Middle- and long-wavelength discrimination declineswith rod photopigment regeneration

Vicki J. Volbrecht,1,* Janice L. Nerger,2 and Armando R. Trujillo1

1Department of Psychology, Colorado State University, Fort Collins, Colorado 80523-1876, USA2College of Natural Sciences, Colorado State University, Fort Collins, Colorado 80523-1801, USA

*Corresponding author: Vicki.Volbrecht@ColoState.edu

Received May 25, 2011; revised August 18, 2011; accepted September 7, 2011;posted October 18, 2011 (Doc. ID 148175); published November 18, 2011

Hue-discrimination functions were derived from hue-naming data (480–620nm, 20nm steps) obtained in 4 minintervals from 4 min to 28 min postbleach at 10° temporal retinal eccentricity. Hue-naming data were also obtainedin the fovea. Hue-discrimination functions derived at the 4, 8, and 12 min intervals were very similar to thosederived in the fovea. As time postbleach exceeded 12 min and rod sensitivity increased, the shape of the hue-discrimination functions changed. Most notably, the minimum between 560–580nm disappeared and the justnoticeable differences (JNDs) for the longer wavelength stimuli increased. The long-wavelength suppression inhue discrimination may be due to rod input in the magnocellular pathway interacting and affecting the long-wavelength sensitivity of the parvocellular pathway. © 2011 Optical Society of America

OCIS codes: 330.0330, 330.1720, 330.5310.

1. INTRODUCTIONThere is a long tradition of measuring the just noticeable dif-ferences (JNDs) between two hues and deriving hue- orwavelength-discrimination functions [e.g., [1–5]]. Typically,in these experiments, a bipartite field comprised of two mono-chromatic stimuli (the standard and the variable stimuli) ispresented to the fovea, and the observer adjusts the lumi-nance and wavelength of the variable field until s/he perceivesa JND in hue between the standard and variable fields. Thedifference threshold is then plotted as a function of the wave-length of the standard stimulus. These hue-discriminationfunctions typically show two primary minima betweenapproximately 460–490 nm and 550–610nm with a primarymaximum between approximately 490–540 nm, contingent onobserver and stimulus parameters; and some early studies[2,3] also report two secondary minima between approxi-mately 410–450 nm and 620–640nm with a secondarymaximum between 600–630 nm. In general, though, the discri-mination function is shaped like a ‘w’.

Other researchers have also investigated changes in huediscrimination when stimuli are presented to the peripheralretina. Results have been mixed with some showing moderatedeterioration in hue discrimination with stimuli presented inthe periphery [6–9] while others report functions similar tothose obtained in the fovea [10,11]. Those studies showinga reduction in the ability to discriminate peripheral hues re-ported greater losses with reduced luminance levels [6–9]and/or increased retinal eccentricity [7–9] under dark adapta-tion [6,9] and light adaptation [6–8] conditions as well as inpostbleach measurements associated with the cone plateauof the dark adaptation function [9]. The discrimination losseswere greatest under the dark adaptation conditions. In each ofthese studies, however, the same size stimulus was used in thefovea as in the peripheral retina, even at eccentricities from25° to 70° along the horizontal meridian.

In general, the hue-discrimination functionsmeasured in theperipheral retina maintained the ‘w-shape’ first reported in fo-vealmeasurements [e.g., [1–5]], butwith larger JNDs. As retinalilluminance decreased, the primary maximum of the functionshifted to shorter wavelengths [6–9] and two studies showed aloss of the ‘w-shape’ in the far periphery (40–70°) of the nasalretina under light adaptation and cone plateau conditions,i.e., the hue-discrimination function showed a minimum at ap-proximately 470nm and dramatic increases in JND from 470 to500nm with JNDs no longer changing at approximately500–520 nm [8,9]. Thus, under the experimental conditionsfrom these two studies, discrimination in the blue-green regionof the visible spectrum(approximately 470nm)was quite good,but discrimination for middle and long wavelengths was com-promised in the far periphery.

