Colour constancy in diurnal and nocturnal hawkmoths
Department of Cell and Organism Biology, Lund University, Helgonavägen 3, S-223 62 Lund, Sweden
* Author for correspondence (e-mail: anna.balkenius{at}cob.lu.se)
Accepted 21 June 2004
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Summary |
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Key words: colour vision, colour constancy, insect, hawkmoth, Macroglossum stellatarum, Deilephila elpenor
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Introduction |
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Colour constancy greatly contributes to the usefulness of colour vision,
which is thought to be more reliable for object recognition than intensity
vision since colour is less affected by changes of illumination (Kelber et
al.,
2003a,b
).
Receptor adaptation contributes a large part to constancy
(Komatsu, 1998). The
sensitivity of the receptors decreases as a result of adaptation of the
photoreceptor cells being stimulated by the background spectrum (Neumeyer,
1980
,
1981
;
Komatsu, 1998
). For chromatic
adaptation, it is assumed that the different receptor types adapt separately
depending on the background spectrum (Neumeyer,
1981
,
1998
).
Chromatic adaptation can be described by the von Kries coefficient law,
which scales the signals from the photoreceptors to the background
illumination to keep the colour constant despite changing spectra
(Kries, 1905;
Vorobyev et al., 1999
). The
model adjusts the sensitivity of the three photoreceptors independently in
proportion to their response to the background illumination
(Backhaus et al., 1998
).
However, the von Kries coefficient law does not lead to perfect colour
constancy since the photoreceptors are not completely independent as the model
assumes. Theoretical analyses have shown that the broad spectral sensitivity
and large overlap in the sensitivity curves of the photoreceptors limit colour
constancy (Worthey and Brill,
1986
; Dyer,
1999
).
The second mechanism to achieve colour constancy is lateral interaction.
This process can occur on an opponent stage in the retina or higher up in the
visual pathway (Neumeyer,
1981; Komatsu,
1998
; Rinner and Gegenfurtner,
2000
). Finally, cognition also contributes to colour constancy in
many ways. The awareness of colours and prior experience influence the
perception of colours (Craven and Foster,
1992
; Hurlbert,
1999
). However, the role of cognition in colour perception can
only be tested in humans.
Besides humans, colour constancy has been studied in honeybees
(Neumeyer, 1981;
Werner et al., 1988
),
goldfishes (Dörr and Neumeyer,
2000
; Neumeyer et al.,
2002
) and butterflies
(Kinoshita and Arikawa, 2000
).
In an experiment with goldfish, the backgrounds were black, grey or white and
this influenced how well colour constancy functioned
(Neumeyer et al., 2002
). In
tests with white or grey backgrounds, goldfish showed perfect colour constancy
but, when a black background was used, they failed colour constancy under
saturated illumination. With the grey and white backgrounds, the colour of the
illumination was reflected and caused adaptation of the receptors. The black
background reflected very little light, causing the adaptation cue from the
illumination to be much weaker (Neumeyer
et al., 2002
). Neumeyer
(1981
) found that the choice
behaviour of honeybees was almost the same under training illumination as
under coloured illuminations. The bees were tested with increasingly saturated
yellow and blue illuminations and, under the most saturated illuminations,
colour constancy started to fail
(Neumeyer, 1981
). Bees were
also tested with Mondrian patterns (multi-colour patterns inspired by the
painter Mondrian), showing good colour constancy
(Werner et al., 1988
).
The butterfly Papilio xuthus was trained to recognise a colour in
a Mondrian under differently coloured illuminations
(Kinoshita and Arikawa, 2000).
In a critical test, the butterflies were trained to discriminate a red
rewarded stimulus from orange under white illumination. Under yellow
illumination, orange reflected the same spectrum as red had done under white
illumination, but the butterflies still chose red, thus showing colour
constancy. However, the same behaviour would result if the butterflies had
chosen the `reddest' colour and thus showed relative colour learning.
Colour vision is assumed to be a general ability of hawkmoths (Kelber et
al.,
2003a,b
).
They have trichromatic colour vision with an ultraviolet-, blue- and
green-sensitive receptor type
(Höglund et al., 1973
).
