1.2.1 Opponent theory of colour perception

The section above describes how the light signal is detected by
structures within the eye. However, vision generally and colour
vision in particular is not experienced as a sequence of random spots
of light or colour flashing on and off in rapid succession; instead, we
see whole objects, and whole areas of colour. This tells us that the
firing of nerves in response to cone and rod cell excitation is not the
end of the story by any means. While the mechanism for detection
of light stimulus by the eye is wellunderstood
and uncontroversial,
there is no similarly wellunderstood
mechanism for interpreting
those detected stimuli as complete visual sensations.
One theory accounting for some empirical observations is known as
the opponent theory of colour vision. The observation that it most
clearly explains is the commonlyexpressed
view that some colours
are opposites of each other: red and green are opposites, and so are
yellow and blue. By ‘opposite’ here is meant that there is no such
thing as a greenishred
colour or a bluishyellow,
whereas other
colours are perceived as mixtures of these opponent primaries.
The proposed mechanism for this opponent theory is that the nerves
do not transmit the firing intensities of the different kinds of cone

Colour Vision
cells directly; instead, the brain receives information about the
differences between combinations of these intensities. The
physiological effects of opposed colours was investigated in Theory
of Colours by Goethe3; more recent psychological theories use the 3Johann Wolfgang von Goethe
(1749–1832), German writer,
scientist and diplomat.
mechanism of opposition to explain not only colour vision but also
other sensations such as emotion and addiction. However, even in
the case of colour vision, there is experimental data that suggests
that the opponent theory is not precisely true: under certain
conditions, it is possible to cause people to perceive a colour which
is described as reddishgreen.