JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Interactions Between Photopic Visual Mechanisms Revealed byMixing Conditioning Fields

ROBERT M. BOYNTON, S. R. DAS, AND JEAN GARDINER

Center for Visual Science, University of Rochester, Rochester, N. Y. 14627

(Received 9 March 1966)

The luminances of conditioning fields that are necessary to reduce the visibility of a previously supra-threshold test flash to an increment threshold are determined for each of two conditioning fields of differentcolor. Normalizing each of these luminances to 1.0, subthreshold amounts (e.g., 0.8, 0.6, etc.) of one fieldcomponent are preset by the experimenter; the subject then adjusts the other component until thresholdis again attained. In an eye for which the adaptive state is controlled by the action of light upon only onespectral class of photopic mechanism, the normalized luminance sum must always equal 1.0. We find howeverthat this sum is consistently less than 1.0, indicating a supersummative interaction of adaptive effect be-tween two or more different mechanisms.INDEX HEADINGS: Vision; Color vision.

TO derive the characteristics of his photopic in-Tcrement-threshold "mechanisms," Stiles'- 6 hasmade use of a working assumption that the thresholdis determined by the most sensitive of these mecha-nisms, provided that it is operative under the prevailingcondition of adaptation (which may include zero), inresponse to any test wavelength. This assumption isprobably safe when one mechanism has a substantiallylower threshold than the other. For example, in Fig. 1for low field radiances, the threshold of Ml is shownto be about 1.7 log unit lower than that of M2 , whichmeans that only about 2% as much light is effectivelyabsorbed in M2 as is absorbed in Ml. Even if the out-puts of these two mechanisms were to summate linearly,the extra 2% contributed by M2 to the total responsewould not lower the threshold by a measurable amount,compared to the threshold of an eye containing Mlalone.

In Fig. 1, the log threshold vs log radiance (tvi)curves of the two mechanisms cross at an intermediatefield radiance. An arrow indicates a region where thetwo mechansims have equal sensitivities, or nearly so.Here we may reasonably question the likelihood that,with ascending field radiances, the threshold is deter-mined by M, alone until a value is reached (the cross-over point of the two curves) where M2 suddenly takesover. Stiles",3 has long been aware that this is not likely.Even if M, and M2 were physiologically and statisticallyindependent, probability summation would produce anexperimental threshold about 0.10 to 0.15 log unitlower than the theoretical crossover level. If the out-puts of the two mechanisms were to summate linearly,the experimental threshold would be 0.3 log unit lowerthan the theoretical crossover. This is what is shown in

X W. S. Stiles, Proc. Roy. Soc. (London) B127, 64 (1939).2 W. S. Stiles, Proc. Roy. Soc. (London) B133, 418 (1946).3W. S. Stiles, Documenta Ophthalmol. 3, 138 (1949).

W. S. Stiles, Rev. Opt. 28, 215 (1949)."W. S. Stiles, in Union Internationale de Physique pure et

appliquee. Coloquio sobre Problemas Opticos' de la Vision,(1953), p. 65.

6 W. S. Stiles, Proc. Natl. Acad. Sci. U. S. 45, 100 (1959).

the figure. From the standpoint of fitting M, and M2to the experimental curve-a problem that Stiles hasoften dealt with-the discrepancy is not too seriousin a case like this where M, and M2 are grossly dis-placed in tvi space. But in a case like that illustratedin Fig. 2, the experimental curve would have a smooth,unitary-appearing shape whatever rule of interactionapplied between the two mechanisms, shown here as2r4 and ir5 (the "green" and "red" mechanisms) forwhich this problem of analysis very definitely exists(see Stiles6 , p. 90).

From another point of view, the rules of interactionare of interest because if known they would tell us some-thing about the degree of independence, or lack of it,between mechanisms of differing spectral sensitivityat low levels of stimulation. An experiment by Boynton,Ikeda, and Stiles7 has been reported previously in whichsuch interactions were examined. The strategy of theexperiment, which is very similar to that reported here,is the following: For a given adapting condition, de-termine separately the thresholds for test wavelengths

1<,

C 0

-c

0.-D

0

2

0

N

K2 0 2 4 6

log field radiance (pul

FIG. 1. A threshold vs radiance (tvi) curve for two mechanismsMl and M2 with spectral sensitivities that are well displaced,and Weber fractions that are substantially different. The arrowindicates the restricted range of field radiances where the sen-sitivities of the two mechanisms are nearly equal, and wheresubstantial interaction effects most likely would occur.

