Sight Our Visual Perception

We can see due to our visual system. The visual system is made up of our eyes, the accessory structures, the optic nerves, tracts and pathways. So what are the basic steps that allow us to see?

Sensation and perception

Major Structures Of The Human Eye

How Light Travels through the Eye

Processing the input from our eyes Transduction Once the light has been focussed as an upside down image on the retina the receptor cells (cones and rods) change the light or electromagnetic energy into electrical energy or nerve impulses Transmission The electrical nerve impulses travel in nerve fibres across the surface of the retina and into the optic nerve which carries the impulses to the brain.

Perception: bottom-up processing In the brain the millions of signals sent from each eye stimulate feature detector cells that determine if each tiny part of an image is vertical or horizontal, moving or unmoving, as well as its length and other information. This information is built up into a whole picture, which our brains then make sense of using thought and perception. This is a simplified view of how vision may occur and is referred to as bottom-up processing: making a whole out of meaningful parts.

Perception: top-down processing Top-down processing: our understanding and knowledge of a whole object allows us to fill in gaps in what we see and overcome any imperfections Perception involves the selection, organisation and interpretation of incoming sensory information. We are going to focus on the organisation of information.

Perception involves integration of past and current experience Perceptual constancies allow us to perceive stimuli that change in size, shape, brightness and orientation in a constant, familiar fashion. So our perception stays the same despite sensory changes that are detected by the retina. The more familiar we are with the observed object the more readily we are able to maintain perceptual constancy of it. Perceptual constancies are usually formed in childhood.

Perception constancies Size constancy is the ability to recognise that an object remains constant in size regardless of its distance from the observer or the size of its image on the retina Three factors influence this: Previous experience with the true size of an object The distance between the object and the person The presence of surrounding objects

Perception constancies Shape constancy is the ability of the visual perception system to recognise a shape despite changes in its orientation or the angle from which its viewed

Perception constancies Orientation constancy is the ability to visually perceive the real orientation of objects even though their retinal images may be at a different orientation. Objects such as buildings, trees and chimneys are normally perceived to be in a vertical position regardless of whether they are actually vertical and regardless of the orientation or position of our own bodies.

Gestalt theories Gestalt theories of perception describe the brains organisation of sensory information into meaningful information. We will now go through four of these. 1. Proximity: Elements that are close together are grouped together to form an overall figure How do these two images appear to you? Still separate but closer in proximity, usually perceived as a group Separate shapes

Gestalt theories 2. Similarity: Elements that are similar are grouped together The example above (containing 11 distinct objects) appears as as single unit because all of the shapes have similarity. Unity occurs because the triangular shapes at the bottom of the eagle symbol look similar to the shapes that form the sunburst.

Gestalt theories 3. Closure: An object is perceived as being whole despite it actually being incomplete Although the panda above is not complete, enough is present for the eye to complete the shape.

Gestalt theories 4. Figure-ground organisation: The part of the visual field being focused on is the figure and the surrounds are the ground. Figure and ground are surrounded by a real or imagined contour (line). A form, silhouette, or shape is naturally perceived as figure (object), while the surrounding area is perceived as ground (background).

Understanding illusions Rest of lesson: Complete both sides of the handout on visual illusions 2. Decide which visual illusion you would like to study in more detail as part of your final project and obtain the following information: an image of the illusion information on its creation an explanation of how it works using knowledge of visual perception including any previous experience that is required by the person observing it for it to work