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