We are exploring the ways in which different sources of perceived chromaticity are combined in the brain. Some possible sources of apparent chromaticity include real light, afterimages, spatial chromatic induction and positive afterimages.

Real Light


We use the term ‘real light’ to refer to light which really exists – a physical source with a dominant wavelength which elicits a perceived hue. Most colors you see on a daily basis arise from real light – those on your monitor, your clothing, etc. The figure above is magenta – really!



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An afterimage is a (roughly) complementary color perceived after a colored stimulus is removed from a part of the retina. If you look at the cross in the green square above for a few seconds, then switch to the cross in the white square, you should see a magenta afterimage.


Spatial Chromatic Induction


Spatial chromatic induction is when an achromatic figure with an equiluminant surround appears to take on the color complementary to its surround. This effect has no temporal component, meaning that you don’t need to make an eye movement or change the stimulus you’re looking at to experience it.  The figure and surround must be roughly equiluminant, and induction is strongest with a narrower figure in a large surround. The diamond in the figure above should appear somewhat greenish due to spatial chromatic induction from the magenta surround.


Positive Afterimages

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A positive afterimage is produced by spatiotemporal chromatic induction – effectively a combination of spatial chromatic induction and an afterimage. The figure to the left above is the same as the one used to demonstrate spatial chromatic induction earlier – what will it produce as an afterimage? Fixate on its cross for a few seconds, then switch to the white square to its right. Do you see a magenta figure? Two independent paths are believed to be at work here. The first is the afterimage of the spatial induction. Since the figure appears greenish due to the magenta surround, its afterimage will appear oppositely colored – magenta. There is also spatial chromatic induction from the afterimage of the surround. The magenta surround produces an afterimage of a green surround, which then creates the perception of magenta in the figure through spatial chromatic induction. These two pathways (spatiotemporal and temporospatial chromatic induction, if you will) have been shown to operate in tandem. The result is that a colored surround with an achromatic, equiluminant figure will result in an afterimage with a figure of the same color as the original surround – hence the term ‘positive afterimage’, as the hue is similar rather than complementary to the color used in the adapting stimulus.

We would like to understand how these effects can be combined. Is an afterimage perceptually indistinguishable from real light? What if an afterimage is superimposed on a real light stimulus – could a green afterimage be neutralized by a magenta light source? Could it be neutralized by a spatially induced perception of magenta? We are using psychophysical approaches to explore these and other questions.