIME (msec) stimulation for a color-coding observer (R.H.) and a non-coding obwver (I.M.).

spectral Analysis of the Visualiy Evoked CkciMom

in Man

423

F. How does the cod&g behave whm evoked by the AI tech ? Figure 13 shows the 10 msec dl VEOGs for the same two observers shown in Fig. 12. The coding in R.H. is well-preserved, while no significant changes are noted for I.M. The coding in other observers was slightly hindered. G. In whut mamer does the coding vary as a function of electrode position ? Specifically, can we find a code in a non-coder by varying the positions of the electrodes? For example, observer R.W.J. shows coding best at occipital pole + Scm, observer A-F. shows it best at pole Jr km, while observer T.S. gives his best amplitudes at the vertez And we have found some observers who, conm to the usual report, show krget amplitudes the smaller the separation between the midline bi-polar electrodes. I.M. 0+&m.

o+lOcm

In Fig. X4we show the responses for the non-coder I.M, for three wav&ngtbs using two electrode positions: pole + 6cm and pole + lCkm, thus surrounding the position (pole + 8&m) used throughout the other phases of this study. No coding appears. Thus* although e&c&& position is quite generally importan& this, in its&f, seems alit to turn a non-coder into a coder. If the Vi?t.?Cis i&&d related to color vision, #lienshotdd it not be modtjied in some way in coio~ de$ctent observers ?

To answer this question, we studied six deutans, two protans, and one t&an observer, and compared them to seven color-normal subjects. The apparatus and proc&ure is that used in Experiment 2.

424

T. SHIPLEY. R.

WAYNE

JONESAND

~ELU

FRY

The results for the normals are shown first, in Fig. 15. An A.O.-H-R-R Protan/Deutan (P/D) ratio of zero means no errors.

R+G The - Y

ratio shows the range of tolerable set-

tings on the anomaloscope with the mean R-G proportion at 23 and the mean Y brightness at 55. (These numbers have only instrumental relevence.) The wavelengths indicate the pure-hue settings. N.P. refers to neutral points (i.e. a h match to an approximately 7500°K tungsten white). R.W.J., A.F. and R.H., as we have observed, show the coding; J.M., S.C. and M.S. show equally high amplitudes, but no coding; the results for T.P. fit neither pattern but this observer was somewhat tense during the runs and tension is well-known to inhibit these responses. The results for the deutan observers are shown in Fig. 16. The diagnosis is made on the basis of the orientation of the error axis on the IOO-hue test, the A.O.-H.R.R P/D ratio (i.e. D > P), the anomaloscope ratios (i.e. numbers less than 23 indicate high green values) and the wide anomaloscope ranges (e.g. observer R.T.). Four observers could find nearly acceptable neutral points. The 469nm value for observer R.T. may be only approximate. This observer’s severe dichromacy, as evidenced by the 100 per cent red-green range acceptable on the anomaloscope, ten&d to make him unsure in all his judgements. The N.P. for observer T.S. was not an exact match, this observer definitely being a trichromate. Generally, the monochromatic VEOGs for any one observer are characteristic of that observer, and, although occasional differences between the responses to the various wavelengths do occur, in no instance is there a progressive change in waveform that we could call a code. But, prima facie, the overall amplitudes are sign&antly reduced from the normal, for example: A.K. in both ON-OFF and AZ; T.N. and T.S. in ON; R.M. and R.T. in ON-OFF. A simple statistical test was done by taking the absolute value of the ON-OFF areas under the blue and the red curves shown for each observer in the two samples, normals and deutans, and running a parametric r-test on the significance of the difference between the mean areas. It was found that a difference such as we have obtained has a t-value = 2 x 10 -4. Finally, Fig. 17, shows comparable results for two protan observers and one tritan observer. Note the shift in the A.O.-H-R-R P/D ratio and the marked change in the Rayleigh equation for the protans; and the orientation on the lOO-hue for the tritanr4. Here the VEOGs of subjects Z.M. and SM. are very much reduced in amplitude, but subject C.P.-clearly protan-gives non-coded response amplitudes comparable to those of our non-coding normals. Summary of the results of experiments I,2 and 3 In order to make our comparisons and our discussion clear, we present together (Fig. 18) the ON-OFF data for a coding and a non-coding color normal, and that for two protans and two deutans. The difference between normals and deficients is impressive; moreover, the hindrance of the deutan observers is greater than that of the protan observers. 14 The tritan subject consistently missed one or two of the A.O.-H-R-R B-Y screening plates, as well as one of the %tartan” plates. He also failed some P and D plates, as the figure shows. His anomaioscope reading is deutan; his NO-hue error orientation as well as his major subjective complaint is clearly Man. His diagnosis is questionable because of these inconsistencies. Were he an acquired tritan. however, his vision should have been affected by this time, but he is entirely free of symptoms.

