Vision Rev. Vol. 32, No. IO, pp. 1947-1953, 1992 Printed in Great Britain. All rights reserved
Copyright
0
0042-6989/92 85.00 + 0.00 1992 Pergamon Press Ltd
Color Mixing in the Pigeon (Columba livia) II: A Psychophysical Determination in the Middle, Short and Near-UV Wavelength Range ADRIAN
G. PALACIOS,*,t
FRANCISCO
J. VARELA*
Received 5 March 1991; in revised form 28 January 1992
Pigeons were trained to discriminate between spectral lights and additive mixtures in the 350460 nm spectral range using a successive “autoshaping” discrimination procedure [introduced in Palacios, Martinoya, Bloch & Varela, Vision Research, 30,587-596 (1!290)]. Dichromatic mixtures were found in the short and near UV region, but not in the middle-wave region. Our results suggest that color vision in the pigeon involves the active participation of five different primary mechanisms, which are differentially active in the yellow- and red-sensitive retinal fields. Animal psychophysics
Pigeon
Color matching
UV vision
INTRODUCTION
The multiplicity of cone photopigments and oil droplets present in the avian retina poses a challenging problem for the characterization of the color space of birds. For our experimental animal, the pigeon (Columba liviu), absorption spectra obtained by microspectrophotometry provide direct evidence for three cone photopigments, with maximum absorption (A,,,) at 460,514, and 567 nm (Bowmaker, 1977). The presence of several kinds of carotenoid-containing oil droplets in the inner segments of avian cones adds a further complication, as these organelles function as long-pass spectral filters. Bowmaker (1977) showed that the three visual pigments he found were paired with oil droplets in such a way as to produce more than three spectral classes of cones. Evidence for the presence of two additional spectral types of cone comes from electrophysiological measurements of spectral sensitivity peaking at 370 and 415 nm, in the near-UV and short wavelength region (Norren, 1975; Govardovskii & Zueva, 1977; Chen, Collins & Goldsmith, 1984). The pigeon therefore possesses at least six spectral cone mechanisms in both the frontal (red) and lateral (yellow) retinal regions, as summarized in Fig. 1 (A-C). Little is known about the behavioral significance of all these potential primary chromatic mechanisms (Varela, Palacios & Goldsmith, 1992). In Palacios, Martinoya, Bloch and Varela (1990b) we presented a psychophysical study of color mixture in the long-wavelength spectral range of the pigeon (58&640 nm), showing that pigeons require at least two *Institut des Neurosciences, Universitk de Paris 6-CNRS, 9 quai Saint Bernard, 75005 Paris, France. tTo whom all correspondence should be addressed at: Department of Biology, Yale University, New Haven, CT 06520, U.S.A. “R
32:,&G
Visual pigments
Pentachromacy
(and possibly three) primary mechanisms in this narrow range. The purpose of the present paper is to continue the characterization of the color space of the pigeon with dichromatic color mixtures involving the middle, short and near-UV wavelength range, between 350-560 nm. In analyzing the pigeon’s visual capabilities in this spectral region, the diversity of potential primary processes complicates the choice of both the appropriate monochromatic reference wavelengths (S- ) and wavelengths to be tested for dichromatic matches (S+). (In these experiments the animals were trained to S+ and tested against a mixture of two reference wavelengths S-.) Our choices for the test wavelengths were guided by the characteristics of the pigeon’s wavelength discrimination function (An/n) (Emmerton & Delius, 1980; Palacios, Bonnardel & Varela, 1990a) [Fig. l(D)]. We selected wavelengths close to the minimal threshold values so as to be sure that we tested animals where their wavelength discrimination is best and to maximize the likelihood of including all of the potentially active mechanisms present in the 350-560 nm spectral interval. We have found dichromatic color matches in the short and near-UV wavelength range, but not in the middle range. We have also analyzed these behavioral data in terms of the relative activations of potential primary mechanisms. METHODS Apparatus
and procedure
The animals, optical apparatus, methods, and data analysis were described in our previous paper (Palacios et al., 1990b). In brief, four animals (Red Carneaux, Coiumba liviu), exposed to a light-dark cycle (7.00/19.00) were kept in individual cages at 80% of their ad libitum 1947
1948
ADRIAN
G. PALACIOS and FRANCISCO
J. VARELA
Chromatics mechanisms P370
1
P370
P415
P460
P514
P567
! P415
.1
~
0
Palahs
.
