(part 3 of 6)
There are a number of different views of visual perception in addition to the psycho-physiological one we have taken thus far. These tend to deal more with perception-as-phenomenological-experience or perception-in-information processing than with perception-as-registering-and-coding-sensation. Examples are the Gestalt, the constructivist, and the ecological approaches.
The Constructivist view: The process of seeing is an active process in which our view of the world is constructed from information in the environment and from previously stored knowledge. So, what we see is not a replica of the world in the same way that a photograph is, despite the commonly held view that the eye is like a camera. Instead, the visual system constructs a model of the world by transforming, enhancing, distorting, and discarding information. In doing so, the visual system provides us with a much more constant view of the world than if we were simply to "see" the images produced on our retinas. So, when we move about (e.g. walk down a street), we see buildings as being stationary and people being approximately the same size and shape - despite the fact that their actual relative images on the retina may be quite different. In the same way, our ability to perceive objects displayed on computer screens, for example, whether they are text or graphics, 2-D or 3-D representations, is a result of our prior knowledge or our expectations as to what should appear, as well as what impinges on our retinas.
The Ecological view: The process of seeing is greatly influenced by what uses (affordances) the object perceptions suggest. Also called direct perception, it stems from work with pilots. Takes as its starting point not a retinal image which is passively sensed. Talks of the ambient optical array that the observer samples. Perception and actions are seen as tightly interlocking and mutually constraining. Emphasised texture.
Movement and spatial perception
Movement can be detected either by moving ourselves (even though the image falls on the same part of the retina) or by staying still whilst the image moves across the retina. The processing of the bodily stimuli and visual stimuli are combined since we can track a still object with our eyes while moving our bodies and yet be sure that it is still.
Ask someone to wave a lighted cigarette (or better yet, a match) around slowly in a darkened room and follow it with your eyes. The movement of the light will be seen even though no image is moving across the retina. The periphery of the retina is the area most sensitive to movement, but it is very difficult to identify an object at the far periphery of the visual field. Movement in the periphery of the eye is what usually initiates the movement of the eye in pursuit of an object so that is can be brought to focus on the fovea. Movement may also be perceived, however, when both eye and retinal image are stationary in the form of afterimages.
Above a certain speed of movement the eye will spontaneously track a moving object (I believe this can be suppressed, however).
Spatial perception comes from the two eyes (muscular activity and discrepancy in image). We can simulate depth perception using the image discrepancy, and several other ways.
After-effects After viewing certain stimulus configurations in motion, we see illusions of movement commonly known as aftereffects. The most marked of these phenomena is the "waterfall effect". If you look at a nearby waterfall for several minutes and then direct your eyes to the bank or any fixed object, it will seem to move in a direction opposite to the flow of the water. This occurs only if there is a relative movement in different parts of the retina, not if the moving field covers the entire retina and moves as a whole. This requires the actual movement of images across the retina and operates independently of eye movements.
Depth cues
Designing for computer screens means using a 2-d surface. Therefore, when we want to display a 3-d image, we have to employ some techniques to based on a 2-D image. The most important of these are the use of perceptual depth cues, for example, size, interposition, contrast, clarity and brightness, shadow, and texture. In the real world, motion parallax may be one of the most important cues that enable us to perceive distance and depth. It occurs when we move our heads from side to side. The objects we see are displaced at different rates. Objects that are further away appear to move more slowly than objects that are closer. In screen design, the trick is to move the viewpoint of the "camera" so that the image on the screen moves according to the principles of motion parallax.
Size - the larger of two otherwise identical objects appear to be closer than the smaller one
Interposition - if one object partially obscures a second object then the blocked object is perceived to be behind and beyond the blocking object.
Contrast, clarity and brightness - sharper and more distinct objects appear to be nearer, and duller objects appear to be further away. In the diagram the sharper rocks appear close and the duller mountains further away.
Shadow - shadows cast by an object provide some cues to the relative position of objects
Texture - as the apparent distance increases the texture of a detailed surface becomes less grainy.
Motion Parallax: when moving one's head from side to side the objects one sees are displaced at different rates.
Stereoscopic depth cues: two images of the same object from slightly different angles are presented separately to each eye. Induce perceived depth through the processes of fusing those images. This is often used in so-called virtual reality.
Perceptual organisation
Gestalt principles of grouping: This approach is a rebellion against the simplistic notion that perception could be analysed in terms of its component sensations (the view of associationism that was dominant in the late 19th and early 20th centuries) and that complex ideas were the result of associating together simpler ones. Gestaltists focused on the fact that there are important aspects of form and structure. A tune is recognisable despite being played in a different key or at a different speed. The spatial and temporal relationships between elements are as important as the absolute size, location or nature of the elements themselves, and a sensation-based account of perception fails to capture this.
We see the world as composed of discrete objects of various sizes that are seen against a background comprised of textured surfaces.
illustrate with squareness