Macroglossum stellatarum is a diurnal hawkmoth
(Kelber and Hénique,
1999
), and Deilephila elpenor is the first nocturnal
animal proven to use colour vision (Kelber
et al., 2002
). Hawkmoths use their colour vision system to find
and forage from suitable nectar flowers. Most hawkmoths are active at dawn and
dusk, when light spectra change most. It would therefore be advantageous for
them to have colour constancy (Kelber et
al., 2002
; Land and Osorio,
2003
). We also tested whether a mechanism operating according to
the von Kries coefficient law can account for colour constancy in both the
nocturnal D. elpenor and the diurnal M. stellatarum.
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Materials and methods |
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The moths learned to retrieve a small amount of 20% sugar solution through
a 3 mm-wide hole in the centre of an artificial flower. Testing started on the
second day. No reward was present during tests, and testing was followed by a
feeding session. Testing continued over 10 days but not all animals continued
to cooperate during this period. A visit was defined as each time the moth
touched the colour patch with the proboscis. Each test lasted for as long as
an animal continued to make choices, which was 10-20 min. The positions
of the colour patches were changed randomly to avoid place learning
(Balkenius et al., 2004
). All
tests were performed on individual animals.
As stimuli, we used different coloured disks of 30 mm diameter on a light grey background. Stimuli and backgrounds were printed with an Epson colour printer (Model P952A) on Ink Jet paper. Experiment 1 was a multiple choice test in the short-wavelength range where eight D. elpenor were trained to discriminate five different bluish colours: blue (b), blue-green (bg), green (g), violet (v) and blue-violet (bv) (Figs 1C, 2A). Experiment 2 was a multiple-choice test in both the short- and the long-wavelength range. Ten D. elpenor and five different yellowish colours were used: yellow (y), yellow-orange (yo), orange (o), dark green (dg) and light green (lg) (Figs 1D, 2A). The moths were tested under white, blue (Schott filter, FG-3) and yellow (Schott filter, FG-13) illuminations. Experiments 3 and 4 were dual-choice tests on D. elpenor and M. stellatarum. Ten D. elpenor and 10 M. stellatarum were trained to turquoise as the rewarded colour, and another 10 specimens of each species were trained to green (Fig. 1E). They were tested under white and yellow illumination (Schott filter, FG-13; Fig. 1A,B).
|
To exclude the possibility of relative colour learning, we performed a fifth experiment. Six M. stellatarum were trained to green as the rewarded colour and turquoise as the unrewarded colour. After one week of training, the moths were tested with green and a `yellower' colour (lime; Fig. 1E).Using relative colour learning, moths trained to choose green and not turquoise should prefer lime to green. Absolute colour learning should result in a high percentage of choices for green in all tests. Another group of six M. stellatarum were trained with green as the rewarded colour and lime as the unrewarded colour and tested on green and turquoise. In the Maxwell colour triangle, green lies between turquoise and lime (Fig. 2B).
Calculation of quantum catches and colour loci
The spectral composition of the light reflected from the paper flowers was
measured with a calibrated spectrophotometer (S2000; Ocean Optics). The
quantum catch (Qi) of each photoreceptor is the number of
photons absorbed by the receptor and is calculated as:
![]() | (1) |
where I() is the spectrum of the illumination,
S(
) is the reflectance of a surface and
Ri(
) is the fraction of the incident light
absorbed by a specific type of photoreceptor i for each wavelength
(
) for the three receptor types sensitive to ultraviolet, blue and
green light (Fig. 1F). The
spectral sensitivities of the photoreceptors were calculated from the recorded
sensitivity maxima (Höglund et al.,
1973
) using the Stavenga-Smits-Hoenders rhodopsin template
(Stavenga et al., 1993
).
The von Kries coefficient law assumes that the signals of the
photoreceptors adapt to the background. The scaling factor,
ki, depends on the illumination spectrum
(Vorobyev et al., 1999), and
the quantum catch after adaptation,
i, is calculated
according to equations 2, 3:
![]() | (2) |
![]() | (3) |
where I() is the spectrum of the light reflected from the
background, SB(
) is the reflectance of the
background and Ri(
) is the fraction of the light
absorbed by a specific type of photoreceptor for each wavelength.
