PERCEPTION AND ILLUSION

WHEN SEEING GOES WRONG
One way of uncovering the processes of seeing is to look at the circumstances in which they go wrong. For example, returning to the ‘cat in the garden’ problem, suppose that, when you were walking past your neighbour’s garden, you were on your way to the station. When you arrived, you boarded your train, then you waited, and waited . . . At last you sighed with relief as the train started to move . . . but then the back of the train on the adjacent track went past your carriage window and you saw the motionless platform opposite. Your train had not moved at all, but your brain had interpreted the movement of the other train – incorrectly – as caused by your own movement, not that of the object in the world. Why are we fooled? How does your brain decide what is moving in the world and what is not? What can we discover from the train experience about how seeing works? As we look at a scene full of stationary objects, an image is formed on the retina at the back of each eye. If we move our eyes, the image shifts across each retina. Note that all parts of the image move at the same velocity in the same direction. Similarly, as we look through the window of a moving train, but keep our eyes still, the same thing happens: our entire field of view through the window is filled with objects moving at a similar direction and velocity (though the latter varies with their relative distance from the train). In the first case, the brain subtracts the movements of the eyes (which it knows about, because it caused them) from the motion in the retinal image to give the perception of its owner being stationary in a stationary world. In the second scenario, the eyes have not moved, but there is motion in the retinal image. Because of the coherence of the scene (i.e. images of objects at the same distance moving at the same velocity), the brain (correctly) attributes this to movement of itself, not to that of the rest of the world.

To return to the situation in which we may be fooled by the movement of the other train into thinking that our train is moving – notice that, although the visual information produced by the two situations (your train stationary, other train moving, or vice versa) is identical, other sensory information is not. In principle, the vestibular system can signal self-motion as your train moves. However, slow acceleration produces only a weak vestibular signal, and this (or its absence, as in the present case, if we are in fact stationary) can often be dominated by strong visual signals. Of course, objects in the world are not always stationary. But objects that do not fill the entire visual field cause patterns of movement which are piecemeal, fractured and unpredictable. One object may move to the right, another to the left, and so on, or one object may move to partially obscure another. So lack of coherence in the pattern of motion on the retina suggests the motion of objects, instead of (or as well as) motion of the observer. Think back to what happened as you were walking past your neighbor’s garden. The patterns of movement in the retinal images caused by the movements of your body and your eyes were mostly coherent. The exceptions were caused by the movements of the long grasses in the breeze and the tiny movements of the cat as it stalked a bird, which were superimposed on the coherent movements caused by your own motion. The visual system needs to detect discrepancies in the pattern of retinal motion and alert its owner to them, because these discrepancies may signal vital information such as the presence of potential mates, prey or predators (as in the case of the cat and the bird). Indeed, when the discrepancies are small, the visual system exaggerates them to reflect their relative importance. Contrast illusions and after-effects Some further examples of perceptual phenomena that result from this process of exaggeration are shown in the Everyday Psychology box. These are known collectively as simultaneous contrast illusions. In each case the central regions of the stimuli are identical, but their surrounds differ. Panel A (figure 8.1) lets you experience the simultaneous tilt illusion, in which vertical stripes appear tilted away from the tilt of their surrounding stripes. Panel B shows the luminance illusion: a grey patch appears lighter when surrounded by a dark area than when surrounded by a light area. Panel C shows the same effect for colour: a purple patch appears slightly closer to blue when surrounded by red, and closer to red when seen against a blue background. There is also an exactly analogous effect for motion, as well as for other visual dimensions such as size and depth. Suppose your train finally started and traveled for some time at high speed while you gazed fixedly out of the window. You may have noticed another movement-related effect when your train stopped again at the next station. Although the train, you, and the station platform were not physically moving with respect to each other, the platform may have appeared to drift slowly in the direction in which you had been traveling. This is another case of being deceived by the mechanisms in our nervous systems. This time what is being exaggerated is the difference between the previously continuous motion of the retinal image (produced by the train’s motion) and the present lack of motion (produced by the current scene of a stationary platform), to make it appear that the latter is moving. Such effects are known as successive contrast illusions, because visual mechanisms are exaggerating the difference between stimuli presented at different times in succession (compared with simultaneous contrast illusions, in which the stimulus features are present at the same time). A famous example of this effect is the ‘waterfall illusion’, which has been known since antiquity, although the first reliable description was not given until 1834 (by Robert Addams: see Mather et al., 1998). If you gaze at a rock near a waterfall for 30–60 seconds and then transfer your gaze to a point on the banks of the waterfall, you will notice a dramatic upward movement of the banks, which lasts for several seconds before they return to their normal stationary appearance. Because the first stimulus induces an alteration in the subsequently viewed stimulus, this and other similar illusions are often known as after-effects. [after-effect change in the perception of a sensory quality (e.g. colour, loudness, warmth) following a period of stimulation, indicating that selective adaptation has occurred] Several further examples of successive contrast are given in the Everyday Psychology section of this chapter. In each case the adapting field is shown in the left-hand column and the test field is shown on the right. Now look at figure 8.2. Panel A lets you experience the tilt after-effect, in which vertical stripes appear tilted clockwise after staring at anti-clockwise tilted stripes, and vice versa.

Panel B offers the luminance after-effect: after staring at a dark patch, a grey patch appears lighter, and after staring at a white patch the grey patch appears darker. Panel C shows the colour after-effect: after staring at a red patch a yellow patch appears yellow-green, and after staring at a green patch a yellow patch appears orange. Like the simultaneous contrast illusions, these after-effects demonstrate that the visual system makes a comparison between stimuli when calculating the characteristics of any stimulus feature. These illusions are not just for fun, though. They also give us vital clues as to how we see, hear, touch, smell and taste under normal circumstances. Indeed, there are three general theories about how we perceive, and these illusions help us to decide between them.