Basic Visual Processes Colour vision I: history, colour mixture, colour anomalies, measuring spectral absorption

Lennie’s view A portrait of the visual cortical systems based on simple considerations of area

“Specificity” of rf properties

Contrasting views

The history of colour vision Isaac Newton, the colour circle and the barocentric model George Palmer and the first glimmerings of trichromacy Thomas Young and the principle of univariance Herman Helmholtz and the beginnings of serious colorimetry James Clerk Maxwell, overlapping curves and imaginary primaries Modern trichomaticity theory and measuring the real curves

Isaac Newton’s experimentum crucis

Spectral composition of white light Pass white light through a prism and produce a spectrum of colours Pass through a second prism and can get no further separation of colours “The light whose rays are all alike refrangible, I call simple, homoegeneal and similar; and that whose rays are som more refrangible than others, I call compund, heterogeneal and dissimilar” “The colours of homogeneal lights, I call primary, homogeneal and simple….”

The beginning of perceptual psychology? “For the rays to speak properly are not coloured. In them there is nothing else than a certain Power and Disposition to stir up a Sensation of this or that Colour”

Newton’s barocentric model “Do not several sorts of Rays make Vibrations of several bignesses, which according to their bignesses excite Sensations of several Colours, much after the manner that the Vibrations of the Air, according to their several bignesses excite Sensations of several Sounds?” Newton tried some rudimentary colour mixing experiments and though his equipment was not up to the task (he couldn’t make whites from two primaries), his real stroke of brilliance was to turn the spectrum into a circle -seven primaries (VIBGYOR) based on notes of the musical scale  the spectrum appears continuous

Newton’s colour mixture theory Primaries lie on outside of circle. Saturation is represented by distance from centre. To predict the outcome of any combination of two colours, simply draw a line between them. Location on line depends on relative contribution from each colour.

George Palmer and trichromacy “The superficies of the retina is compounded of particles of three different kinds, analagous [sic] to the three rays of light; and each of these particles is moved by his own ray.” (1777)

Thomas Young “Now, as it is almost impossible to conceive each sensitive point of the retina to contain an infinite number of particles, each capable of vibrating in perfect unison with every possible undulation, it becomes necessary to suppose the number limited…and that each of the particles is capable of being put in motion less or more forcibly by undulations differing less or more from a perfect unison” Helmholtz’s graphical expression of Young’s idea – the principle of univariance

From Helmholtz to Maxwell Helmholtz was skeptical about trichromacy because of the failure of his own colour mixture experiments Maxwell’s insight was that if the sensitivity curves for the three receptors overlapped, the colour mixture of the kind described by Newton and tested by Helmholtz would not work.

Maxwell’s “imaginary primaries” If selected primaries do not fall into region of overlap, then any test light can be matched by some combination of the primaries

Maxwell’s imaginary primaries If chosen primaries fall into region of overlap, then some test lights cannot be matched by sum of primaries. But they can be matched by subtraction.

Can describe ‘algebraically’ T=aL+bM+cS (this just means that the appearance of the test light can be matched by the sum of a amount of primary L, b amount of primary M and c amount of primary S). In cases like the one just shown, it would be: T=aL+bM-cS Or T+cS=aL=bM In other words, the match can be made but the primary must be added to the test stimulus

What does this look like as a barocentric model? Circle represents the spectrum of visible colours, as in Newton’s model Primaries are located off the circle, meaning that it is impossible to activate just one of the processes

Modern colorimetry Adjust the three primaries until it is impossible to distinguish the two sides – that is, until a metameric match is obtained

More modern colour mixing functions

Chromaticity coordinates Now the numbers are normalized so that the values of the three curves sum to 1 at any point.

Colour spaces depend on choice of primaries Both of these spaces are based on the same colour mixture data but with different primaries and one can ‘easily’ be derived from the other.

The CIE colourspace

JND’s in CIE colour space Each ellipse is drawn at 10 x standard deviation of values obtained for metameric match to colour represented at crosses. This means that CIE space is not necessarily a good psychological tool.

Munsell-Farnsworth colour discrimination test Designed for equal psychological distances for normal colour vision

Measuring the ‘real’ primaries Direct physical measurement Retinal densitometry Microspectrophotometry Suction electrophysiology Psychophysical measurement Using colour anomalous observers

A retinal densitometer -the main point to get here is that one is measuring light reflected back out of the eye -by selectively bleaching different types of cones, we can limit absorption to unbleached species.

Can also use colour deficient observers Protanopia – lacking long wavelength pigment Deuteranopia – lacking middle wavelength pigment Tritanopia – lacking short wavelength pigment Genes for long and middle wavelength pigments located on X chromosome Genes for short wavelength located on chromosome 7.

Colour deficiency and retinal densitometry

Microspectrophotometry

MSP results

Suction electrophysiology -tends to give more stable recordings than MSP but results are quite similar

Psychophysical measurements of absorption curves

A comparison -psychophysics State of the art retinal densitometry using adaptive optics

One more look at the receptor mosaic

Individual variability Both images from the same area in two different retinas from people with normal colour vision.