7 R. M. Boynton, M. Ikeda, and W. S. Stiles, Vision Res. 4,87 (1964).

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VOLUME 56, NUMBER 12 DECEMBER 1966

I I I I

B OYNTON, DAS, AND GARDINER

XI tand X2. Call each of these lumiinances 1.0. Then ad-ditively mix the two test wavelengths in various pro-portions of their threshold amounts. For example, if0.5 units of each component were mixed together, thecombined flash would still be at threshold onlv if theindividual contributions were to summate linearly.Typically, such a combined flash was found to be belowthreshold, and the experimental procedure was thento increase the luminance of each component whileholding the ratio at 1:1 until a new threshold was ob-tained. This procedure was repeated for several otherratios, for many combinations of wavelengths, and fortwo principal conditions of adaptation. It was laterextended to other adapting conditions by Ikeda.5

Although the results of. these experiments were verycomplex, they may be summarized as follows. (1) Inter-action, and not independence among mechanisms is therule: probability summation is seldom observed. (2)This interaction is very often inhibitory; more light isrequired in the mixture at threshold than probabilitysummation would predict. (3) The interaction is some-times summative; most often, but not always, this

I I.- I -. I -1 - - 11!---- 1 ! 1 .1 -. - -- -occurs 11uponentsnism. BPconditiorstimulat(is neverthe mixtresponse.

Alpernment wiiled themamong S482) ".indepencitest flasi

C

- .0

01

4

2

0

Fio. 2..identical Isitivities.mechanisrhigher one

8 M. Ik(9 M. Ali

//I--Condifion, eq -tg

| Stimulus t -

/ I

TestISliuu dv -~a - I\ I

(c) in -

ThresholdLevel

O~rj

Sleesetlus C vt

(d) I

FIG. 3. Models to illustrate alternative conceptions of adaptationof photopic mechanisms (left) and of the reaction of these mecha-nisms in response to a test stimulus (right), where the output of amechanism or mechanisms must reach some level (indicated bythe dotted line) in order to produce a threshold response. Themodels at the top half of the figure are free of interactions be-tween mechanisms: the adaptive loops (arrows) are strictly withinmechanisms; the threshold model at the top right assumes thatonly the most sensitive, and therefore the most excited, mechanismcontributes. The models at the bottom permit complex inter-actions among mechanisms to occur.

ander conditions where light trom the two corm- raises this threshold only by stimulating 7r5 in the sur-iS likely to be absorbed by a common mecha- round. The extent to which 7r4 and irn are also stimulatedit summation was also found to occur under is quite irrelevant. Similarly if the test excites 7r4 or i7ris contrived to allow the two test flashes to at threshold, the after-flash effectiveness depends solely

; separate mechanisms. (4) Supersummation upon the stimulation of 7T4 or ir, in the surround."found. That is, less than 1.0 threshold unit in They also feel that this statement applies equally wellture flash never suffices to produce a threshold to the steady-state condition used by Stiles.

Their model is represented in Fig. 3(a) and (c). Intand Rushton0 recently reported an experi- Fig. 3(a), light from the conditioning stimulus is ab-:h "after flashes" as conditioning fields, which Zto conlude that -. sorbed in different amounts in three mechanisms, ac-to conclude that complete independence exists cording to their spectral sensitivities at wavelength A.

;tiles's photopic mechanisms. They state: (P. The small arrows running out of and back into the. each of the Stiles's colour mechanisms acts boxes represent an adaptive effect which somehow re-

Lently in these after-flash experiments. If the duces the sensitivity of each mechanism. The adaptivei at threshold excites irs, then the after-flash effect is specific within each of the three mechanisms,

meaning that light absorbed in one of them has noeffect upon the sensitivities of the other two. In 3(c),the test stimulus has a similar independent action. Italso stimulates all three mechanisms, generally in adifferential way. In the example shown, the threemecha-nisms are assumed to have had their sensitivities setby the adapting stimulus in the manner of 3(a).Mechanism 7r5 has a greater sensitivity to X than do 7ri

or or2; therefore its output (shown as the longest dotted74 arrow) is greater. Threshold is reached when the out-

put of 7ri attains a critical level. The subthresholdoutputs from 7r, and 7r2 are assumed to have no effect.

