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First published online August 9, 2007
Journal of Experimental Biology 210, 2795-2800 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.007377

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Colour perception in a dichromat

Lina S. V. Roth^1,*, Anna Balkenius² and Almut Kelber¹

¹ Department of Cell and Organism Biology, Vision Group, Lund University, Helgonavägen 3, S-22362 Lund, Sweden
² Department of Plant Protection Biology, The Swedish University of Agricultural Sciences, Sundsvägen 14, S-230 53 Alnarp, Sweden

View larger version (16K): [in this window] [in a new window]   Fig. 1. The chromatic space of a trichromat and a dichromat. (A) Chicks were left with a trichromatic vision by excluding UV light from the illumination. They were trained to two positive stimuli (red `+') and they generalized onto a novel intermediate test colour (red circle) in all cases but one. When tested with blue and yellow, which lie on opposite sides of the grey point, the chicks did not generalize onto the intermediate test colour, grey. They treated the grey point as achromatic. Redrawn from (Jones et al., 2001). (B) In dichromats, one hypothesis suggests that the neutral point divides the chromatic space into two colour categories (Jacobs and Deegan, 1994; Vienot et al., 1995). (C) A second hypothesis (Hemmi, 1999) proposes that dichromats perceive a continuous scale of colours. We trained horses in two experiments. (D) As a control, two horses were trained to two positive colours (red `+') with colour loci situated on the same side of the neutral point and tested with a novel intermediate green colour (red circle). The black `–' is the negative colour. (E) To test the first hypothesis – whether the neutral point operates as a categorical boundary in the dichromatic colour space – three horses were trained to two positive colours with colour loci situated on different sides of the neutral point and tested with a novel intermediate grey stimulus corresponding to the neutral point. (F) All colours are visualized and named as they appear to humans and with respect to the corresponding wavelength. The symbols shown in F represent all used colours and their corresponding wavelength, i.e. the wavelength of a monochromatic colour that has the same hue. All stimuli are marked with the same symbols in Fig. 2 (see Materials and methods).

View larger version (7K): [in this window] [in a new window]   Fig. 2. Stimulus colours. All colours were measured with a photospectrometer and brightness is visualized as the quantum catch for S and L cones, calculated according to Eqn 1. The ratio between the absorption of L cones and the sum of both cone types determines the location of each colour on the x-axis. Below the chart, the corresponding wavelength is visualized.

View larger version (98K): [in this window] [in a new window]   Fig. 3. Experimental apparatus. The horses were released at a distance of 3 m from the two stimuli placed on the doors of the experimental apparatus. A wooden divider forced them to choose a door at a distance of at least 40 cm from the stimuli.

View larger version (34K): [in this window] [in a new window]   Fig. 4. Experiments I and II. The results bars are located corresponding to the chromatic space shown at the top, and the black vertical line corresponds to the neutral point in horses. Different shades of grey in the bars signify different horses (binominal tests; *P<0.05, **P<0.01). (A–F) In Experiment I, two horses were trained to positive grey and yellow stimuli (red `+'). Blue was the negative stimulus (black `–'). (A,B) Both horses reached high choice levels in both training combinations. In tests (C) and (D), they treated the novel green stimulus (red circle) as a positive colour and showed a significant preference for green when it corresponded to the longer wavelength of the stimuli presented. In test (E), the horses again showed preference for the stimulus corresponding to the longer wavelength. In a final test (F), one horse chose the novel green test colour as if it was a positive stimulus. (G–L) In Experiment II, blue and green were positive training stimuli (red `+') and yellow was the negative training stimulus (black `–'). (G,H) All three horses reached high choice frequencies for both training combinations. In tests (I,J) with one positive training stimulus and the novel intermediate stimulus, grey, corresponding to the neutral point, grey was treated as a positive stimulus in the experiment. In test (I), a significant preference for the colour corresponding to the shortest wavelength was seen. (K) When both positive colours were presented to the horses a strong preference for blue was again seen. (L) Two horses tested with the negative yellow stimulus and the novel colour grey significantly preferred grey.

View larger version (12K): [in this window] [in a new window]   Fig. 5. Experiment III. The results bars are located under the corresponding colour in the chromatic space shown at the top (binominal test; **P<0.01). The black vertical line corresponds to the neutral point. Two horses were trained on positive green stimulus (red `+') and negative grey stimulus (black `–'). (A) Both horses reached high choice levels for the training combination. (B) In tests with the positive green stimulus and a novel yellow stimulus (red circle) no significant difference could be seen. (C) In tests with the negative grey stimulus and a novel blue stimulus (red ring) both horses preferred the negative grey stimulus corresponding to the longest wavelength in the combination.