It is that both veridical and illusory perceptions are aspects of consciousness, a state of mind that is and always has been among the central problems of psychological science. Both Wilhelm Wundt and William James, two of the great pioneers of modern psychology, identified the nature of consciousness as the primary issue for psychological enquiry. Perception is essentially a conscious process and therefore offers a route into studying the central problem of contemporary psychology.
Illusion, an impression of a real stimulus, received through one of the senses, which does not agree with other sense impressions or with objective tests. For example, a straight stick half submerged in a glass of water appears to be bent at the water line. However, if the observer runs a finger along it, the stick will feel straight. The bent appearance is called an optical illusion.
These illusory phenomena are more than just interesting visual tricks; they reveal something about the behavior and structure of the human visual system, and therefore can be used to better understand the nature of vision and perception. Thus, psychologists and vision scientists have tried to explain these illusions at a much lower level--in terms of sensory mechanisms within the visual system itself (such as receptive fields and simple feature detectors).
The basic principle to account for these illusionsis the concept of muscular strength. According to Delboeuf, the eye judges angles and lengths by the instinctual impression of the muscular effort it must exert to move from one point to another of the object, while incorporating the fact that some of the effort is expended in the transition from rest to motion and from motion to rest. These changes in muscular sensations enable us to judge differences in extent. Delboeuf argues that all causes which tend to increase fatigue should lead to a judgment of greater to-be-measured extent.
The question as to whether these illusions also occur in touch was asked during the first half of the 20th century, in particular by the Gestalt psychologists. These researchers maintained that systematic errors relating to the estimation of the size or shape of certain figures were not linked to one particular sense but were a result of the functioning of the nervous system itself and particularly of the interactions (called field effects) between the different parts of the figure.
Scene-based theories (Gillam, 1978, 1980, 1998; Gregory, 1963, 1965, 1967, 1968, 1974; Gregory & Harris, 1975) assume that visual illusions reflect the operation of processes that normally serve to enable development of a scene representation from the natural perspective information present in the 2-D retinal image. When processes evolved for the analysis of natural perspective are applied to the linear perspective information present in a 2-D drawing, they produce consistent interpretation biases that, although appropriate for natural perspective, are inappropriate for linear perspective. Visual illusions are the consequences of such biases.1
The illusions are so compelling that they go unnoticed in daily life. If human beings were intrinsically rational, they should make only sporadic errors, similar to slips of the tongue. If they used invalid rules of inference, which would explain the illusions, they would be intrinsically irrational. But the illusions have no such implication: People understand the explanation of their errors.
"We perceive a world that can be divided into objects with boundaries," said Shapiro. "According to Gestalt psychology, the objects constitute the foreground, while the rest of the world acts likes the background for these objects. Our illusion illustrates that the visual system can organize the world based on the transition between the foreground and the background."
Instead perceiving the bar at once in its full length, it appears to be drawn-out from the location of the previously flashed spot. In psychology this phenomenon has been named 'line-motion illusion' since motion is perceived even though both stimuli are displayed stationary. Thus, brain processes that initiate widespread activity propagation may be partly responsible for this motion illusion.
In the ''moon illusion,'' the moon rising or setting over the horizon appears much bigger than the moon directly overhead, even though both moons are exactly the same size. By allowing viewers to move simulated moons toward and away from each other, the scientists demonstrated that the so-called moon illusion seems to be caused by the brain's inability to estimate huge distances in the empty night sky. The brain has many similar perceptual difficulties, they said, which produce a variety of convincing optical illusions.