In contrast to the Weale [8] and Stabell and Stabell [9] stu-dies, Van Esch et al.[11] found that the ‘w-shape’ of the discri-mination function could be maintained in the far periphery (upto 80° in the nasal retina) if the size of the stimulus was scaledaccording to the cortical magnification factor [12]. Further-more, Van Esch et al. [11] showed at 25° retinal eccentricity,if they used a stimulus smaller than that computed with thecortical magnification factor, the hue-discrimination functionlost its ‘w-shape’ and appeared more similar to those mea-sured by Weale [8] and Stabell and Stabell [9], as describedabove.

None of these studies, however, examined the transition ofthe wavelength-discrimination function as the amount ofavailable rod photopigment to capture light increases withpostbleach time. Using traditional psychophysical methodsto obtain hue-discrimination functions between the time per-iods associated with the cone and rod plateaus of the darkadaptation curve can be difficult and quite tedious. Fortu-nately, others have demonstrated that hue-naming responsesacquired in a scaling procedure can be transformed to gener-ate hue-discrimination functions [13–17]. These derived foveal

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hue-discrimination functions have been compared to fovealhue-discrimination functions obtained in the same laboratorywith the same observers using the method of adjustment[16,17] or to data collected in other laboratories using tradi-tional psychophysical procedures [13]. These comparisonsshowed that the derived discrimination functions from fovealhue-naming data closely resemble foveal data obtained withtraditional psychophysical procedures.

More recently, our laboratory [18,19] employed the hue-naming procedure to investigate changes in hue perceptionafter termination of a bleaching field in the peripheral retina,between and including time periods associated with the coneand rod plateaus of the dark adaptation function. In theseexperiments, a monochromatic stimulus is presented at a de-signated time period postbleach and the observer scales his/her perception of the stimulus by assigning percentages to thefour elemental hue terms (blue, green, yellow, and red) and tosaturation. As Abramov et al. [20] have noted, data can begathered quickly with this procedure and provide similar in-formation about the visual system as traditional psychophysi-cal procedures. Unlike our previous studies, which focused onhue-naming functions at shorter wavelengths [18] and uniquehue loci [19], this study examines changes in hue-discrimina-tion functions under mesopic conditions across a wide rangeof the visible spectrum and at various time intervals followinga rod-bleaching field.

2. METHODA. ObserversFour color-normal observers (three females, 21, 49, and 51years of age; one male, 22 years of age) served as participantsin this experiment. Color vision was assessed using theFarnsworth–Munsell 100-Hue, D-15 and desaturated D-15 pa-nel tests as well as the Neitz anomaloscope (OT-II). Observerswere practiced psychophysical observers but naïve with re-spect to their own data until all sessions were completed.Two of the observers (VV and LB) were aware of the purposeof the study and the other two (AK and KY) were unaware ofthe purpose of the experiment.

B. ApparatusThe optical apparatus is shown in Fig. 1. All stimuli were pre-sented in a three-channel Maxwellian-view optical system. A300W xenon (5500 K) arc lamp (S) regulated by a dc powersupply at 290W projected light from two ports of the lamphousing. Light exiting the first port formed Channel 1 of theoptical system, and a beamsplitter (BS) placed in the light pathoriginating from the second port created Channels 2 and 3 ofthe optical system. The light from the ports passed throughheat absorbing filters (HF) and was collimated. Light was thendirected through a series of focusing and collimating lenses(L), front-surface mirrors (M), and beamsplitters (BS). Alllenses (L) were achromatic doublets. In Channel 1, light wasfocused onto a grating monochromator (MN; Instruments SA,H-20, 4 nm half amplitude bandpass) to create the test stimu-lus (480–620nm, 20nm steps). Light exiting the monochroma-tor was then focused onto a 2:0 log unit neutral density wedge(W). This wedge along with neutral density filters (F) con-trolled the retinal illuminance of the test stimulus. A shutter(SH; Uniblitz, Model T132) controlled the duration of the teststimulus and a field stop (FS) determined the stimulus size. A

field stop (FS) in Channel 2 generated the broadband (5500K)fixation points. Neutral density filters (F) controlled the inten-sity of the fixation points. Channel 3 produced the bleachingfield (5500K). A field stop (FS) in Channel 3 defined the size ofthe bleaching field while neutral density filters (F) set the ret-inal illuminance of the field. Apertures (A) in Channels 2 and 3restricted light transmission to the arc of the xenon lamp. Abeamsplitter (BS) combined Channels 2 and 3 and a thirdbeamsplitter (BS) combined this new path with Channel 1.The light then passed through an artificial pupil (AP) and wasfocused by the final lens onto the observer’s pupil. The finalsize of the focused image was less than 2mm. A bite bar as-sembly permitted the observer to align to the optical axis ofthe Maxwellian-view system.

