| Symposium on How the Brain Constructs Reality |
| 14 and 15 Dec, 2000 |
Department of Ophthalmology, University of Auckland, Auckland, New Zealand
Colour Appearance: Evidence for a Cortical Third Processing Stage Beyond Opponent Cells
The extent within isoluminant colour space that observers perceive red, green, blue and yellow was measured. Colour scaling using large spots, activating all cone types, was compared to that with tiny central foveola spots, where S cones and thus S opponent (So) cell activity is largely absent. The addition of So input to that from L and M opponent cells changes the chromatic appearance of all colours, affecting each primary colour in different chromatic regions in predictable directions. Shifts from white to the various chromatic stimuli used produced sinusoidal variations in cone activation as a function of colour angle for each cone type. However the psychophysical colour-scaling functions for 2° spots were nonsinusoidal, being much more peaked, fitted by sine waves raised to exponents between 1 and 3. Using single cell recording techniques the same difference was found to be true for a large population of LGN and striate cortical cells. Therefore, narrow colour tuning, discrepancies between the spectral loci of the colour-scaling function peaks and those of LGN cells, and changes in colour appearance produced by eliminating So input provide evidence for a cortical processing stage where the perceptual colour axes are rotated by a combination of So cell outputs with L and M opponent cells.
This page was last updated on 16 Feb 2001.