Spectral Analysis of the Visually Evoked Occipitogram in Man

425

. . , w

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Fro. 16. Monochromatic VEOGs in six deutan observers for ON-OFF, ON, and d1 stimulation, together with their scores on a select group of color vision tests (see text).

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Spectral Analysis of the Visually Evoked Oocipitogram

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T. SHIPLEY.R. WAYNEJONESAND AMELIA FRY

428

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DISCU!WON C~nsidc~ this VEOG color coding, which some color-normal observers display under our test conditions, and the sign&ant loss in overall response amplitudes from the deutan observers, it would seem that a rather complex relationship between the VEOG and both color evocation and receptor physiology is beginning to emerge. It is premature to speculate on the meaning of this relationship, but it may be helpful to try to formulate the directions which our work ought to take in the future: shorter flash duration; smaller, but not much smaller, field sixes; narrow band-pass amp&r-s; better isolated scalp electrodes; the use of random stimuli rates; and, most important, the use of some type of Ah technique instead of the AZ technique. This means that we should study: (1) waveforms evoked by intensity matched metameric colors in the normal eye and intensity matched confusion colors in the del%zient eye; and (2) waveforms evoked by intensity matched complementary colors, with various adaptation durations, so that the Ar\ can then interact with an after-image of identical hue. Then, since latencies are inherently more reliable than amplitudes, a simple method must be devised to allow us to study the time patterns of the individual wavelets as well as the amplitude patterns. The Fourier analysis is certainly the most revealing way to do both these things at once. We would like now, to make some comments on methodology. Fiiy, the success of scalp electrodes will always be limited until their responsiveness can somehow be restricted to voltage changes occurring in the cortex directly beneath. But even then, the inference to immediately subjacent cortical regions will not be unconstrained (cJ HEHH and GALBIU~TH,1966). Secondly, we can show that reliable non-random components in the VEOG can occur as late as 750 msec after a flash duration even as short as 10 msec. This is quite unlike the ERG, which is over long before this. Consequently, stimulus periods which are faster than 750 msec will simply mask these long latency VEOG wavelets. There are many papers in the literature which report VEOGs obtained with very high frequencies, even lOc/s or more, and the authors often fail to point out that their VEOGs are necessarily distorted from those in which the complete response is allowed to take its course. In most