C. StimuliThe 500ms, 20 td monochromatic test stimuli measured 2:55°in diameter; this stimulus size was selected because previousresearch has demonstrated it is large enough to fill the percep-tive fields for blue, green, yellow, and red hues under darkadaptation conditions for a 20 td stimulus presented 10° inthe temporal retina [21]. The retinal illuminance of the stimu-lus fell within the mesopic range and is, therefore, within theresponse limits of both rods and cones [21–23]. As shown inthe inset of Fig. 1, three pinhole-size points in the fixationarray formed an isosceles triangle. The test stimulus was cen-tered between the two vertical fixation points. The observerviewed the third fixation point on the right so that the test sti-mulus fell 10° on the horizontal meridian of the temporal re-tina. When foveal measurements were made, the peripheralfixation point was covered and the observer fixated betweenthe two vertically displaced points. The observer set the ret-inal illuminance of the fixation array so that s/he could justdetect its presence and thus mitigate any adaptation effects[24]. The retinal illuminance of the 10° broadband (5500 K)bleaching field was 6:55 log scot td and was estimated tobleach approximately 85%-87% of the rod photopigment fol-lowing a 10 s exposure [25,26]. The observer fixated on thefar right fixation point before presentation of the bleachingfield; the bleaching field was centered with respect to theretinal location and due to its size extended beyond thetwo vertical fixation points.

Fig. 1. Schematic of three-channel Maxwellian-view optical system.Key: S, xenon light source; HF, heat absorbing filter; L, lens; MN,monochromator; W, neutral density wedge; FS, field stop; M, mirror;F, neutral density filters; SH, shutter; BS, beamsplitter; A, aperture;AP, artificial pupil. The upper right inset presents a schematic ofthe test stimulus and fixation point arrangement.

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D. ProcedureObservers commenced each test session by adapting to thedark for 10 min. Following dark adaptation, for peripheralmeasurements, observers viewed the bleaching stimulus.Upon termination of the bleaching field, test stimuli were pre-sented from 4 to 28 min postbleach in 4 min intervals. Stimuliwere, thus, observed at time periods associated with the coneplateau, intermediate times between the cone and rod pla-teaus, and the rod plateau of the dark adaptation function.Since the time of the postbleach test presentation was critical,the observer was encouraged to view the test stimulus onlyonce at each time period. If the observer was unable to makea judgment following stimulus presentation, the stimulus wasimmediately presented a second time. If the observer couldnot make a judgment after two presentations, the experimentadvanced to the next time period. There were only one to twoinstances across all observers and all conditions when a sti-mulus needed to be shown a second time and no instanceswhen the observer was unable to make a judgment afterthe second presentation.

Following presentation of the test stimulus, the observerused the “4þ 1” scaling method [27] to describe the color ofthe stimulus. First, observers assigned percentages to the fourhue terms—blue, green, yellow, and red—with the constraintthat the percentages needed to sum to 100%. Second, obser-vers rated the saturation of the test stimulus from 0% (com-pletely achromatic) to 100% (completely chromatic). At notime were the hue terms red and green or blue and yellowused simultaneously to describe a hue perception. The wave-length for the test stimulus was pseudorandomly selected fora particular postbleach time with the restriction that onewavelength did not appear more than twice in a given 28 mintesting interval. Three responses were obtained for eachwavelength at each of the seven postbleach times for each ob-server. Foveal judgments were obtained using the same pro-cedure with the exception that a bleaching field was notemployed.