Emmerton and Delius 1980
et al. 1490
hnm FIGURE 1. (A) Photopigment absorptions in the pigeon retina. P460, only found in the red field, P514 and P567 correspond to the microspectrophotometric data of Bowmaker (1977); P370 is suggested from Chen et al. (1984); and P415 by Govardoskii and Zueva (1977) and Norren (1975) from electroretinographic (ERG) techniques. There is no information about the red and yellow field distribution of these two ERG mechanisms, we have assumed they are present in both fields. A 1% of maximal sensitivity limit was imposed in drawing these functions (see text). (B, C) The same mechanisms as in A after adjustment by the oil droplets. The following are the 1,, of the pigment, + (in parenthesis) the 3,values for 50% transmission (&,o) of the oil droplets, and the I,, of the resulting receptor (Bowmaker, 1977). Combinations for the red field are P567 + R(610 nm) = 2, 619 nm, P514 + C(570 nm) = A,, 575 nm, P567 + B(554 nm) = A,, 589 nm, and P460 + A(476 mn) = 1_ 485 nm. Combinations for the yellow field are P567 + R(600 nm) = I,, 613 nm, P514 + C(562 nm) = 1,, 567 nm, P567 + B(499 nm) = A,,,,, 567 nm, and P5 14 + A(470 run) = 1, 525 nm. A transparent oil droplet was assumed to be associated with P370 and P415 (Goldsmith et al., 1984). (D) Behavioral wavelength discrimination functions redrawn from Emmerton and Delius (1980) and Palacios et al. (1990a).
body weight, with free access to water. Experiments were performed in the morning (6 days/week) in a modified Skinner box. Stimuli were presented through a central hole (2 mm 0) in a plastic gray key, which the birds selected by pecking. The key was fitted with a plastic diffuser transparent to UV (Altuglas M25). Previous to the experimental sessions, the animals were light adapted for l-2 hr under photopic illumina-
tion levels (fluorescent strip light, Philips 40/W-I daylight, 150 cd/m’) in the experimental room. A similar light source (Philips TL 6W/33) was used in the experimental box. Monochromatic lights were obtained with a quartz-halogen lamp (24 V/25OW) and interference filters (Corion; half band width: 10 + 2 run, transmission spectra measured with a Shimadzu UV260 spectrophotometer). Intensities were adjusted with a variable
COLOR
MIXING
aperture controlled by a stepping motor. Energy calibrations were done with a Tektronix radiometer 516 fitted with 56502 and 56503 probes. Measurements with these probes require a compensating correction for its sensitivity spectrum in the 350-400 nm range. Further details about the optical set-up can be found in Fig. 1 in Palacios et al. (1990b). During the pre-experimental period, pigeons were trained with autoshaped trials (15 blocks of 10 trials per session) consisting of a presentation for 10 set of a “white” light [the reinforced stimulus (S+)] followed by 5 set of access to food. The unilluminated key was the non-reinforced stimulus (S-). For each session we counted the total number of pecks per trial, and the animal’s performance was then defined as the ratio of responses to S+ over the total number of responses [S+ + S-1. A criterion of 85-90% correct responses during 3 consecutive sessions was attained in 5-6 sessions, on the average. Spectral
sensitivity
Prior to the experiments on color mixtures, and in order to calibrate the monochromatic lights for equal luminosity for individual pigeons, we determined for each animal the spectral luminosity function (V,) at 20 mm intervals between 350 and 560 nm (same experimental conditions as in Palacios et al., 1990b). The quanta1 flux of monochromatic S+ was varied in successive sessions from high to low values, while keeping a fixed value during each session, until performance dropped below 70%. In these experiments, S was the unilluminated key. A 75% threshold value was finally obtained by linear interpolation. Color mixtures
For color matching experiments, the positive (reinforced) stimulus S+ was a monochromatic wavelength and S, the non-reinforced stimulus, was an additive mixture of two monochromatic reference wavelengths [x% i, + y% A,] combined in a bifurcated optical fiber bundles. The end of the optical bundle fit a 15 cm aluminum tube that served to homogenize the mixture and prevented chromatic boundaries at the output (Neumeyer, 1985). We employed autoshaping training (Brown &Jenkins, 1968) in which a 10 set presentation of S+ was followed by 3 set of reinforcement with food regardless of the animal’s response, but a 10 set presentation of S was never reinforced. A session consisted of 11 blocks of ten trials, each block with 5 S+ and 5 S- in a pseudo-random presentation. The first block for each session was considered a warm-up and was subsequently ignored. The sequence of test wavelengths S+ and the corresponding reference wavelengths in the mixture comprising S [brackets] was as follows: 520 nm [470, 5601, 450 nm [430, 4701, 450 nm [410, 4701, 400 nm [370, 4501 and 390 nm [350, 4301. After verifying that the animals were able to discriminate each S+ from each wavelength comprising S, we looked for color matches in a systematic way. For each wavelength used as S+, the pro-
IN THE
1949
PIGEON
portion of the two reference wavelengths in S- was varied in random order, but using only one proportion [e.g. 10% I, + 90% A,] per session. A metameric match was indicated by 50% (chance) performance.
