Psychologists typically make a distinction between sensation and perception. Sensations are uninterpreted sensory impressions created by the detection of environmental stimuli, whereas perception refers to the set of processes whereby we make sense of these sensations. Perception enables us to literally navigate through the world, avoiding danger, making decisions, and preparing for action. Visual perception has received the most attention from researchers, and then speech.
The problem of visual perception
At the back of the eye (see also: human eye) is an area called the retina, consisting of light-sensitive neurons. However, the retina is not light sensitive at the point where the optic nerve leaves the eye and this results in a blind spot in our visual field. However, most of the time we are unaware of the existence of the blind spots associated with each eye, except where a special procedure helps draw it to our attention (see here, for example). It seems that our brains perform a "best guess" operation and try to use context to fill in the gap.
Light from an object is inverted as it falls on the retina. The same pattern of light could be caused by an infinite number of different objects, yet our brains usually manage to make the correct interpretation. This is known as the inverse projection problem. How do we make sense of visual sensations? How do we distinguish one object from another? How do we perceive depth from the array of light stimuli that strike our retinas? Consider the picture shown to the right . What is the object shown here? (for the answer, see footnote1). Most people do not "see" (i.e. perceive) the object immediately, though usually do once told what it is.
The transformative nature of perceptual processes is also suggested by the existence of perceptual illusions. Several of these are shown below. In the Ponzo illusion2 people tend to see the top horizontal line as longer than the one below, whereas they are both the same length. In the Ebbinghaus illusion3 people tend to see one of the central circles as larger than the other, although they are both the same size. In the cafe wall illusion4the horizontal lines between the bricks are all parallel, but are perceived not to be. Also shown below is a British road safety commercial, shown in 2008, which used visual illusions to show how easily misperceptions can occur (I don't vouch for the specific statistics stated in the ad).
Gestalt principles of visual perception
In the 20th century, Gestalt psychologists developed the idea - in opposition to structuralism - that our perception of an object is more than simply the sum of individual sensations pieced together. They developed various laws, or principles, according to which our minds group objects in the environment. According to the Law of Pragnänz, a kind of meta-principle, out of the many various ways that visual stimuli could be organised, we perceive the stimuli in the simplest way. The Law of Pragnänz translates as the Law of Good Figure or the Law of Simplicity. Other Gestalt laws are described here. The Gestalt laws can be considered "best guess" solutions to the problems of percepetual organisation, and are not really laws or principles at all but, rather, heuristics5 (cognitive shortcuts). Essentially, the Gestalt theory of perception seems to describe aspects of perception rather than truly explain them.