E. Data AnalysisHue-discrimination functions were derived from color-namingdata according to the methods developed and outlined byAbramov et al.[16,17,20,27]. First, means of hue values foreach observer were calculated in the following manner:(1) An arcsine transformation was performed on each hueand saturation percentage. (2) For each hue judgment at eachwavelength, hue percents were scaled to the percent satura-tion so that the sum of the hue percents equaled the percentsaturation for that judgment and the ratio between the huepercents remained unchanged. (3) Means were computedfrom the three transformed and scaled hue judgments at eachwavelength for each time period, observer, and retinal loca-tion. (4) If there were more than two mean hue percentsfor a given wavelength (e.g., 70% green, 10% blue, 4% yellow),the smaller value was reapportioned into the two larger huepercents with the constraint that the hue ratio between thetwo larger percents was the same before and after this com-putation. This situation occurred for example when anobserver reported the stimulus as bluish green on one trialand yellowish green on another trial.

Second, mean values from each observer underwent asmoothing procedure: (1) A negative sign was arbitrarily

assigned to the blue and red percents while the percent valuesfor yellow and green remained positive, thus creating two di-mensions: yellow/blue and red/green. (2) New mean percentswere calculated at each wavelength for each observer by tak-ing the mean at that wavelength for a particular hue dimensionwith the respective percent at the adjacent wavelengths. Thepercents from the two adjacent wavelengths for each hue di-mension were weighted by a factor of 0.5 before computingthe new mean. [Note: Percents for 480nm and 620nm stimuliwere weighted by the percents of a single adjacent wave-length, i.e., 500nm and 600nm, respectively.] (3) Weightedmean hue percents as a function of wavelength for each ob-server and for each time period in the peripheral retina as wellas for the fovea were fitted with a cubic spline function. Per-cents for each hue dimension could then be ascertained in1 nm steps between 480 and 620nm. This analysis convertedthe data into the uniform appearance diagram (UAD) de-scribed by Abramov et al. [16,17,20] and created a city-blockmetric for defining distances between the various wave-lengths along each hue dimension.

Third, the wavelength discrimination function for each ob-server was derived from the UAD in the following manner:(1) At each of the eight wavelengths for each time period,the distance from the particular wavelength to its neighbor,1 nm away, along each hue dimension was determined usingthe city-block metric. For 480 and 620nm, there were two dis-tances, one for the yellow/blue dimension and one for the red/green dimension while for the other wavelengths there werefour distances, two for each hue dimension. (2) The reciprocalof the mean of the distances was computed for each wave-length at each time period as well as for each wavelengthin the fovea. (3) A criterion of four units [20] was selectedto define a JND in the UAD. This value was multiplied withthe reciprocal mean distance value to create JNDs that werecomparable to JNDs measured in nanometers from traditionalpsychophysical studies of hue discrimination. (4) The JNDswere specified as a function of wavelength, thus, generatinghue-discrimination functions for each time period in the per-ipheral retina as well as for the foveal data for each observer.

Mean JNDs were also computed from the individual JNDsfor each wavelength at each time period and in the fovea.

3. RESULTSFigure 2 compares individual hue-discrimination functions tomean hue-discrimination functions for the fovea (upper leftpanel) and from three time periods at 10° temporal eccentri-city along the horizontal meridian—a time associated with thecone plateau (8 min postbleach, upper right panel), a time be-tween the cone plateau and rod plateau (16 min postbleach,lower left panel), and a time associated with the rod plateau(28 min postbleach, lower right panel). In each panel thederived hue discrimination is plotted as a function of wave-length with the thick, solid line representing the mean hue-discrimination function across observers and the differentdashed lines denoting the four different observers; the errorbars represent �1 standard error of the mean (SEM) acrossobservers.

In general, the mean functions from the fovea and the 8 minand 16 min time periods show the traditional ‘w-shape’ withminima between approximately 480–500 nm and 560–580nmand a maximum at approximately 520nm. Because the

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shortest wavelength used in our study was 480 nm, the com-plete ‘w-shape’ of the function is not observed. At 28 min theshape of the hue-discrimination function changes with thedisappearance of the maximum at 520 nm and the minimumbetween 560–580 nm. This loss of the maximum and minimumis similar to what others [8,9] have reported for peripheralmeasures.

