Depth Perception and top down processes

Depth Perception 

•  ability to perceive 3-d space and accurately judge distance
• Can be difficult as image on retina is 2-d. Optical illusions highlight this difficulty.

Cue approach to depth perception

• The brain learns to identity info in retinal image correlated with depth. Repeat exposure makes association automatic.
• Two types: Monocular Cues and Binocular Cues

Information about depth (pictures, 2-d images) reliant on one eye

• Accomodation ~ lens changes shape when we focus on objects at varying distances.
• Linear Perpective ~ Parrallel lines appearing to converge cue distance
• Relative size ~ Of two objects in similar size, we perceive the object farthest to be smaller vice versa.
• Relative Height ~ We perceive objects higher in our visual field to be farther away than those lower.
• Interposition (Occlusion) ~ when one image blocks another you perceive the blocked image to be farther away.
• Texture Gradient ~ textuere is denser in distant objects vice versa. Also changes in texture imply depth discontinuity (i.e ART)
• Atmospheric perspective ~ distant objects have tint appearing more blurry, thus farther
• Light & Shadow ~ shadows enhance 3-dimensions of object, nearer objects reflect more light than distant objects
• Motion Parallax ~closer objects move past fast, vice versa.Objects at great distnces suce as moon sun appear to move forwards.

Cues arising from both eyes working together, sensing the position of the eyes and muscle tension.

• Convergence ~ eyes move inwards as we focus on something closer, vice versa, this signals distance.

• Retinal Disparity ~ both eyes see slighlty different views of the world. Distance between the eyes determines the size of the visual field. Prey usually have eyes on eother side of head, predators both eyes on front giving large binocular view.

• Stereopsis ~ impression of depth due to two different image on retina. (Wheatstone) made stereoscope - differnt view of same image shown to each eye, brain integrates them into one image.

(Immanuel Kant) ~ INNATE? knowledge comes from innate knnow how of organising sensory experiences.

• Gibson & Walk found babies( 6-14 mnths) wouldn't walk over "cliff" table.
• Campos et al found infants perceive depth at 2 mnths
• Tsutsi found mokeys have cells responding to pictorial cues and binocular disparity

(John Locke) ~ LEARNT? we learn to perceive the world through our expereineces

• (Blakemoore & cooper) found kittens reared in lined chamber for 5 mnths, were blind to other orientations

Some things are innate (Binocular disparity) 

Otehrs are learnt (Linear persepective and interposition)

• To recognise that an object is the same, relies on top down processing

• Shape Constancy ~ an object remain the same despite changes in viewing angle

• Size Constancy ~ perceiving objects as being same size regrdless of change in distance

• Colour constancy ~ perceiving objects to have the same colour regardless of lighting changes BUT context also plays role in expereince of colour (i.e rubiks cube with different lighting)

• Perceived size = retinal image size x perceived distance

Nurture?

• (Pedersen and Weller) found african who lived in square houses (having expereinced edges etc) were susceptible to the muller lyon illusions, other group didn't see it as they lived in round houses.

Consrtuctivist Vs Direct Perception

Constructive view ~ perceptions are built ( empiricist view) mix of stimulus info, expectations etc.

• optic array proveides info about objects in space (CANNOT EXPLAIN ILLUSIONS!)
• Motion provides information (motion gradients are changes in optic flow, within the optic array) (DEPTH PERCEPTION POSSIBLE WHEN MOTIONLESS!)
• Affordances ~ objects potential uses are directly observable (TOO SIMPLISTIC!)

Direct View ~perceptions are based on uninterpreted info taken direct from environment (nativist view)

• Unconcious Inference Theory (Helmholtz) ~ combining retinal images with assumptions

• Constructivist Theory (Richard Gregory) ~ People actively construct perceptions using information based on expectations and past experiences.

 Evidence for CT:

• context effects, and perceptual set
• We project our knowledge of the world onto our perceptions (visual illusions, context effects & perceptual set)

Perception is not automatic from raw stimuli. Information from the eye is partial and ambiguous. Human visual perception requires interpretation and projection from stored knowledge. It needs to be supplemented from knowledge about objects (through stored representations) in order to make sense of the data.

A synthesis between top-down and direct approaches:

Relevant schemata are collections of representations from past experience. They direct perceptual exploration, which leads to sampling of the stimulus environment. If this does not match the relevant schemata then CHANGE schemata