Not all of Gregory’s explanations are accepted- e.g. Muller Lyer Last week we looked at Gregory’s Top Down theory of perception. (Indirect theory) Gregory Nature/ Nurture (perception is influenced by learned knowledge and contextual cues) Supporting evidence from visual illusions e.g. perceptual set, Diener (1990)- see textbook Not all of Gregory’s explanations are accepted- e.g. Muller Lyer Over emphasised perceptual errors- we’re usually pretty good. (Generalisable to real world?) X X

Objectives To explain what is meant by the phrase ‘Bottom Up’ processing To Outline James Gibson’s theory of perception. Invariants Affordances To make links between Gregory, Gibson and the nature-nurture debate.

Eyes Brain or Eyes Brain Bottom Up / Direct theory Top Down/ indirect theory

Gibson’s Bottom up theory of Perception- a Direct Theory We “work upwards in our analysis of the visual world from basic sensory inputs at the bottom level towards the higher, more cognitive levels of the brain. “

Gibson’s main Points Use REAL WORLD to study visual perception (training pilots in WW2). Found all the info the pilot needed to land plane was the the pattern or light that reached the eye. Gibson proposed a bottom-up theory of perception.

Optic Array Gibson proposed that the pattern of light reaching the eye could be thought of as an optic array. This contains all the information we need to make sense of the world.

The optic array The structure of light in the environment, unambiguous information about the layout of objects in space. We don’t see static ‘retinal images’ but we move around in a complex environment. If you move your eyes, your head or your body, the optic array changes- the optic array is ambient.

Invariant Information in an Ambient Optic array Despite the changes, there are some things in the optic array that are invariant (don’t change). We automatically extract this from the input data and it is used to gauge depth, distance and orientation. This invariant information leads directly to perception (being able to see). (i.e. there is no need to interpret the information to make sense of it)

Gibson Gibson suggested that the invariant information came in a number of forms. Texture Gradient Optic Flow Pattern Horizon Ratio

Optic Flow This is the changes in light patterns that reach us when we move. As we move forward objects move past us. How the objects move provide us with information about our speed and direction. Bicycle ride

Optic Flow Optic flow also includes motion parallax that gives us depth information. As we move objects appear to move past us. The speed that they seem to move gives us information about how far away they are. The further away they are the slower they move. Motion parallax

Optic Flow Patterns

Optic Flow Patterns Johansson (1973) found that a black-clad actor wearing lights on his joints walking in a darkened room, was seen as a moving person, but standing still, he was not perceived as a stationary person. Maher and West (1993) filmed the movement of black-clad animals with light on their joints and found that participants could recognise the species of animal. This demonstrates the strength of movement information in determining optic flow patterns and shows that there is enough sensory information for perception to occur directly.

Depth Cues Texture gradients provide us with depth cues. The world is made up of surfaces with different textures. The closer the object the more detail we will see.

Here, texture gradient is used in the form of cobblestones that get progressively smaller to show that the street recedes.

Texture Gradient

Texture Gradient Gibson and Bridgeman (1987) showed participants photographs of surface textures. Participants could correctly identify objects, state their colour, identify the light conditions and say whether they were lying flat etc., suggesting that there is sufficient sensory information in surface textures to permit direct perception. Frichtel et al. (2006) presented participants with a film of a car driving through scenery. Evidence was found that infants as young as 4 months old could perceive using texture gradient, implying that the ability is innate.

Depth Cues Parallel lines are known to converge as they move further away. A classic example of this is railway lines, but you can see the same effect in other things such as roads.

Horizon Ratio The proportion of the object above and below the horizon line is constant for objects of the same size standing on the same ground

Affordances One of the more contentious aspects of Gibson's theory is his idea of ‘affordances’ The meaning of an object can be directly perceived, and can tell you what to do with that object.

Affordances Gibson suggested that objects could ‘tell’ us what they are for. His suggestion was that our visual system has evolved to inform us directly of what the function of an object is, e.g. a chair is for sitting on. Or a chair ‘affords’ sitting down.

Organisms and the environment they are adapted to are inseparable Organisms and the environment they are adapted to are inseparable. (An ecological theory)

Hmm, I could sit on that! This stone affords sitting This log affords sitting

Arrrrgh, that box affords hidin me treasure in! That box affords standing on

Affordances Warren (1984) studied whether participants could judge whether staircases portrayed with differently proportioned steps could afford to be climbed. Whether they actually could depended on the length of a participant’s leg. It was found that participants were sensitive to the affordance of ‘climbability’, and according to Gibson this would be achieved by the light reflected from the staircase. Bruce and Green (1990) found that the idea of affordances could be used to explain the visually controlled behaviour of insects.

Objectives To explain what is meant by the phrase ‘Bottom Up’ processing To Outline James Gibson’s theory of perception. Invariants Affordances To make links between Gregory, Gibson and the nature-nurture debate. PLTS Independent Enquirer Decide which theory you find more convincing, based on the evidence you have.

The Usefulness of Perception Gibson believed perception isn’t just about detection of information but detection of USEFUL information The fact that objects have affordances enables an animal to respond in an adaptive way to it’s environment.

Evaluating Gibson Gibson’s theory has practical applications – such as road markings. Horizontal lines are painted on the road, becoming closer together as driver approach junctions. These markings are often seen on the exit roads from motorways and serve to create the illusion of increasing speed, which causes the driver to slow down before joining a non-motorway road.

Evaluating Gibson Tan et al. (2006) used Gibson’s idea of optic patterns to investigate the effects of motion on car drivers’ perception, where cars travelling towards a viewer appear to increase in speed and size, even though their real speed and size is constant. This perceptual problem is increased by the fact that the viewer is also in motion. A camera mounted on a wing mirror was used to determine real-world speed. The data gained from such studies is used to determine speed limits and advise on braking distance. As well as contributing to knowledge of perception, Gibson’s theory also saves lives in preventing / lessening the impact of traffic accidents.

Evaluation AO2 Biological motion http://www.lifesci.sussex.ac.uk/home/George_Mather/Motion/BM.HTML Johansson (1973) showed that humans ( and animals!) could perceive this sparse information as human movement- suggesting an innate ability AO2

Evaluation AO2 Innate Abilities Lots of perceptual abilities do seem to be innate (more info to follow!) AO2

Evaluation AO2 Affordances Has been applied to robots which are able to imply the meaning of objects and use them appropriately BUT knowledge about objects is down to cultural expectations and knowledge. AO2

Affordances MUST involve some top down perception.

Evaluation Illusions, Muller Lyer doesn’t work if a 3D model is made of it, suggesting that movement around an object is important AO2

Gregory Vs Gibson Maybe they are both right? What and Where Systems

Spatial perception and movement Ventral stream deals with object recognition (Top down) Dorsal stream deals with spatial/movement perception (bottom up)

Outline and evaluate two theories of visual perception. (24) Outline and evaluate two theories of perceptual organisation (24)

Objectives To explain what is meant by the phrase ‘Bottom Up’ processing To Outline James Gibson’s theory of perception. Invariants Affordances To make links between Gregory, Gibson and the nature-nurture debate. PLTS Independent Enquirer Decide which theory you find more convincing, based on the evidence you have.