Figure 3 compares the mean hue-discrimination functionsfrom the seven postbleach time periods and the fovea. Forease of comparison across all time periods, the top panelshows the fovea (solid circles), 4 min (open circles), 8 min(open squares), and 12 min (open triangles) functions; themiddle panel replots the fovea (solid circles) and 12 min(open circles) functions with the 16 min (open squares)and 20 min (open triangles) functions; and the bottom panelreplots the fovea (solid circles) and 20 min (open circles)functions with the 24 min (open squares) and 28 min (opentriangles) functions. Error bars represent �1 SEM.

The top panel represents postbleach times associated withminimal rod input and there is little difference among thethree functions (open symbols). These three functions alsoare similar to the foveal function. The middle panel of Fig. 3illustrates time periods where rod activity is increasingas more rod photopigment has regenerated and is availablefor light capture. As can be seen, the three peripheral func-tions (open symbols) are most similar at the shorterwavelengths, but progressively diverge at the longer wave-lengths. As time postbleach increases, there are greater devia-tions between the peripheral functions and the fovealfunction. The bottom panel shows little difference betweenthe 20 min (open circles), 24 min (open squares), and 28 min(open triangles) functions, postbleach times associated withmaximal probability of rod input. All three functions no longer

have a minimum at approximately 580nm and show the great-est divergence from the foveal function compared to the otherpostbleach functions presented in the top and middle panels.

When the foveal function (solid circles) is compared to re-presentative time periods associated with the cone plateau orminimal rod input, 8 min function (open squares, top panel),and the rod plateau or maximal rod input, 28 min function(open triangles, bottom panel), the differences in the JNDsassociated with the shorter wavelengths (480–540 nm) arethe smallest. The mean of the absolute differences betweenthe 8 min and 28 min JNDs from 480–540 nm is 0:6nm whilethe mean of the absolute differences between the foveal and8 min JNDs for the same range of wavelengths is 0:75nm. Thegreatest differences among these three functions occur at thelonger wavelengths, 560–620 nm, where rod input has reduceddiscrimination at the longer wavelengths. The mean of the ab-solute differences between the 8 min and 28 min JNDs from560–620 nm is 2:5 nm while it is 0:35 nm between the fovealand 8 min JNDs across the same range of wavelengths. Lastly,one feature that is consistent across all three panels for allpostbleach times and retinal locations is that the JND at600nm is similar, but typically larger, than the JND at 620nm.

4. DISCUSSIONIn this study hue-naming measurements were obtained atpostbleach times associated with the cone and rod plateausof the dark adaptation function as well as at intermediatetimes. It was assumed that as time postbleach increasedand photopigment regenerated, rods would become progres-sively more sensitive and consequently alter the shape of thehue-discrimination function. As the results illustrate, theshape of the peripheral hue-discrimination functions are simi-lar to each other at 4 min, 8 min, and 12 min postbleach, times

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Fig. 2. JNDs derived from foveal and peripheral (8 min, 16 min, 28 min postbleach) hue-naming data are plotted as a function of wavelength.Different dashed lines represent each of the four observers. The bold solid line is the mean hue-discrimination function across observers. The errorbars are �1 standard error of the mean (SEM).

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associated with the cone plateau of the dark adaptation func-tion (see Fig. 3, upper panel). These three functions also re-semble the hue-discrimination function obtained in the fovea.Between 12 and 20 min, the hue-discrimination functions

enter a transitional phase as rod input increases, wherebythe minimum near 560–580nm commences to disappear withone minimum at approximately 480 nm (see Fig. 3, middle pa-nel). After 20 min postbleach, there is little change in theshape of the hue-discrimination functions (see Fig. 3, bottompanel).

A. Long-Wavelength InflectionAll of our mean hue-discrimination functions, both those ob-tained in the fovea and in the peripheral retina, displayed asecondary maximum at 600 nm and a secondary minimumat 620 nm. This does not seem to be a consequence of aver-aging individual hue-discrimination data since many of ourindividual observer discrimination functions also show theselong-wavelength maximum and minimum (see Fig. 2). Severalearlier studies reported similar inflections in foveal hue-discrimination functions (e.g., [2,3]; cf. [4,5]).

