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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).
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