For an animal with trichromatic colour vision, colours can be represented
as different loci in a Maxwell colour triangle, which is a projection of the
three-dimensional colour space on a plane of equal intensity
(Fig. 2;
Kelber et al., 2003b). This is
possible for animals that disregard intensity and predominantly use the
chromatic aspect of colour as has been shown for both D. elpenor and
M. stellatarum (Kelber and
Hénique, 1999
; Kelber
et al., 2002
). In the Maxwell colour triangle, the colour
loci are calculated as:
![]() | (4) |
Here, QUV, QB and
QG are the quantum catches of the three photoreceptor
types of the moths, and qi represents the projection on
the Maxwell triangle (Kelber et al.,
2003b). Colour loci can also be calculated with von Kries
coefficient law using
i instead of
Qi.
For calculation of the quantum catches, the sensitivity curves were normalised in such a way that the integrals equal 1 (Fig. 1F).
The Euclidean distance d(x, y) between the colour
coordinates in the Maxwell triangle was used as a measure of similarity:
![]() | (5) |
These colour distances were calculated for different colours and illuminations with and without a von Kries coefficient law (Tables 1, 2).
|
|
For the dual-choice experiments (experiments 3 and 4), we selected two colours that require a colour constancy mechanism to be distinguished under the changed illumination (Fig. 1E; Table 2). In yellow light, the turquoise colour generated almost the same quantum catch in the different photoreceptor types and occupied almost the identical colour locus as the green colour did in white light (Figs 2A, 3).
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Results |
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Experiment 2: multiple-choice test in both the short- and the long-wavelength range
In experiment 2, D. elpenor were rewarded at the yellow (y) colour
under white illumination during the training sessions. Under the white, yellow
and blue illuminations, they selected yellow most frequently
(Fig. 5). The choice
distribution for the colours did not differ under the white and yellow
illumination (2-test, P>0.05;
Fig. 5) and it differed from
chance (
2-test, P<0.001). Under the blue
illumination, the moths chose orange less frequently compared with the white
and yellow illumination, and the choice distribution differed from chance
(
2-test, P<0.001). Under the yellow illumination,
light green (lg) resulted in almost the same quantum catch as the rewarded
yellow under white illumination (Fig.
5; Table 1). Orange
(o) was selected more frequently than yellow-orange (yo) under white
illumination even though the yellow-orange was more similar to the rewarded
colour than the orange (not shown). The behaviour changed under blue
illumination, when the yellow-orange was chosen more frequently than orange
(o).
|
Experiment 3: dual-choice tests on D. elpenor
In experiment 3, we tested whether the von Kries coefficient law can
explain colour constancy in D. elpenor. If the moths did not have
colour constancy, turquoise would occupy almost the same colour locus as the
green did under white illumination (Fig.
3A). Still, the moths chose green. With turquoise as the rewarded
colour, D. elpenor selected turquoise in 100% of the trials under
white illumination and in 98% under yellow illumination
(Fig. 6A). With green as the
rewarded colour, the moths selected green in 96% of the cases under white
illumination and in 98% under the yellow illumination
(Fig. 6B). The choices of the
moths can be explained by the use of von Kries coefficient law.
|
Experiment 4: dual-choice tests on the diurnal hawkmoth M. stellatarum
We performed the same experiment on the diurnal species M.
stellatarum. The experimental design was identical to that of the
previous experiment, with the exception that the light intensity was
104 times higher. With turquoise as the rewarded colour, the moths
selected the turquoise in 87% of the trials under white illumination and in
99% under the yellow illumination (Fig.
7A). When green was rewarded, the moths chose correctly in 100% of
the cases under white illumination and in 84% under the yellow
(Fig. 7B). This indicates that
M. stellatarum also has colour constancy.
|
Experiment 5: test of relative colour learning
The dual-choice experiments might have allowed the animals to perform
relative choices. They might have chosen the `bluer' or `yellower' colour
rather than the learned colour. Therefore, a group of six moths were trained
on a rewarded green colour and an unrewarded `yellower' colour (lime) and
tested with the green and a `bluer' colour (turquoise;
Fig. 2). They selected green in
91% of the cases (Fig. 8A) even
if it was the `yellower' of both test colours. The second group of six moths
were trained to green as the rewarded and turquoise as the unrewarded colour
and tested with green and lime. They selected the green in 85% of the cases
(Fig. 8B, white bars). These
moths were also tested with green and lime colours, for colour constancy,
under white and yellow illumination. The moths selected the green in 85% of
the cases under white illumination and 91% under the yellow illumination
(Fig. 8B, grey bars;
2-test, P<0.001). These results exclude relative
colour learning as a possible explanation for the results of experiments 3 and
4.
