Lightness and Brightness and Other Confusions

the sensory data pouring in from all directions. The brain – and the whole
living organism – must choose what is relevant (see also Natural Experiences
and Physical Abstractions, p. 23 in this volume). Perceptual psychology
abounds with tests and demonstrations of visual attention showing how selective
our vision is. In given situations our brains can choose to ignore huge
stimuli in preference for much smaller or weaker ones – and not always to
our advantage. Most of the time our visual system works just fine – largely
due the process of selection and filtering, which keeps most of the irrelevant
and confusing sensory data out of our visual experiences. 36 But there is more
going on in our brains and minds when we see. While the brain discards irrelevant
stimuli, it enhances others. It achieves this on mainly two levels: the
inbuilt automatic level of visual processing and the more conscious level of
visual attention. An example of the automatic processing is edge detection;
another is simultaneous contrast, which is sometimes involved in the former.
37 These kinds of ‘hard-wired’ mechanisms help us to separate objects
from their backgrounds and edges of objects from the borders and gradients
of light and shadow. All this happens without the intervention of our consciousness.
An example of visual attention is keeping your eyes (and hopefully
your mind!) focused on the words and lines of this text amid all the
myriads of stimuli surrounding you.
How the brain chooses what is relevant for each situation is a subject that is
beyond the scope of this article. It is worthwhile, though, to consider for a
while why it does this and what are the consequences for our perception of
colour and light. Starting from the notion that the proximal stimuli for vision,
the patterns of radiant energy on the two retinas (that are sometimes called
retinal images) 38 , are physically some two or three centimetres diameter in
size. They are also two-dimensional, although projected onto a convex spherical
surface. From these two proximal stimuli we, our brains and bodies, are
able to create a perception of the world that is not only spatial, but surrounds
us entirely, is infinite. It is in our ecology that our perceptions are integrated
into this spatial whole in a way that provides us with information primarily
concerning our position in and relation to the spatial whole. This spatial per-
36
Zeki 2002, pp 5–6.
37
Simultaneous contrast, also known as colour induction, is the perceived hue or lightness
shift of a colour when juxtaposed spatially with another colour. The ‘subjective’
contrast colour occurs simultaneously with the ‘objective’ stimulus colour. Simultaneous
contrasts are most apparent in juxtapositions of the centre–surround type.
38
The notion of the retinal image was first criticized by James Gibson (Gibson 1986).
Gibson says that the spatial features of the world are perceived directly. According to
this view the staring point for perception is in the optic array surrounding us. Our
movement through this array creates, among other things, an optic flow rather than a
series of static retinal image for analysis by the brain. There are no retinal images,
static or otherwise, argues Gibson. Instead there is a constantly moving pattern that is
the proximal stimulus for integrating space, colour and objects through our interaction
with the world.
35