Thomson and Trezona [28] attributed the presence or ab-sence of the long-wavelength maximum and minimum tothe psychophysical procedure used to obtain data. In particu-lar, in the middle portion of the spectrum, observers typicallyadjust the variable stimulus starting with either a shorter orlonger wavelength than the standard and a mean is takenof the JNDs obtained when starting shorter and longer thanthe standard stimulus. At the shorter wavelengths and longerwavelengths JNDs may only be measured with the wavelengthof the variable stimulus set longer or shorter, respectively.Consequently, the mean JND for a long-wavelength standardstimulus would consist only of measures obtained with thewavelength of the variable stimulus set shorter than the stan-dard stimulus. The mean hue-discrimination function wouldthus represent a combination of the two different methodsof deriving the mean JND. Thomson and Trezona [28] sug-gested the inflection at the longer wavelengths representedthe location where one method of computing mean JNDsended and the other began.

In this study, hue-discrimination functions were derivedfrom hue-naming data and were constrained at the wavelengthend points used in this study, i.e., the distance between neigh-boring stimuli was computed in only one direction for the 480and 620nm stimulus while all other stimuli had neighbors inboth the shorter and longer wavelength directions. If thiscomputational difference contributed to the long-wavelengthinflection, then one might expect a similar inflection between480 and 500nm. This did not occur. Furthermore, the meanfoveal hue-discrimination functions from Abramov et al.

[16,17,20], derived in the same manner as ours from hue-naming data, do not show an inflection at the longer wave-lengths. It seems unlikely, then, that Thomson and Trezona’s[28] explanation can account for the long wavelength inflec-tions in our study.

One important difference between our study and others[4–10,16,17,20] is that our stimulus was larger. Our stimulussize (2:55°) was selected to fill all four perceptive field sizesof the elemental hues with rod input. The smaller stimulusfrom the other studies would not have filled the perceptivefield for green in the peripheral retina [21,23,29], which mayaccount for some of the differences in the long wavelengthfindings. The Van Esch et al. [11] study, also, used larger sti-muli and some of their observers show an inflection at ap-proximately 600 nm, but this inflection is not consistent

Fig. 3. Mean JNDs derived from peripheral hue-naming data areplotted as a function of wavelength. The top panel compares fovealand peripheral functions from the 4 min, 8 min, and 12 min post-bleach times; the middle panel fovea and 12 min, 16 min, and20 min postbleach times; and the bottom panel fovea and 20 min,24 min, and 28 min postbleach times. Solid symbols within each paneldenote the fovea and open symbols the different postbleach times.Error bars represent �1 SEM.

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across all eccentricities or with all observers. The significanceof the long-wavelength inflection remains unknown, but thedata from this study indicate it is real and warrants furtherinvestigation at even longer wavelengths.

B. Mediators of Hue DiscriminationEarly researchers [3,28,30] attributed the ability to discrimi-nate between wavelengths to differential stimulation of thethree cone photoreceptors, with the best discrimination(i.e., smallest JND) occurring where the relative sensitivitiesof at least two cone types are changing rapidly. For example,the spectral sensitivity of one cone type is rapidly increasingwhile another cone type is showing a decrease in sensitivity inthe same part of the visible spectrum. Consequently, a smallchange in wavelength produces a large change in the ratio ofthe neural signal between the two cone types, i.e. consistentwith the basis of the trichomatic theory of color vision. Thistheory as well as studies with tetrachromatic observers [e.g.,[31]] suggests that the presence of a fourth photoreceptor,such as the rods, could improve hue discrimination. Thus,at 28 min postbleach with maximal rod input, it might beexpected that JNDs should become even smaller between ap-proximately 500–560nm, the region of decreasing rod sensi-tivity and increasing M- and L-cone sensitivities. Instead,there is little change in JNDs between 500 to 540 nm for anyof the hue-discrimination functions, regardless of when theywere measured postbleach, and at 28 min postbleach there isan increase in the JND at 560nm (see Fig. 2). As Stabell andStabell [32] have shown, though, stimuli become desaturatedwith rod input and this loss of hue perception is greatest in theregion of the visible spectrum where the presence of the rodreceptor might be expected to improve hue-discrimination. Itis quite possible that this perceptual loss of overall hue (i.e.loss of saturation) counteracts any benefits that are derivedfrom the presence of a fourth receptor.