RESULTS Spectral
sensitivity
The mean of four individual Vj. functions in the 350-560 nm range is shown in Fig. 2. Values were obtained by linear interpolation of performance to a criterion of 75% correct; the linear regression coefficients varied between r = 0.65 and r = 0.99. The shape of this spectral sensitivity curve, particularly at short wavelengths, agrees with previously published data for the yellow but not the red retinal field (Remy & Emmerton, 1989). Furthermore, although our video records show that the animals fixated on the luminous spot with both frontal and lateral gazes, the birds appeared to use lateral fixation preferentially (and thus the yellow retinal field) in performing the present task. Dichromatic color matches Middle wavelength range. Figure 3 shows the discrimi-
nation performance for four birds when S+ was 520 nm, and S- was an additive mixture of 470 and 560 nm. In contrast to the results obtained with dichromatic mixtures in other regions of the spectrum, the pigeons were able to distinguish all mixtures of 470 and 560 nm from 520 nm and thus appear to be more than dichromatic in this spectral region. Short and near UV-wavelength range. Figure 4(A) shows the results when S+ is set at 450 nm and S is an additive mixture of 430 and 470 nm. A matching point at [900/, 430 + 10% 470 nm] is quite clear for each bird. The results were similar when 410 nm replaced 430 nm in the additive mixture [Fig. 4(B)], but for one animal the matching point was displaced towards 470 nm by 10%.
.Ol 340
380
420
460
Wavelength
500
540
580
(nm)
FIGURE 2. Spectral luminous efficiency sensitivities for four experimental animals. Values are averaged after normalization relative to sensitivity at 430 nm. Absolute values of energy at 560 nm for the different animals range between l-7 x 10” photons cm-* set-‘.
1950
ADRIAN 6. PALACIOS and FRANCISCO
I
iii
1%
r
,
60
80 20
40
g
I ?_!O;:q
FIGURE 3. ~ndi~d~i color matching data in the middle-wave spectral range for S+ 520 nm. The matching primaries were 560 nm and 470 nm, mixed in varying percentages, as indicated along the horizontal axis. 50% performance indicates a match to SC. In this example there is no evidence for a match.
Note that in both Fig 4(A) and 4(B), the matching function for one of the birds had a broad profile (see Discussion below). Figure 4(C), shows the birds’ performance when S+ was 400 nm and S- was an additive mixture of 370 and 450 nm. In this experiment performance dropped to chance level, but in contrast to the relative sharpness of most of the matches in Fig. 4(A) and 4(B), it did so with a broad profile.
We explored the near-UV region by taking S’ at 390 nm and S- as additive mixtures of 350 and 430 nm. Figure 4(D) shows that a reasonably sharp match point was found at [lo% 350 + 90% 430 nm] for two animals and [30°h 350 + 70% 430 nm] for a third. The fourth animal was not tested. While keeping S’ at 390, we also attempted to find other match points using mixtures (S-) of 370 and 410 nm, but this was not possible, as the animals’ behavior did not stabilize to a baseline performance. An important conclusion from these experiments is that wavelength discrimination in the near-UV range appears to be a more demanding task for pigeons than it is in the middle and long wavelength spectral regions.