-2 0 2 4 It is possible to imagine a much more complex situ-log field radiance (p.) ation, alongthelinespreviouslyconsideredbyBoynton."0

The results of the Boynton, Ikeda, and Stiles experi-A tvi curve for two mechanisms, 7r4 and 7r5, which have t7 .imiting Weber fractions and overlapping spectral sen- ment7 fit into the scheme illustrated in Fig. 3(d), sug-Here the possibility of interaction between the two gesting that threshold depends upon the level of outputns is very great at all field radiances-particularly the following some interaction among the mechanisms.

*5 Therefore it is not safe to assume that only the most.da, Ph.D. dissertation, University of Rochester (1962).iern and W. A. H. Rushton, J. Physiol. 176, 473 (1965). 10 R. Mi. Boynton, J. Opt. Soc. Am. 53, 165 (1963).

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December1966 INTERACTIONS BETWEEN

sensitive mechanism determines threshold: thresholds,compared to those determined by the more sensitivemechanism alone, may in fact be raised or lowered as aresult of light absorbed by another mechanism in sub-threshold quantities, amounting to one-fourth or evenless of threshold intensity. In Fig. 3(b), the feedbackwhich sets the sensitivity of each mechanism comesnot only from the mechanism itself, but also from theother two classes of mechanism. The conclusion7 thatthis complex system exists clearly contradicts the con-clusion reached by Alpern and Rushton.

We are left with the following situation. Alpern andRushton7 suggest that models (a) and (c) are correct.7

Boynton, Ikeda, and Stiles7 have strong evidence thatscheme (d) is generally correct, although (c) may applyin special cases. The purpose of this experiment is to usethe mixture method to examine scheme (b), to testpossible interactions among adaptive mechanisms. Con-ditions of wavelength, adapting luminance, field size,and flash duration have been deliberately chosen tomakes the conditions comparable to those used byAlpern and Rushton.

Experimental Conditions

The conditioning field was circular, with a diameterof 100, and was presented continuously. It consistedof various additive combinations of the following wave-lengths: 625, 527, and 456 nm. The test field was 10square, centrally fixated, and exposed for 10 msec. Itconsisted of 600, 500, or 580 nm.

Experimental Procedure

Our procedure was as follows: (a) The absolutethreshold for a given test wavelength was determined.(b) The test luminance was set at either 4(log=0.6)or 25 (log= 1.7) times the absolute threshold, not to bevaried within an experimental session. (c) The subjectadjusted the luminance of the conditioning field so asto cause the test flash to be at its threshold. Very greatcare was taken to ensure that the adaptive state of theeye had stabilized at the final level set, since failure toallow sufficient adaptation time could produce sub-stantial artifacts with this method.

In a second set of experimental sessions, mixtures oftwo conditioning fields were used. Discussion of therationale for this, and the method of data analysis, ispostponed to a later section.

APPARATUS

The apparatus used was a five-channel maxwellian-view system. This equipment, and the procedures usedto calibrate it, have been described in detail by Ingling.1'The source is a 900-W high-pressure xenon arc, whoseimage is brought to a focus thrice in each channel; the

'C C. Ingling, Ph.D. dissertation, University of Rochester (1966).

PHOTOPIC MECHANISMS 1777

final images in all five of the channels are superposed atthe pupil of the eye. An interference wedge is locatedat the first image in each channel, a variable neutral-density wedge and solenoid shutter is near the secondimage in each. The solenoid shutter is controlled by anelectronic timer; the characteristics of the test-flashexposure were calibrated by use of a photomultipliertube at the subject's eye position; its signal was de-livered to an oscilloscope. Luminance was calibrated fora single wedge position in one optical channel, by flickerphotometry against a diffuse field lit by a calibratedlamp to a luminance determined by use of the inverse-square law. The "neutral" wedges were calibrated byrelative radiometry with a vacuum thermopile at every0.50, using a reference wavelength of 540 nm; the de-parture from neutrality was determined separatelyfor each wedge so that wedge densities could be readfrom tables of values appropriate to 540 nm, and thencorrected by applying a single multiplicative factorappropriate for the wavelength actually used. For thepresent experiment, the first and second channels wereused as conditioning fields, the third channel deliveredthe test flash, the fourth channel provided the fixationpoint, and the fifth channel was not used.

b V,,,4 , 5,

2-J

500 600 500 600Xt(nrn)

FIG. 4. Results of single conditioning fields for subject SRD(left) and RMB (right). The upper families of curves relate topoints obtained for the low-threshold criterion, the lower familyto the high criterion. The curves are for Stiles's 7r4 and 1r5 mecha-nisms, displaced arbitrarily in the vertical direction. The symbolsencode the wavelength of the test stimuli used: * O-X = 600 nm:*ECl-X =S500nm; *AA-X=S580Onm.