Spectral Analysis of the Visually Evoked Occipitogram in Man

429

of the records which we have reported here, for example, we have excellent reliability up to 400 msec. Had we used a stimulus rate of 2/set or just faster, these latter components would have been masked. It is like studying the ERG with tlash periods shorter than that of the b-wave and then failing to point out that, thereby, the b-wave is distorted or seen only as a contaminant upon the succeeding u-waves. We think this point needs particular emphasis because we do not yet have any clear idea of the maximum latency with which some non-random wavelets may appear in the VEOG. We have cited the value of 750 msec because this has been our experience; others (e.g. in the auditory area-cf: footnote 12) have cited longer latencies, but no one has yet reported a systematic study on this issue. Thirdly, in our previous work (SHIPLEY, JONESand FRY, 1966), we have shown that the threshold after a 32 flash VEOG run does not change (within about O-1 log units) from what it was before the run; but we suspect that this might not be so if we extended our runs to include the greater number of flashes (100-200) typical of most other studies. For this reason, we have always sought to constrain the stimulus conditions so as to obtain the maximum amount of information from the minimum number of flashes. Observer fatigue, and eye-movement shifts, as well as retinal adaptation, are also important contaminating factors in long runs. One cau readily observe large alpha bursts between successive &tshes in many observers, especially near the end of a session. This is largely a fatigue effect, though also partly a consequence of our low flash rate. In any case, the longer event epochs invite alpha intrusion and, since both the EEG and the alpha characteristics vary from observer to observer, it is often difficult to specify what an appropriate control should be (as has been well described by PERRY, 1966). In our initial work, for example, the control was an N = 32 brain sweep taken with the subject in darkness, eyes either open or closed. (This did not seem to matter.) Controls were often obtained which were clearly contaminated by alpha. Such a control is shown in Fig. 19 as the non-random sweep. Subsequently we realized that in a true control the

100 r

NON-RANDOM

RANDOM

SWEEPS

SWEEPS

-looOI

TIME (msec) FIG. 19. The problem of the specithtion of an appropriate control.

T. SHIPLEY,R. WAYNEJONESAND AMELIA FRY

430

excitation condition should be exactly the same as that occurring during a run. From the observers’ point of view, nothing should be changed. This means that the only change must be a randomization of the brain sweeps with respect to the stimulus inputs. The subject still receives the 32 flashes in Maxwellian view. Such a control does indeed properly eliminate the alpha intrusion, as is also shown in the figure (as the random sweep). It seems to us, then, that the specification of an appropriate control is not a trivial matter, and that some serious consideration ought to be given to this whenever a new experimental procedure is introduced. And, lastly, we would like to emphasize that reliable data of the kind that we are trying to obtain cannot as yet be evoked casually from untrained observers. Consequently, it is difffrut to evaluate studies which are aimed at examining possible correlations between selected faotors and the VEOGs of large numbers of untrained observers (c_ WERREand SMITH, 1964; especially KOOI and BAGHI, 1964b, p. 267).1s This, of course, is the goal of muoh of our work, but it does seem to us that until our techniques for the evocation of these scalp corti~grams are very much improved, the va~ability in the evoked responses may well be so great as to speciously mask any relationship that they may have to possible correlative physical or physiological factors. This calls for some special reserve in the interpretation of negative tidings. Atiwlez&mear-We are particularly grat&l to our subjects in these experiments, primarily medical school staff and students, for their interast and their many car&U hours of observation. BALDWIN,I.

=REIwES T. and TINSLEY, R. W. (1965). Gamma-mowmen t and the pupil r&x.

Am. J. Psychof. 78,

-9.