|
Discrimination of illumination
The moths did not change their preferred colour when the spectra of the
illumination changed and thus showed colour constancy. However, the total
numbers of choices differed under different illuminations
(Fig. 9). The difference could
be observed in all experiments, but only in one experiment did a sufficiently
large number of animals (N=4) make choices on eight subsequent days.
In this experiment, M. stellatarum were trained with turquoise as the
rewarded and green as the unrewarded colour. Moths made fewer choices the
first day when they saw the changed illumination. This implies that they
noticed the change in illumination even if they compensated for it
(Fig. 9). There was a
significant difference in the total number of choices made under white and
yellow illumination (day 1, one-tailed t-test, P<0.04).
On days 2-5, they made similar numbers of choices under the yellow and the
white illumination (day 5, two-tailed t-test, P>0.82).
After five days, the number of choices under yellow illumination decreased
again and differed significantly from the number of choices under white
illumination (day 8, one-tailed t-test, P<0.03).
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Discussion |
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Colour constancy
In the dual-choice tests, green under white illumination and turquoise
under yellow illumination resulted in almost identical quantum catches in the
photoreceptors and it would have been impossible to recognise the green colour
under yellow illumination without colour constancy
(Fig.
3A,B;Table 2).
The moths did not show perfect colour constancy under the blue illumination in the second multiple-choice experiment (Fig. 5). They selected the orange and dark green colours less frequently under the blue illumination than under the white illumination. The blue illumination was highly saturated and the moth's visual system is perhaps not adapted to this spectrum.
If chromatic adaptation resulted in perfect colour constancy, we would
expect all colours in the Maxwell colour triangle to show a zero shift when
the illumination changes. In our dual-choice experiments, the von Kries
coefficient law does not completely eliminate the shift. This limitation
depends on the spectral width and the overlap of the different photoreceptor
sensitivities (Worthey and Brill,
1986). The coloured oil droplets in birds are thought to improve
colour constancy since they narrow the spectral sensitivities, which makes the
von Kries coefficient law more effective
(Vorobyev et al., 1998
).
Narrowing the width of the different photoreceptor sensitivities has the
disadvantage of reducing sensitivity, which seems useless in a nocturnal
hawkmoth. In the butterfly Papilio xuthus and in the goldfish, it has
previously been shown that colour constancy is not perfect under strongly
saturated illumination or with a black background
(Dörr and Neumeyer, 2000
;
Kinoshita and Arikawa, 2000
;
Neumeyer et al., 2002
). Humans
do not display perfect colour constancy either. We fail colour constancy under
saturated illuminations and on solitary colour surfaces surrounded by black
(Bäuml, 1999
;
Hurlbert, 2002
). It is known
that the colour constancy in humans is based on several mechanisms. Thus, not
only the adaptation mechanism fails under these extreme conditions.
Possible alternative explanations
Could the moths have used achromatic cues instead of a colour constancy
mechanism? In previous experiments, D. elpenor did not use achromatic
cues to recognise colour (Kelber et al.,
2002) but this does not guarantee that they cannot use achromatic
cues to select the correct flower. In the second multiple-choice experiment,
two colours (orange and dark green; Fig.
5) reflected less light under the blue illumination and the moths
did not select them as frequently as under the white and yellow illuminations.
This might indicate that the moths use achromatic cues when they fail in
colour constancy. However, in the other experiments, no strong intensity cues
were available.
In three of the experiments, one of the unrewarded colours occupied a colour locus closer to that of the rewarded colour when a coloured illumination was used (Tables 1, 2). In all three cases, animals still chose the training colour (Figs 4, 5, 6, 7).