1777

1778BOYNTON, DAS, AND GARDINER

-J

FIG. 5. Results of mixture experiment for subject SRD, forX=625 nm and 527 nm. The upper curves show the logarithm ofthe normalized luminances of the two mixture components (Siand S2) when the increment threshold for X=600 nm, for thehigh criterion, was achieved. X: 625 nm; 0: 527 nm. The bottompoints (A), falling along a horizontal straight line, show that thelogarithm of the total intensity in the mixture (2) is always zero(left ordinate) and that the summation index (a) is 0.3 (rightordinate). All values are plotted as a function of the logarithmof the mixture components.

Results: Single Monochromatic Conditioning Fields

The results for the experiment using single mono-chromatic conditioning fields are shown in Fig. 4 forsubjects SRD and RMB (left and right, respectively).Some of the thresholds shown for SRD are from an ex-periment explicitly designed to test identity of curveshapes, others are from the mixture experiment to bedescribed, for those conditions where only one com-ponent of the mixture was used. Where three valuesare shown, the third is from a supplemental experi-ment. The three points in these cases then representvalues taken on different days, where at least two con-ditioning-field wavelengths were used within each experi-mental session. The values for RMB were obtainedexclusively from special conditions within the mixtureexperiments, where only one component of the mixturewas used.

The ordinate of these graphs represents relative spec-tral sensitivity, plotted as the logarithm of the reciprocalof the relative radiance vs wavelength on a log nmscale.' 2 There has been no normalization of the curves,so all values are comparable to one another, within andbetween subjects. The upper curves in each figure showthe reciprocal of the radiance required for a conditioningfield of the wavelength indicated on the abscissa toelevate the test threshold by a factor of 4 (0.6 log unit).The symbols indicate the wavelength of the test flashused in each case. The lower sets of curves are for testflashes set at 25 times (1.7 log unit above) their ab-solute thresholds; since more conditioning-field radianceis required to raise the thresholds to this higher level,these spectral sensitivity curves are lower.

The experimental points through which the uppersets of curves were drawn are fitted quite well by 7r5,

12 R. M. Boynton, J. Opt. Soc. Am. 53, 641 (1963).

for either of these two test fields. The results for a testwavelength at 580 nm are also fitted by i5; the absolutesensitivity was slightly lower than for 600 nm for bothsubjects. The curve for Stiles's 714 is also shown, andfits nothing. One very low point for subject SRD, fora test flash of 600 nm and a conditioning field of 456nm, indicates that his sensitivity for shortwave con-ditioning stimuli is considerably lower than either 7r4 or7W would predict. We can conclude from this that, forthe long-wave conditioning fields at least, the criterionof spectral sensitivity would indicate that all three testfields stimulate 7r5, and that 7'4 is not importantlyinvolved.

The lower curves, for the high-threshold criterion,tell the same story for test flashes at 600 and 580 nm.But for both subjects, 7ri fails to fit the data for testwavelength 500 nm. The sensitivity function for SRDis narrower than 'ri, but broader than 7rN. For RMB,714 comes close to the experimental points but is stilltoo narrow, while as previously noted, 7'5 provided agood fit for the low-threshold criterion. Therefore, whenthe test wavelength is 500 nm, we find clear evidencethat there must be a change of the shape of the tvicurve between the two threshold levels tested, sinceotherwise the same sensitivity functions would neces-sarily fit the data at both levels of test-flash luminance.For the 600 nm test flash, the 7r5 function fits the datafor both conditions of the experiment.

Results: Mixture Experiments

In the case of an eye containing only a single spectraltype of mechanism, exactly equivalent effects can beobtained with conditioning fields of differing wave-length, provided that adjustments are made for sen-sitivity differences. If threshold is determined by onlythe most sensitive mechanism and there are no adaptiveinteractions of the sort suggested in Fig. I (b), this kindof stimulus substitutability should hold. A critical testof the noninteractive model is therefore to determine theluminance of each of two conditioning fields, when usedalone, to produce the criterion effect, and then to mixthese two conditioning fields together in various ratios.