Others have suggested that neural processes fartheralong the visual pathway (postreceptoral) mediate hue-discrimination. DeValois et al. [33] demonstrated from singlecell recordings in the lateral geniculate nucleus (LGN) of themacaque monkey that hue discrimination is mediated by thechromatic opponent cell (red/green or yellow/blue) that ismost sensitive in that part of the visible spectrum, with thebest hue discrimination occurring where a cell’s responsechanges from excitatory to inhibitory or vice versa. For hue-discrimination functions derived from cellular responses, theshort-wavelength minimum was approximately 490nm, repre-senting the response crossover for the yellow/blue cells, andbetween 560–600nm the response crossover for the red/greencells. The minima in our foveal hue-discrimination functionsas well as those associated with the cone plateau postbeachtime period (e.g., 8 min) correlate well with these findings.This explanation does not, however, account for the loss ofthe minimum between 560–600nm as rod input increases withpostbleach time unless rods effectively inhibit the output ofthe chromatic opponent cells.

C. Rod Influence on Hue DiscriminationThe greatest change in hue-discrimination functions with rodinput occurs at the longer wavelengths (Figs. 2 and 3), result-ing in an increase in JNDs and a loss in discriminability.Helmholtz [34] wrote of a “red” blindness in the peripheral

retina, suggesting that rods induce a protanomaly. To test thishypothesis, we compared our individual hue-discriminationfunctions from 28 min postbleach (maximal rod input) to pro-tanomalous hue-discrimination functions reported by Nelson[35]. None of our four observers showed responses similar toNelson’s protanomalous observers, supporting the argumentof Stabell and Stabell [9] that the “red” blindness or suppres-sive activity of rods on L cones is not comparable to thatassociated with congenital protanomaly.

Suppressive rod-cone interactions (SRCI) have been de-monstrated between rods and L cones for detection of bothflickering [fSRCI; [36,37]] and grating [gSRCI; [38]] stimuli,but the neural mechanisms proposed for fSRCI and gSRCIare different. One of the distinguishing factors is whether anadapting stimulus presented to one eye can suppress the rodeffect in the other eye. Only the gSRCI is suppressed by di-choptic presentation of an adapting stimulus [39].

Sincemeasurements fromour studywere obtainedmonocu-larly, it would be difficult to surmisewhich physiological expla-nation from fSRCI and gSRCI studies could potentiallyexplain the long-wavelength suppression observed in our hue-discrimination functions with rod input. Gur [40], however,measured hue-discrimination functions in the traditional man-ner using a bipartite field. In one condition, both halves of thefield were presented monocularly to the same eye; and in theother condition, eachhalf fieldwaspresented to adifferent eye.He found that there was no difference in hue-discriminationfunctions obtained in the monocular and dichoptic conditions.This finding suggests, then, the neural explanation of gSCRImaybe a candidate to explain the long-wavelength suppressionobserved in our hue-discrimination functions.

Lange et al. [39] has proposed for gSRCI that the magnocel-lular pathway influences the parvocellular pathway at the cor-tical level where inputs from the two eyes are combined.While both rods and L cones provide input to the parvocellularpathway, the rod input is rather weak in the parvocellularpathway and, if present, is often at retinal illuminances lowerthan those used in this study [41–44]. Rod input is more ro-bust, however, in the magnocellular pathway and occurs athigher retinal illuminances, comparable to the 20 td used inthis study [41–44]. Taken together, it is possible, then, that inthe cortex, where information is combined from both eyes, themagnocellar pathway with rod input is suppressing or redu-cing long-wavelength input from the parvocellular pathway.

D. SummaryResults from our study reveal systematic changes in huediscrimination as rod photopigment regenerates, in particularthe loss in hue discrimination at the longer wavelengths withincreasing rod activity. Based on these results as well asprevious research, we suggest that rods may suppress long-wavelength input to the parvocellular pathway via the magno-cellular pathway.

ACKNOWLEDGMENTSWriting of this manuscript was partially supported by NationalScience Foundation (NSF) grant 1 127 711 to VJV and JLN.

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