Tuesday, 28 November 2017

Gibson and walk's visual cliff study


Background/context

Height perception = a form of distance perception with both eyes, so the brain receives information from both visual fields so we can see in 3D

Depth cues let us perceive height, relative size, linear, perspective and texture gradient

If info about direction and velocity of movement is known, motion parallax provides depth information

Infants are more prone to fall, because their visual co-ordination hasn't fully developed


Aim

To investigate how humans and animals perceive height distance and to consider the stage they develop the ability to perceive height distance


Method

Humans

Lab experiment, repeated measures
Iv= whether infant was called by mother from cliff side or shallow side (of the visual cliff apparatus)
Dv= whether the child crawled to it's mother or not


Animals

Quasi-experiment
Iv= species, rat/chick/lamb/kitten
Dv= whether they preferred shallow or deep side of the visual apparatus



Procedure

Sample

36 infants, 6-14 months
Young animals 


Design

Lab experiment, using a visual cliff to test height distance perception safely
It was a board laid across a sheet of glass, 1 foot above the floor and on one side, a patterned material was placed under the glass (shallow side) and the other side of the glass, the same board was on the floor (cliff side) = illusion of depth


Human trial

Each kid placed in the middle of the board, individually tested
Mothers were there to motivate the babies to move, they called from both sides and the child's behaviour was recorded


Animal trial

Animals from cornell behaviour farm, there was also dark reared kittens (reared in darkness for 27 days)


Controls

Number of changes to set up to check for bias, e.g. lighting, pattern and height distance


Separation of visual cues

Researcher included a variation with 2 materials placed under each side

Motion parallax = looking out a side and then front car window

Relative size = smaller it looks, further away it is

To remove cues, placed patterns at the same level

To remove object cue size they used a large pattern on low side of cliff = constant density



Results

Human trials

All 27 infants went to the shallow side, only 3 went to the deep side, many crawled away from mother

Some cried because they thought it would be dangerous to reach their mum on the deep side

Dependent on their vision to determine movement, even if they felt the glass, they'd refuse to change it or cross over it

Several would've fallen off if there had been no glass


Animal trials

Depth perception emerged once an animal became mobile = adaptive purpose = helping them avoid falls in the wild

Dark reared kittens equally likely to move to cliff as shallow and didn't freeze when on the cliff side

After a rock in light, they behaved the same as the other kittens = suggests light is needed for visual maturation


Control trials

Variations found no hidden bias

When researchers placed plain grey surfaces down instead, rats still had no preference, same when patterns were both against and under the glass


Separation of visual cues

Day-old chicks and rats in both conditions = preference for shallow side

Motion parallax = innate visual cue that develops through maturation



Conclusion

Nature + nurture influence development of depth perception

Binocular cues like motion parallax are innate

monocular cues like size constancy are learned

Humans and animals develop some depth perception by the time they're mobile


Evaluation

Stressed babies so some psychological harm, ethical issues with using animals, and subtle cues from mothers could've biased the findings and lowered the validity

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