BMW,

S. S. (1963). Yhjow o study o/its basis. H&m, N.Y.-London (Revision from 1941); especially pp. 156-165. B-T, J. S. (1964)‘ Mathematical analysis of averas response values for nonstationary data. IEEE Runs. Mwned. B~~QT.July, 72-81. c=ruu9, F, C. R. and ~&XX,J. (1965). Evoked potu~tiaIs and into&ewe. Li/t Scf. 4,1319-1322. EASON,R. G., Gmws, P., Wmrs, C. T. and GDBN,D. (1967). Evoked cortical potentials: relation to visual field and handadness. Science, N. Y. lS6, 1643-1646. G. C. (1966). mry evoked responses worded simultaneously from HBATH,R. 0. and Gm, human oartex and scalp. Nature, Land. 212, 1535-1537. HILL,D. and PARR,G. (1963). .&c~wacep~&grq~y. Macmillan, N.Y.C. Kcw, K. A. and B~osi~, B. K. (1964a). Obacrvation on early components of the visual evoked response and occipital rhythms. ~ct~F~. din. Neur~y~io~. 17,638-643. inman. Ann. N. Y. Acad. Sci. 112.254430. KOOK, K. A. mdBAmx, B. K. (1964b). Vi~~ok~~ AWRY, N. (1966). Signal verswsnoise in the evoked potential. S&we, N. Y. 153,1022. Pans, A. M. and NAGAYA,T. (1965). The use of the 0.06 degree red target for evahwtion of fovea1 function. Invesrve.Ophrh. 4, 302309. Roox~, E. A., Grusa~~,J. L., GUDOBBA, R. D. and ZACHARY,G. (1965). Relationship of EEG background rhythms to photic evoked responses. Ekctroenceph. clin. Nwophysiol. 19, 301-304. SAUNDERS,D. T. and LIZXE,E. (1967). Flash@ color and the electroencephalo@am. Newology 17, 157-161, 171. SHIPLEY, T. (1964). Rod-cone duplexity and the autonomic action of ii&. Vision Res. 4, 155-177. SHIPLEY, T. (1965). Some teaching demonstrations in vision. National Science Foundation Report #G22942. SHIPLEY.T.. Jo-. R. W. and FRY.A. (1965). . Evoked visual potentials and human color vision. Science, N. Y: lti, 1162; SHIPLEY, T., JON&S, R. W. and FRY, A. (1966). Intensity and the evoked occipito@am in man. Vision Res. 6, 657-667.

WALTERS, W. G. (1%3). Te~u~~fer~re~at~on. In Will and Parr (q.v.) Chap. 3, pp. 6.5-98. WERRB, P. F. and SMITH,C, 1. (1964). Variability of responses evoked by flashes in man. ~~ec~r~e~ce~~. din. Neuruphysiol. 17, 644652.

13The work of SAND= and LJXE (1967) is exceptional in this connection in that, despite the use of free-viewing,through both open end closed lids, and wide-band color filters, they neverthelessdo confirm the uniquenessof the red response.

Spectral Analysis of the Visually Evoked Ckcipitogram in Man Ab&act-The visually evoked occipitogmm was studied in two color-normal observers over the visible spectrum (380-700 nm) in 16 steps at matched intensities. Conl%ming earlier responses are also reported for other color-normal observcm, which ‘introduces a note of caution into our interpretation of these findings. These individual differencea persist for on-& on and oflevevocation. The VEOG responaca in several colord&cknt observers are also reported, indicating that an amplitude depression exists in the deutan group. Rcaprmc-on ttudie I’occipitogramme CvoquC visuelkment sur deux syiets normaux en vision dcs coukurs dans tout le specm visibk (380-700 nm) pour 16 longueurs d’onde g intens& &gales. En aamd avec lcs travaux ant&ion trouve que la forme dcs r&onses varie avrc la hgueur d’ondc. L’analyse de Fourier indique que ce codage chromatique cst mrtout cmccnti dam lcs compcwank3 4-8 c/s de I’onde. &pendant on constate aussi dca rbponses non cod&s pour d’autm sujets normaw cc qui introduit un tlbent de prudena dam notm intcrpr&ation des lkultats. ces diffti individu~ peEktent dans les hocations uwofl, m et ufl On phente ami les x+ponfss de plusieurs sujets d&cimts en vision des couleurs, avec I’indication d’une dbprcssion d’amplitudc dam le gnmpe dcs dcuthauomaux. ZdeDas

visuell~herv~me Oc+5pitogramm wurde bei zwei farbttkchti~ Beobachtcm fiber das sic&bare Spoktrum (3scrtoo nm) in 16 Stursn bci angqml3ten IntaIsitgtenuntczsucht.DabeiwurdatMLbaeBerichte,~sichdieweilulfomlin AbUngi&it von der We&&&c lndart b&U@. Dii Fowe& darauf hin, daBdiaa&oma&heGdiauaasicflhauntsgdG&aufdic4-8HzKommmcnteninden

431