Did the moths learn relative rather than absolute colour? To our knowledge,
the only animal that has been shown to learn relative colour is a dichromatic
marsupial, the tammar wallaby (Macropus eugenii). Trained to choose a
450 nm stimulus and to avoid a 500 nm stimulus, the wallabies preferred the
stimulus with the shorter wavelength in any pair they were tested with, thus
they preferred 400 nm to 450 nm, the training colour
(Hemmi, 1999).
In our fifth experiment, two groups of moths were tested for relative colour learning. The first group of moths were trained on green and turquoise and tested with green and lime. The lime colour is `yellower' for the moths than the rewarded green colour (Fig. 2), but the moths still selected the original green patch. The second group of moths were trained on green and lime and, when tested with green and turquoise, they still selected the green colour. If the moth had learned to select the `yellowest' or `bluest' colour, they would have switched to the other colour when tested. Instead, they stayed with their original choice, which rules out their use of relative colours. Their choices are consistent with a comparison between the test colour and the recollected rewarded colour assuming they have colour constancy.
It is highly probable that the tetrachromatic Papilio butterflies
do not use `relative colour learning' either. The results of Kinoshita and
Arikawa (2000) are a
conclusive proof of colour constancy in this species as well. Honeybees do not
learn relative colours, otherwise it would have been impossible to obtain
their wavelength discrimination curve
(Helversen, 1972
;
Menzel, 1979
).
Learning of illumination colours
The total number of visits made by the moths changed when the illumination
differed from the illumination used during training. The moths noticed the
changed illumination and reacted to it as a contextual cue. Such a cue
modulates which behaviour the animal performs but does not control it directly
(Mackintosh, 1983).
On the first day that the moths saw the new illumination, they showed an
orienting reaction to the changed environment and thus increased exploration
(Gray, 1975). As a consequence,
they made fewer choices although the choices were still correct. The orienting
reaction to the changed illumination interfered with the trained behaviour.
After some days, the moths habituated to the yellow illumination, and the
number of visits to the patches increased. During this phase, there was no
significant difference between the behaviour of the moths in white and yellow
illumination. Finally, the moths learned to discriminate between the context
of the white illumination, where they were rewarded during training, and the
yellow illumination, where they were never rewarded. Tests under yellow
illumination were generally performed before tests under white illumination,
and decreased motivation can therefore not account for the result. We
therefore propose that moths learned that they were never rewarded under
yellow illumination and therefore did not approach the stimuli.
The intensity remained almost the same under white and yellow illumination. In earlier experiments, moths did not respond to much larger intensity changes (2-3 log units), although they stopped flying when it became too dark (A.K., unpublished). We cannot exclude that they learned to react to the small change in intensity instead of the changed spectrum but, even if this is the case, they still treated this as a context change.
Colour constancy mechanisms
Either of two mechanisms, receptor adaptation or lateral interaction, can
explain the colour constancy in the experiments. However, the moths can react
to the colour of the illumination even though they have colour constancy.
There are two possible explanations of this behaviour. One is that the colour
constancy mechanism does not work instantly, which allows the animals to react
to the colour change while adapting to it. This strongly implicates receptor
adaptation that works slowly as the underlying colour constancy mechanism.
Lateral interactions are assumed to work instantly and can therefore be ruled
out. Another possible explanation is that colour constancy involves
higher-level mechanisms that can distinguish between the colour of the
illumination and the colour of an object.
Studies with human infants have shown that colour constancy is not an
inborn property and that humans show individual differences. Children develop
colour constancy between the ages of two and four months
(Dannemiller, 1989). In our
experiments, the moths were tested after a very short training period where
the illumination never changed, which indicates an innate mechanism for colour
constancy. Receptor adaptation most certainly works innately and without
learning. Higher cognitive levels of colour constancy in humans are probably
learned, but it is difficult to test if animals also have these abilities.
Conclusions
Colour constancy is necessary if colour vision should be used to recognise
objects solely by their colour. It has been suggested that all colour vision
systems have colour constancy and that colour constancy could have been the
driving pressure for the evolution of colour vision
(Campenhausen, 1986). Receptor
adaptation most probably existed in animals even before colour vision evolved.
Our results prove that D. elpenor and M. stellatarum have
colour constancy and this fits well with this picture.
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Acknowledgments |
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