03 0.6--10 -08 -0.6 -04 -0.2 0 02 04 0.6 0.8 1.0 1.2

FIG. 6. Same plot as Fig. 7, for RMB.

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December1966 INTERACTIONS BETWEEN

This is the method previously applied by Boynton,Stiles, and Ikeda7 using test-flash mixtures, appliednow instead to conditioning-field mixtures. It is alsoessentially similar to the method used recently byGuth,1 3 who used the absolute threshold of the mixtureas his response criterion.

If we let Lio be the threshold luminance of Stimulus1 when presented alone, and if L 2 0 is the thresholdluminance of Stimulus 2 when presented alone, and ifLim and L2AI are the luminances of the two stimuli atthe threshold of the mixture, new units are defined,called S units, such that

S= LlM/Llo

S2= L 2m/L 20

and a summation index is defined as o=0.3-log(S1+S2)Readers are referred to the paper of Boynton, Ikedaand Stiles, for further details.7 In words, we use Si andS2 to specify the luminance of each component in termsof its threshold when used alone, and then determinethe sum of S units required in various mixtures to pro-duce the criterion effect of raising the test thresholdeither 0.6 or 1.7 log unit. A sum of S and S2 eithergreater than or less than 1.0 implies an interactionamong mechanisms. A sum less than 1.0 indicatesenhancement.

The procedure here was to determine the thresholdsof each component alone three times within each experi-mental session. Subthreshold amounts of one or theother component would then be set by the experimenter,following which the subject adjusted the other com-ponent until a new threshold was obtained. The orderin which the various subthreshold amounts were intro-duced was varied randomly.

The results for SRD and RMB for X = 600 nm (high)are shown in Figs. 5 and 6. In this case, mixtures of625 and 527 nm conditioning fields were used. In theupper parts of the figures, the X's show the logarithmicamounts of the 625-nm component in the mixture, theO's show the 527 nm component. The vertical separa-tions between the curves represent the logarithms of theratios of the two components, and all values are plottedas functions of the logarithm of that ratio. In thelower part of the figure, the summation index is shownon the right, and the logarithm of the number ofthreshold units in the mixture is shown on the left, alsoas function of the logarithm of the ratio of the luminanceof 625 nm to 527 nm in the mixture. All of these curvesexhibit, in different ways, exact substitutability andcomplete summation: One componentcanbe substitutedfor the other in any ratio whatever without changingthe effect of the mixture upon the test threshold. Thisis powerful evidence, for this condition, that only oneadaptive mechanism is at work. On the basis of thespectral sensitivity data presented earlier, this mecha-nism is clearly 7r5.

13 S. L. Guth, J. opt. Soc. Am. 55, 718 (1965).

PHOTOPIC MECHANISMS 1779

& A "A A

A A

-12 -06 0 06 12log r

FIG. 7. This figure corresponds to the bottom parts of Figs. 6and 7. Except for k and 1, X1=527 nm and X2= 6 2 5 nm. SubjectSRD is represented by the open triangles; RMB by the filledones. The experimental conditions represented are (values in nm):

a and b: X=600 (low); c and d: X=500 (high);e and f: X = 500 (low); g and h: X = 580 (high);i andj: x=580 (low); kandi: X=600 (high),

(pj=456, andu p= 6 00 nm).

To save space, the S-unit functions are not shownfor the remainder of the data to be presented. In Fig.7, these results are shown in a single graph, where eachpanel corresponds to only the bottom part of Figs. 5 and6. Fig. 7(a) shows what happens for X= 600 nm (low),for the same two conditioning-field mixtures. For SRD,the required number of S units for the mixture isgenerally less than unity, often about 0.05 log unit,of 12% less than unity. RMB behaves differently: hisS units sum to about 0.1 log unit more, or 25% morethan unity for an equal mixture of the two components,but about 0.10 log unit less for the unbalanced mixturestested. It is seen from the remainder of Fig. 7 that theresult for SRD in Fig. 7 (a) is typical of results for mostconditioning fields and both subjects, and that thethree points for RMB in Fig. 7 (b) showing a summationindex less than 0.3 are among the very small number ofresults below this value obtained in the entire experi-ment. Despite the differences between the two subjectsunder this condition, it is clear that for both, morethan a single mechansim must be at work. Thus, the

B OYNTON, DAS, AND GARDINER

fact that the 7r, cCurve fits the data for each componentalone cannot be taken as proof that n1o adaptive inter-action occurs, even for this test wavelength.

