primeview - PRIME - page 8 (Physiology)

( Prime - page 8 of 32 )

PRIME: PHYSIOLOGY, continued

Receptors sensitivity to colour

It is not enough to say that an object's colour is derived from the wavelength it reflects while absorbing others. A red object will still appear red when it is essentially reflecting predominantly middle wavelength or short wavelength light. In a series of experiments, Edwin Land, showed that the brain registers the varying intensities of light, or lightness, in each of the long-, middle-, and short-wave bands from all of the coloured surfaces in a scene, compared them simultaneously to produce three lightness records, and then compared those lightness records to construct colours. It is receiving striking confirmation from physiological studies of single cells in the visual cortex, most notably by Semir Zeki (footnote 5.) and from studies of patients who had become colour blind as a result of injury to the brain.

Diagram of the structure of the retina

(1) choroid; (2) pigmented epithelium; (3) and (4) sensitive epithelium (layer of rods and cones) ; (5) necleus of cones; (6) necleus of rods; (7) synaptic layer; (8) horizontal cells; (9) layer of bipolar cells; (10) inner associative cells; (11) synapses; (12) layer of ganglion cells; (13) fibers of optic nerve that carry the impulses to the brain.

The Eye ( partial section )

The physiological study of the eye is fascinating for a number of reasons. Firstly, the retina of the eye appears to be an extension of the brain. The retina can be described as a net of a complex organisation of rods and cones, which are identified by the shapes of the light-receptive terminal members, sensitive to specific frequencies of light. The rods are generally sensitive to lightness and darkness, and are capable of intensifying their response in very poor lighting conditions. The eye and brain, automatically adjusts, and the rods become an extremely sensitive detector in conditions of low illumination. The cones operate however, at relatively high light intensities and are particularly sensitive to colour.