ANALYSIS
108
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70
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68
60
e
50
50
48
40
40f I&
20 80
.
I 20 80
40 60
.
I 40 60
4”o
1
. z
!z
.
1 ;:
AND DISCUSSLON
Based on the assumption that discrimination of color mixtures depends on the relative activations of cones with different combinations of visual pigments and oil droplets, in Palacios et al. (1990b) we concluded that color mixtures in the 580-640 nm range were compatible with the presence of two (or perhaps three) receptor mechanisms with A,,,,, at 575, 589 and 619nm, as measured by microspectrophotometry @owmaker, 1977). Attempting to extend this analysis to the near-UV and blue region range, however, is not straightforward as there are a number of unknown factors, such as the precise location of a,,,. Importantly, in birds the
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I&
1 100 0 [370] [4501
20 80
40 60
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loo 0 14781 14101
1 80 20
loo 0 f4381 [3501
FIGURE 4. Individual color matching data in the short and near-UV spectral range, plotted in the same way as in Fig. 3. Note the presence of a match for each pair of S+, particularly in (A) and (B).
COLOR
MIXING
description of chromatic mechanisms requires knowledge of both the absorption spectra of the visual pigments of the cones as well as the transmission spectra of the cone oil droplets. The former is technically difficult to acquire due to the small size of avian cones (l-2 pm diameter). Detailed characterization of the oil droplets is complicated by high absorbances of pigment (see Goldsmith, Collins & Licht, 1984). Some information on spectral sensitivities has been obtained by selective chromatic adaptation of the ERG, which has revealed mechanisms with maxima of sensitivity at 370 and 415 nm (i.e. Introduction). However, little is known about regional differences in the distributions of short wavelength receptors in the red and yellow fields of the pigeon (for discussion see Remy & Emmerton, 1989). For the present analysis we assumed that the various monochromatic stimuli employed activated cone pigments whose absorbance spectra are approximated by the function in Fig. l(A). (These curves were calculated from a polynomial function that approximates rhodopsin spectra; G. Bernard, personal communication.) The absorbance functions were corrected for the transmittance of cone oil droplets [Fig. l(B) and (C)] using a cut-off function of appropriate slope and the 50% transmission values (A,,,) reported by Bowmaker (1977). For the receptor sensitivities [Fig. l(B) and (C)l, absorptions less than 1% were ignored. Absorbance by the ocular media (other than the cone oil droplets) was assumed to have no effect (Emmerton et al., 1980).
A
Red Field
IN THE
1951
PIGEON
Middle wavelength range The absence of a dichromatic match in the middle wavelength range (Fig. 3) reflects the likelihood that 520 nm (S+) is absorbed by three receptors, with I,,, at 485, 575 and 589 nm in the red field [Fig. l(B)], and 1,,, at 525, 567 + C and 567 + B nm in the yellow field [Fig. l(C)]. In contrast, the mixture of 470 + 560 nm always stimulates five different receptor mechanisms. Thus a metameric match with only two wavelengths in the mixture is predictably excluded and is confirmed by the behavioral results. Short wavelength range Figure 5(A-C) show calculated Maxwell triangles for the relative activation of the three receptors (370 nm, 415 nm, and a third that is different in each of the three triangles, A-C). For each triangle the calculated positions of each of four wavelengths is plotted as a solid square, 410, 430, 450, and 470nm. In the red field, S+ (450 nm) and S- (470 + 430 or 410 nm) are both absorbed by three receptor mechanisms with A,,,,, at 370, 415 and 485 nm [Fig. 5(A)]. For the case in which 430 was used in S, the locus point for the reinforced wavelength S+ falls close to the line joining 430 and 470 nm and permits a prediction: 450 = [85% 430 + 15% 470 nm], which is close to the experimental data. In contrast, no mixture is predicted for 450 = 410 + 470 nm, and yet we found little behavioral difference between these two sources. For the yellow field, four mechanisms with II,,, at 370, 415, 525 + A and 567 + B nm could be potentially activated
Yellow Field
B
C P415
D 1.0
Red Field
E Yellow Field
410/430 r
.450
0.8 vi
0.6.
