Lightness and Brightness and Other Confusions

ception is entirely integrated with and dependent on movement or the possibility
of movement. It is also highly dependent on the perception and apprehension
of the directional quality of light, affording objects their plasticity
though shadow-formation.
The human visual sensory apparatus, the whole eye-cortex mechanism, is a
marvellous tool. Yet it has its limitations and peculiarities. It has evolved to
operate mainly in photopic conditions, that is in daylight, which allows us
perception of detail and colour. We are diurnal animals and our vision serves
purposes of orienting and acting in the daytime. Our ability to perceive detail,
colour and contrast at night, in scotopic conditions, is heavily compromised
in favour perceiving movement and large forms. In between these two extremes
there is the mesopic zone of dim lighting, of dusk and early dawn,
where acuity of vision is low, perception of contrast and movement are diminished
and colours of objects and surfaces are distorted in comparison to our
daylight experience. There are many other constraints to our vision and these
constraints contribute to our experience of the constancy of the world. 39
Our visual organism has evolved over millions of years to provide us with
information about a three-dimensional, dynamic world with objects and
spaces in light and shadow. Much of this is tied up with the way we have adapted
as a species to the process of natural selection. This does not mean,
however, that we are mechanical slaves of the inbuilt mechanisms of our
brains. The brain of course does nothing of its own accord. It is we who use
our brains and not the other way round. We have considerable freedom in
choosing what to attend to. But this freedom has its limits, which probably
works for our benefit. Nevertheless, there is always – even after filtering and
enhancement – much more available to us in a scene than we need for appropriate
actions. We are able to extract several levels of information and meaning
from the same visual stimulus. Some of these levels are concerned with
alternative scenarios of action or interpretation, others with alternative
meanings and aesthetic qualities.
Training the eye
Let’s imagine three round green apples on a table. They are arranged at
slightly varying distances from one’s vantage point. We see that they are
roughly spherical, although the retinal images created by them are flat discs.
We see that they are green all over, although due to light and shadow they
are, say, lighter and yellower on one side and darker and greener on the other
– plus all the transitions in between – and there is a near white highlight on
39 There are very good (ecological and economical) reasons for these limitations, of
course, but it is seldom that we pause to consider their benefits for a common experience
of the world and hence our ability to appreciate such forms of non-verbal communication
as art. For a further discussion of this see Zeki 2002.
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