Visual Contrast; the importance of mutual inhibition in visual perception.

This aspect of the mechanics of vision is particularly interesting and relevant. Photo receptors in the retina are wired up to accentuate contrast; this happens through a reduction/suppression of electrical activity in receptors exposed to lower levels of light wherever they are situated next to receptors that are exposed to higher light levels. This increases the difference between the two signals. In colour perception, hue and saturation are also processed through a contrast-enhancing mechanism.

'This 'contrast only' form of vision meant that the eye could detect small local variations in light intensity, even though ambient light varies a million fold between sunlight and starlight.'
(P 205)


'Only if we rob and object of all context can the eye be fooled. A full moon at night, lit by a sun we cannot see, and suspended in the non-reflecting vacuum of space appears white -yet moon dust is black.'
(P 231)

'Context is everything. The eye has no interest in absolute levels of illumination, nor colour values. Every colour is perceived in relation to every other, just as every patch of light is perceived in relation to every patch of shade. In the real world this makes colours remarkably stable.'
 (P 236)

'Humans are the descendants of night dwellers, and our excellent night vision is one of the consequences of our spell in the dark'
(P 257)

'If we are surprised to learn that blue and yellow are perceived as mixtures, how much more startling it is to discover that red- most vibrant and unsettling of colours; the colour of danger, blushes, poison, blood, and sexual arousal- is not directly perceived, but constructed whenever yellow cones are excited and blue cones fall silent. Indeed the sensation of red can be generated by removing all bluish wavelengths and enriching the yellowish green part of the spectrum. The fact that red is patently more than a formula is striking evidence that colour is neither an objective property of objects nor of the light they reflect; it is a construct of the mind.'
(P 228)

Hartline 'mutual inhibition'

Lands 'retinex theory'

Mach Band illusion

Gordon Lynn Walls - The Vertebrate eye and its Adaptive Radiation.

All references taken from
The Eye; A Natural History.
Simon Ings
Bloomsbury 1997.