PERCEPTION OR HALLUCINATION

Most observers perceive an inverted ‘whiter-than-white’ triangle with clearly defined edges filling the space between the black discs, each with a sector removed. This inverted triangle is illusory, since the white paper on which it is perceived is of the same luminance as that outside the triangle. It is as though, faced with the incomplete black discs and line corners, the visual system makes the best bet – that this particular configuration is likely to have arisen through an overlying object occluding complete black discs, and a complete outline triangle. In other words, since the evidence for an occluding object is so strong, the visual system creates it. A rather different example is depicted. A small spot is projected onto a large frame or screen, which is then moved. What the observer sees is the spot moving on a stationary screen. Again, this appears to reflect an assessment of relative probabilities. Small foreground objects are more likely to move than large background objects, and s this is what the observer sees.