ii .470
0.4 -
0.0
0.2
0.4 0.6 P485+A
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
P525+A
FIGURE 5. (A
Copyright
0
0042-6989/92 85.00 + 0.00 1992 Pergamon Press Ltd
Color Mixing in the Pigeon (Columba livia) II: A Psychophysical Determination in the Middle, Short and Near-UV Wavelength Range ADRIAN
G. PALACIOS,*,t
FRANCISCO
J. VARELA*
Received 5 March 1991; in revised form 28 January 1992
Pigeons were trained to discriminate between spectral lights and additive mixtures in the 350460 nm spectral range using a successive “autoshaping” discrimination procedure [introduced in Palacios, Martinoya, Bloch & Varela, Vision Research, 30,587-596 (1!290)]. Dichromatic mixtures were found in the short and near UV region, but not in the middle-wave region. Our results suggest that color vision in the pigeon involves the active participation of five different primary mechanisms, which are differentially active in the yellow- and red-sensitive retinal fields. Animal psychophysics
Pigeon
Color matching
UV vision
INTRODUCTION
The multiplicity of cone photopigments and oil droplets present in the avian retina poses a challenging problem for the characterization of the color space of birds. For our experimental animal, the pigeon (Columba liviu), absorption spectra obtained by microspectrophotometry provide direct evidence for three cone photopigments, with maximum absorption (A,,,) at 460,514, and 567 nm (Bowmaker, 1977). The presence of several kinds of carotenoid-containing oil droplets in the inner segments of avian cones adds a further complication, as these organelles function as long-pass spectral filters. Bowmaker (1977) showed that the three visual pigments he found were paired with oil droplets in such a way as to produce more than three spectral classes of cones. Evidence for the presence of two additional spectral types of cone comes from electrophysiological measurements of spectral sensitivity peaking at 370 and 415 nm, in the near-UV and short wavelength region (Norren, 1975; Govardovskii & Zueva, 1977; Chen, Collins & Goldsmith, 1984). The pigeon therefore possesses at least six spectral cone mechanisms in both the frontal (red) and lateral (yellow) retinal regions, as summarized in Fig. 1 (A-C). Little is known about the behavioral significance of all these potential primary chromatic mechanisms (Varela, Palacios & Goldsmith, 1992). In Palacios, Martinoya, Bloch and Varela (1990b) we presented a psychophysical study of color mixture in the long-wavelength spectral range of the pigeon (58&640 nm), showing that pigeons require at least two *Institut des Neurosciences, Universitk de Paris 6-CNRS, 9 quai Saint Bernard, 75005 Paris, France. tTo whom all correspondence should be addressed at: Department of Biology, Yale University, New Haven, CT 06520, U.S.A. “R
32:,&G
Visual pigments
Pentachromacy
(and possibly three) primary mechanisms in this narrow range. The purpose of the present paper is to continue the characterization of the color space of the pigeon with dichromatic color mixtures involving the middle, short and near-UV wavelength range, between 350-560 nm. In analyzing the pigeon’s visual capabilities in this spectral region, the diversity of potential primary processes complicates the choice of both the appropriate monochromatic reference wavelengths (S- ) and wavelengths to be tested for dichromatic matches (S+). (In these experiments the animals were trained to S+ and tested against a mixture of two reference wavelengths S-.) Our choices for the test wavelengths were guided by the characteristics of the pigeon’s wavelength discrimination function (An/n) (Emmerton & Delius, 1980; Palacios, Bonnardel & Varela, 1990a) [Fig. l(D)]. We selected wavelengths close to the minimal threshold values so as to be sure that we tested animals where their wavelength discrimination is best and to maximize the likelihood of including all of the potentially active mechanisms present in the 350-560 nm spectral interval. We have found dichromatic color matches in the short and near-UV wavelength range, but not in the middle range. We have also analyzed these behavioral data in terms of the relative activations of potential primary mechanisms. METHODS Apparatus
and procedure
The animals, optical apparatus, methods, and data analysis were described in our previous paper (Palacios et al., 1990b). In brief, four animals (Red Carneaux, Coiumba liviu), exposed to a light-dark cycle (7.00/19.00) were kept in individual cages at 80% of their ad libitum 1947
1948
ADRIAN
G. PALACIOS and FRANCISCO
J. VARELA
Chromatics mechanisms P370
1
P370
P415
P460
P514
P567
! P415
.1
~
0
Palahs
.
