Visual Perception

Visual Perception is the ability of our sense organs to translate physical energy from the environment into electrical impulses processed by the brain. For example, light, in the form rays, causes receptor cells in our eyes to activate and send signals to the brain. But we do not understand these signals as pure energy. The messages are then split up in the brain and images are sorted into parts of the brain depending on size shape and colour. This process of visual perception allows us to interpret them as objects, events, people, and situations. (Grivas, Down, 2002).

The brain orgainises it messages using three main categories Gestalt principles, depth/distance principles and perceptual constancies. The Gestalt principles figure/ground, (the tendency to segregate a scene into figure and ground.) proximity, (the tendency to group close objects together as groups) similarity, (the tendency to group similar items together) closure, (the tendency to complete objects that are missing some feature) good continuation, (the tendency to see lines flowing in a continuous direction and even if partially occluded we see one object) and common fate, (the tendency to see objects moving together as a group).

Depth perception helps influence how we see objects. Depth cues are sources of information from the environment (external cues) or from within our body (internal cues) which helps to guess distance. The retina (part of the eye located near the fovea) actually holds two dimensional images from the world around us, but we normally perceive the world in three dimensional forms. Depth cues provide the information to translate two dimensional images to three dimensional are so the ability to see a front and back of an object, or in three dimensional reality. Depth cues can be categorized in terms of whether they are binocular (using both eyes and mostly use to work out short distances) or monocular (using only one eye but also operating with both eyes helps work out longer distances).

Perceptual constancies ensure that images look the same regardless of how light hit the retina (lying on couch watching TV) Perceptual Constancies help view objects that are small, or behind other objects, are assumed further away. Also Parallel lines appear to join, as they get further away.

The eye is an incredible tool that many animals rely on for daily needs especially humans in the twentieth century however nothing is perfect and there are some problems like blind spots and optical illusions. An optical illusion occurs when our perception of an object differs from physical reality. There are eight main forms of illusions: illusion of length (Muller Lyer), illusion of shape (, illusion of size, illusory contours, impossible figures, illusory motion, illusions of touch and temperature and reversible figures

There are many well known illusions such Ponzo illusion moon illusion, Muller Lyer illusion etc. Moon illusion is different because it is a naturally occurring illusion. Muller-Lyer illusion is another well known illusion, two lines of equal length are perceived to have different lengths. The difference in perceived length occurs because the arrowheads on the two lines have different angle endings.


The most popular explanation of the Muller-Lyer illusion is that our brain makes assumptions about the relative depths of the two lines. We are used to seeing outside corners of buildings with lines sloping towards the two distant perspective points on the horizon. The brain knows that the vertical line forming the outside corner is the closest part of the building. The brain realises that this line is not really longer than the other corner lines of the building. We are also used to seeing the inside corners of rooms with the lines of the roof and floor sloping outward away from them. The brain realises that this line is really longer than it appears when compared to the rest of the room. So the brain's visual system makes compensations / predictions for lines in almost every case for example like in the orientation constancy.( it means when a subject watches the television on the side it looks upright, the brain transposes the image) There are many variations of Muller Lyers illusion


Will the variations of muller lyer illusion have the same effect the standard? Will the variation (no line) have more errors on the in than the