Figure 1.
Figure 2.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjD0qzGlCh9hR8NfNFpBb3fCuDXsYy5A2KfSYRkkvkpUVCDcWHv7QQamR32H_Inv8Zzp5PQ4AvxYvm1j1U9LrkXEfXkmhu1PiIPjUBl_nmdRUQ0OChacbNe4uSzBYiWKzdWeGBTIX-4bX9v/s320/unionjackafterimg.jpg)
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.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiLXeIr4OEmzSjk_ugZNq4fTFpJ4jW9dL7WQRZWPITr6G2ZPrwLWX8WClaigxBffsxKCJdzNeO4EnfUFxY4FNyhURvglN4iKaO6u-lT4jVqHt0epZoLB94b4vOQIlYJpyMs8S7-G7woKELQ/s320/shimojo_stim_2001.gif)