Emmerton and Delius 1980
et al. 1490
hnm FIGURE 1. (A) Photopigment absorptions in the pigeon retina. P460, only found in the red field, P514 and P567 correspond to the microspectrophotometric data of Bowmaker (1977); P370 is suggested from Chen et al. (1984); and P415 by Govardoskii and Zueva (1977) and Norren (1975) from electroretinographic (ERG) techniques. There is no information about the red and yellow field distribution of these two ERG mechanisms, we have assumed they are present in both fields. A 1% of maximal sensitivity limit was imposed in drawing these functions (see text). (B, C) The same mechanisms as in A after adjustment by the oil droplets. The following are the 1,, of the pigment, + (in parenthesis) the 3,values for 50% transmission (&,o) of the oil droplets, and the I,, of the resulting receptor (Bowmaker, 1977). Combinations for the red field are P567 + R(610 nm) = 2, 619 nm, P514 + C(570 nm) = A,, 575 nm, P567 + B(554 nm) = A,, 589 nm, and P460 + A(476 mn) = 1_ 485 nm. Combinations for the yellow field are P567 + R(600 nm) = I,, 613 nm, P514 + C(562 nm) = 1,, 567 nm, P567 + B(499 nm) = A,,,,, 567 nm, and P5 14 + A(470 run) = 1, 525 nm. A transparent oil droplet was assumed to be associated with P370 and P415 (Goldsmith et al., 1984). (D) Behavioral wavelength discrimination functions redrawn from Emmerton and Delius (1980) and Palacios et al. (1990a).
body weight, with free access to water. Experiments were performed in the morning (6 days/week) in a modified Skinner box. Stimuli were presented through a central hole (2 mm 0) in a plastic gray key, which the birds selected by pecking. The key was fitted with a plastic diffuser transparent to UV (Altuglas M25). Previous to the experimental sessions, the animals were light adapted for l-2 hr under photopic illumina-
tion levels (fluorescent strip light, Philips 40/W-I daylight, 150 cd/m’) in the experimental room. A similar light source (Philips TL 6W/33) was used in the experimental box. Monochromatic lights were obtained with a quartz-halogen lamp (24 V/25OW) and interference filters (Corion; half band width: 10 + 2 run, transmission spectra measured with a Shimadzu UV260 spectrophotometer). Intensities were adjusted with a variable
COLOR
MIXING
aperture controlled by a stepping motor. Energy calibrations were done with a Tektronix radiometer 516 fitted with 56502 and 56503 probes. Measurements with these probes require a compensating correction for its sensitivity spectrum in the 350-400 nm range. Further details about the optical set-up can be found in Fig. 1 in Palacios et al. (1990b). During the pre-experimental period, pigeons were trained with autoshaped trials (15 blocks of 10 trials per session) consisting of a presentation for 10 set of a “white” light [the reinforced stimulus (S+)] followed by 5 set of access to food. The unilluminated key was the non-reinforced stimulus (S-). For each session we counted the total number of pecks per trial, and the animal’s performance was then defined as the ratio of responses to S+ over the total number of responses [S+ + S-1. A criterion of 85-90% correct responses during 3 consecutive sessions was attained in 5-6 sessions, on the average. Spectral
sensitivity
Prior to the experiments on color mixtures, and in order to calibrate the monochromatic lights for equal luminosity for individual pigeons, we determined for each animal the spectral luminosity function (V,) at 20 mm intervals between 350 and 560 nm (same experimental conditions as in Palacios et al., 1990b). The quanta1 flux of monochromatic S+ was varied in successive sessions from high to low values, while keeping a fixed value during each session, until performance dropped below 70%. In these experiments, S was the unilluminated key. A 75% threshold value was finally obtained by linear interpolation. Color mixtures
For color matching experiments, the positive (reinforced) stimulus S+ was a monochromatic wavelength and S, the non-reinforced stimulus, was an additive mixture of two monochromatic reference wavelengths [x% i, + y% A,] combined in a bifurcated optical fiber bundles. The end of the optical bundle fit a 15 cm aluminum tube that served to homogenize the mixture and prevented chromatic boundaries at the output (Neumeyer, 1985). We employed autoshaping training (Brown &Jenkins, 1968) in which a 10 set presentation of S+ was followed by 3 set of reinforcement with food regardless of the animal’s response, but a 10 set presentation of S was never reinforced. A session consisted of 11 blocks of ten trials, each block with 5 S+ and 5 S- in a pseudo-random presentation. The first block for each session was considered a warm-up and was subsequently ignored. The sequence of test wavelengths S+ and the corresponding reference wavelengths in the mixture comprising S [brackets] was as follows: 520 nm [470, 5601, 450 nm [430, 4701, 450 nm [410, 4701, 400 nm [370, 4501 and 390 nm [350, 4301. After verifying that the animals were able to discriminate each S+ from each wavelength comprising S, we looked for color matches in a systematic way. For each wavelength used as S+, the pro-
IN THE
1949
PIGEON
portion of the two reference wavelengths in S- was varied in random order, but using only one proportion [e.g. 10% I, + 90% A,] per session. A metameric match was indicated by 50% (chance) performance.