For a test wavelength of 500 nm (high), using thesame conditioning-field mixtures, both subjects agreewell and show that about 25%o fewer S units are neededin the mixture to produce the criterion effect, than foreither component alone. These results are shown inFig. 7 (c) and (d). This would well support the idea thatmore than one mechanism determines the threshold.Figs. 7 (e) and (f) show equivalent data for a test wave-length of 500 nm (low). Here, RMB shows little dif-ference from the high condition, while SRD shows moreor less linear summation. Judging from the earlier sen-sitivity data, this would suggest that 7r5 mainly de-termines threshold for SRD for this condition, andthat or. and 7r5 are both involved for RMB.

Figs. 7 (g) through 7 (j) show that at least two mecha-nisms determine the threshold at 580 nm, for both lowand high criteria, even though the sensitivity dataagree well with 7r5. Again, the effect of the mixture ismore powerful than is either component alone.

Finally, 456 nm was mixed with 625 nm, instead ofthe 527-625 mixture shown in all other curves. For this,the effect is moderate for RMB and spectacularly largefor SRD, for whom an equally balanced mixture is morethan 140% as effective as either component used alone.These results are shown in Figs. 7 (k) and 7 (1).

Discussion

These results show that the orange test flash at X = 600may be considered to stimulate only 7r5, and that onlylight absorbed by 7r5 is an important determinant ofthe increment threshold, provided that the thresholdcriterion is 25 times (1.7 log units above) absolutethreshold, and the conditioning fields are of wavelengthslonger than 527 nm. But for the lower criterion, despitethe fact that all data for unitary conditioning stimuliare well fitted by 7r5, there is a clear interaction effectthat means either (a) that 7r5 is not a unitary mechanismand/or (b) a second mechanism is affected by the con-ditioning field and influences the threshold. For thehigh criterion, when a blue (456 nm) conditioning fieldis used, the interaction effects are severe and the sen-sitivity data do not fit 7r5 at all well.

; i f , * , . - % .-* Ib I s at i t 1 '~ 4 'ts r C a1 AiC': 3X

For X=500 nm, the sensitivity measured is morenearly that of 7r5 than that of 7r4: indeed, 7r5 fits thedata very well for unitary conditioning fields requiredto produce the low-threshold criterion. The interactioneffects found with the mixture stimuli show clearly thata unitary mechanism is not at work. The reason whythe 7r5 sensitivity fits the results for the 500-nm testflash is almost certainly that a brief duration of thetest flash was used. Ikeda and Boynton"4 have pre-viously shown how spectral sensitivity may be alteredby changes of test-flash duration, and have sought toexplain this in terms of different threshold-vs-durationcurves for the different mechanisms underlying thethreshold. Even for Stiles's condition of a 200-msectest flash, r.4 sensitivity exceeds that of 7r5 at X= 500 nmby only about 0.3 log unit: this advantage is evidentlywiped by out using so short a test flash.

These results also strongly suggest that Stiles's 74

and 75 mechansims are not unitary mechanisms. Rather,they appear to be made up of underlying mechanismswhose spectral sensitivities remain reasonably well fixedwith respect to one another, but the interaction be-tween these components clearly shows up in the mixtureexperiments (see also Boynton'"). Threshold is by nomeans determined solely by the most sensitive mecha-nism; when the field-sensitivity method is used, thereis an enormous amount of summation of adaptive effectfrom the component mechanisms. In almost all casesit exceeds complete linear summation. The mixturecurves show many instances in which the thresholdamount of the dominant component of a mixture islowered 10 to 15 percent by the addition of as little asT-6 of the threshold amount of the secondary component.Thus, even when the spectral sensitivity of a secondmechanism is y or less than that of a primary mecha-nism, the adaptive contribution of its activity cannotbe neglected.

Finally, it seems probable from these results thatinteractions of the type schematically shown in Fig.3(b) must now be considered, along with those pre-viously demonstrated in Fig. 3(d). Adaptative effectsare by no means restricted to what goes on withinindividual types of mechanisms.

14 . Ikeda and R. M. Boynton, J. Opt. Soc. Am. 52, 697(1962).

Joseph Reader, NBS, and Sumner P. Davis, University ofCalifornia and an Associate Editor of the Journal, at the 50thAnniversary Meeting of Optical Society, Washington, D. C.

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