Where are coloured afterimages generated?

A facinating study by Shimojo et al., (2001) has demonstrated something unexpected about how colour afterimages are generated in the brain. Chromatic after images are common and easy to produce. See for yourself - try staring at fig 1 for 30 seconds and then shift your eyes to the blank region on the right. You may find it takes a couple of seconds to start, but you should find yourself experiencing a coloured afterimage. Can you see a face? Theories of colour vision suggest that these after images are generated through opponent processes in the retina and brain. The opponent colours come in three pairs, red - green, blue-yellow, black-white. If you try the test again on figure 2 you should experience a correctly coloured union jack as the afterimage. Notice the range of colours in the original consist exclusively of the complementary opponents.

Figure 1.










Figure 2.












There is also another factor that is involved when we see coloured after images. You will notice that the effect requires you to stare at the original for a certain length of time. This implies that stimulating the visual system with the same coloured pattern for a length of time causes a change to tale place. This change is neural adaptation. Constant excitation of a neuron causes it to become fatigued and fire less to a subsequent input.

It is commonly believed that the primary (if not only) site of neural adaptation causing colour after effects is the retina - at the level of the photoreceptor. The photo pigments are believed to become bleached, causing adaptation. With a simple but effective visual trick Shimojo et al have shown otherwise. By constructing a stimulus which generates the effect of "perceptual filling in" -that is a region where the brain believes there should be colour and fills in the gaps, even though there is no physical colour being presented - they have shown that afterimages are also perceived for this "illusory" coloured region. That is, a perceived region of colour not presented to the eye, can causes an after image - This means that adaptation must be taking place at a cortical site and not in the eye. A simple but effective experiment!

Try it for yourself by staring at the white dot underneath the red region in figure 3 for 30 seconds. Then shift your gaze to the white dot on the right. You may see one of the patterns shown on the bottow row (B).

Figure 3.

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