RESULTS Spectral
sensitivity
The mean of four individual Vj. functions in the 350-560 nm range is shown in Fig. 2. Values were obtained by linear interpolation of performance to a criterion of 75% correct; the linear regression coefficients varied between r = 0.65 and r = 0.99. The shape of this spectral sensitivity curve, particularly at short wavelengths, agrees with previously published data for the yellow but not the red retinal field (Remy & Emmerton, 1989). Furthermore, although our video records show that the animals fixated on the luminous spot with both frontal and lateral gazes, the birds appeared to use lateral fixation preferentially (and thus the yellow retinal field) in performing the present task. Dichromatic color matches Middle wavelength range. Figure 3 shows the discrimi-
nation performance for four birds when S+ was 520 nm, and S- was an additive mixture of 470 and 560 nm. In contrast to the results obtained with dichromatic mixtures in other regions of the spectrum, the pigeons were able to distinguish all mixtures of 470 and 560 nm from 520 nm and thus appear to be more than dichromatic in this spectral region. Short and near UV-wavelength range. Figure 4(A) shows the results when S+ is set at 450 nm and S is an additive mixture of 430 and 470 nm. A matching point at [900/, 430 + 10% 470 nm] is quite clear for each bird. The results were similar when 410 nm replaced 430 nm in the additive mixture [Fig. 4(B)], but for one animal the matching point was displaced towards 470 nm by 10%.
.Ol 340
380
420
460
Wavelength
500
540
580
(nm)
FIGURE 2. Spectral luminous efficiency sensitivities for four experimental animals. Values are averaged after normalization relative to sensitivity at 430 nm. Absolute values of energy at 560 nm for the different animals range between l-7 x 10” photons cm-* set-‘.
1950
ADRIAN 6. PALACIOS and FRANCISCO
I
iii
1%
r
,
60
80 20
40
g
I ?_!O;:q
FIGURE 3. ~ndi~d~i color matching data in the middle-wave spectral range for S+ 520 nm. The matching primaries were 560 nm and 470 nm, mixed in varying percentages, as indicated along the horizontal axis. 50% performance indicates a match to SC. In this example there is no evidence for a match.
Note that in both Fig 4(A) and 4(B), the matching function for one of the birds had a broad profile (see Discussion below). Figure 4(C), shows the birds’ performance when S+ was 400 nm and S- was an additive mixture of 370 and 450 nm. In this experiment performance dropped to chance level, but in contrast to the relative sharpness of most of the matches in Fig. 4(A) and 4(B), it did so with a broad profile.
We explored the near-UV region by taking S’ at 390 nm and S- as additive mixtures of 350 and 430 nm. Figure 4(D) shows that a reasonably sharp match point was found at [lo% 350 + 90% 430 nm] for two animals and [30°h 350 + 70% 430 nm] for a third. The fourth animal was not tested. While keeping S’ at 390, we also attempted to find other match points using mixtures (S-) of 370 and 410 nm, but this was not possible, as the animals’ behavior did not stabilize to a baseline performance. An important conclusion from these experiments is that wavelength discrimination in the near-UV range appears to be a more demanding task for pigeons than it is in the middle and long wavelength spectral regions.
ANALYSIS
108
8
98
98
8
8g
88
a $
70
70
?z se:
68
60
e
50
50
48
40
40f I&
20 80
.
I 20 80
40 60
.
I 40 60
4”o
1
. z
!z
.
1 ;:
AND DISCUSSLON
Based on the assumption that discrimination of color mixtures depends on the relative activations of cones with different combinations of visual pigments and oil droplets, in Palacios et al. (1990b) we concluded that color mixtures in the 580-640 nm range were compatible with the presence of two (or perhaps three) receptor mechanisms with A,,,,, at 575, 589 and 619nm, as measured by microspectrophotometry @owmaker, 1977). Attempting to extend this analysis to the near-UV and blue region range, however, is not straightforward as there are a number of unknown factors, such as the precise location of a,,,. Importantly, in birds the
loo
11$
J. VARELA
I&
1 100 0 [370] [4501
20 80
40 60
4OI I&
1 ::
$
z
20 80
I 40 60
loo 0 14781 14101
1 80 20
loo 0 f4381 [3501
FIGURE 4. Individual color matching data in the short and near-UV spectral range, plotted in the same way as in Fig. 3. Note the presence of a match for each pair of S+, particularly in (A) and (B).
COLOR
MIXING
description of chromatic mechanisms requires knowledge of both the absorption spectra of the visual pigments of the cones as well as the transmission spectra of the cone oil droplets. The former is technically difficult to acquire due to the small size of avian cones (l-2 pm diameter). Detailed characterization of the oil droplets is complicated by high absorbances of pigment (see Goldsmith, Collins & Licht, 1984). Some information on spectral sensitivities has been obtained by selective chromatic adaptation of the ERG, which has revealed mechanisms with maxima of sensitivity at 370 and 415 nm (i.e. Introduction). However, little is known about regional differences in the distributions of short wavelength receptors in the red and yellow fields of the pigeon (for discussion see Remy & Emmerton, 1989). For the present analysis we assumed that the various monochromatic stimuli employed activated cone pigments whose absorbance spectra are approximated by the function in Fig. l(A). (These curves were calculated from a polynomial function that approximates rhodopsin spectra; G. Bernard, personal communication.) The absorbance functions were corrected for the transmittance of cone oil droplets [Fig. l(B) and (C)] using a cut-off function of appropriate slope and the 50% transmission values (A,,,) reported by Bowmaker (1977). For the receptor sensitivities [Fig. l(B) and (C)l, absorptions less than 1% were ignored. Absorbance by the ocular media (other than the cone oil droplets) was assumed to have no effect (Emmerton et al., 1980).
A
Red Field
IN THE
1951
PIGEON
Middle wavelength range The absence of a dichromatic match in the middle wavelength range (Fig. 3) reflects the likelihood that 520 nm (S+) is absorbed by three receptors, with I,,, at 485, 575 and 589 nm in the red field [Fig. l(B)], and 1,,, at 525, 567 + C and 567 + B nm in the yellow field [Fig. l(C)]. In contrast, the mixture of 470 + 560 nm always stimulates five different receptor mechanisms. Thus a metameric match with only two wavelengths in the mixture is predictably excluded and is confirmed by the behavioral results. Short wavelength range Figure 5(A-C) show calculated Maxwell triangles for the relative activation of the three receptors (370 nm, 415 nm, and a third that is different in each of the three triangles, A-C). For each triangle the calculated positions of each of four wavelengths is plotted as a solid square, 410, 430, 450, and 470nm. In the red field, S+ (450 nm) and S- (470 + 430 or 410 nm) are both absorbed by three receptor mechanisms with A,,,,, at 370, 415 and 485 nm [Fig. 5(A)]. For the case in which 430 was used in S, the locus point for the reinforced wavelength S+ falls close to the line joining 430 and 470 nm and permits a prediction: 450 = [85% 430 + 15% 470 nm], which is close to the experimental data. In contrast, no mixture is predicted for 450 = 410 + 470 nm, and yet we found little behavioral difference between these two sources. For the yellow field, four mechanisms with II,,, at 370, 415, 525 + A and 567 + B nm could be potentially activated
Yellow Field
B
C P415
D 1.0
Red Field
E Yellow Field
410/430 r
.450
0.8 vi
0.6.
ii .470
0.4 -
0.0
0.2
0.4 0.6 P485+A
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
P525+A
FIGURE 5. (A
