WJEC PY2 - Gibson and Walk - Depth Perception.

Depth perception is our ability to perceive how close or far an object is from us. It is just one element of our visual capabilities, but it is an essential perceptual ability to have as we have to negotiate our way around our world. If we had no depth perception, we would probably not survive very long – just think about the everyday activity of ascending/descending stairs.

Nativists (people who support the Nature side of the Nature/Nurture debate) believe that we are born with certain capacities such as the ability to perceive depth and this is either present at birth or develops through infancy.

In contrast, empiricists support the nuture debare and argue that depth perception is learny by experience and is therefoe not an innate characteristic.

On the other hand, the interactionists offer a compromise and believe there is room for both arugments by arguing that the exsistance of depth perception isinnate but it then develops as we interact with our environment

Mobility is an important factor as it is seen that depth perception itself only becomes important when a human or non human infant is able to move around by itself. Whether the infant has the ability to perciece depth as soon as it is mobile or whether it is learnt or modified by experience is therefore another important area of research. As human infants are not usually independently mobile until they are around six months old, whereas some non human infants such as chicks are independently mobile from birth and others are mobile within weeks, suggests further evolutionary differences.

Furthermore, as the development of perception itself is seen to be linked to the development of the optic nerve in infants (which is shorter and narrower in infancy comapred to adulthood) also raises questions whether depth perception can be innate a product of maturation or learnt from experience.

Lashley and Russell (1934) provided experimental evidence that depth perception was indeed innate. They used light-reared and dark-reared rats and placed them on a 'jumping stand', The rats had to jump from the stand onto another platformwhich was placed at varying distances from the stand. As the amount of force exerted by both groups of rats correlated with the distance they had to leap, L & R concluded that depth perception was innate.

However Gibson and Walk cite evidence from other investigators using dark reared kittens which suggested that they are not born with innate movement and appear to stare straight ahead. Therefore depth perception appears to vary between species of non-human animals but G & W wanted to find out more.

Gibson and Walk wanted to assess whether an infant's ability to percieve depth was an innate characteristic or learnt from experience or whether it was both. Therefore they aimed to objectively test for this within a controlled setting so as to imply cause and effect. 

Moreover they felt that conclusions drawn from human infants alone would not provide enough evidence as to whether depth perception was innate or not they also aimed to discover whether depth perception varied between independently mobile human infants and a range of independently mobile non-human animal infants (e.g lambs and chicks) as a comparison. Thus Gibson and walk would then be able to really test if depth perception was innate or not.

The study took place at Cornell University, USA. 

The apparatus enabled the researchers to control optical, auditory and tactile stimulus and also protect the participants.

The visual cliff consisted of a large glass sheet which was supported 30cm or more above the floor. 

On one side a sheet of patterned material was placed directly beneath the glass.

On the other side, the patterned material was laid on the floor, this side of the apparatus formed the visual cliff.

The patterns on the material underneath the glass gave visual clues that one side was 'shallow' and the other was 'deep'

The participants - infants or young animals were placed on the centre board that lay between the shallow and deep sides and then encouraged to move across the shallow and deep sides in order to observe whether they could refuse to crawl over the drop.

Participants

Human infants

• total of 36 infants aged 6-14 months were tested. All the infants were able to crawl.

Non human infants

• Chicks, lambs and baby goats mobile at one day old.
• Kittens mobile at four weeks old.
• Kittens that had been reared in the dark for 27 days (which prevented them from learning depth cues)
• Rats movile at four weeks old, some wore hoods so they were using their whiskers- touch rather than visual cues
• pigs, dogs and aquatic turtles.

There are two possible cues that the young animals may have used to perceive dept. Firstly the size/spacing of the pattern would indicate depth. A second cue was motion parallax.

Human infants

Quantitative findings:

• All 27 of the infants who moved off the centre board crawled out on the shallow side at least once. 

Qualitative findings:

Only three attempted to crawl on the deep side (cliff side).

Many of the infants crawled away from their mother when they called to them from the deep side, others cried when their mothers stood there because they could not get them without crossing the deep side.

The infants often patted the glass with their hands to see if there was a solid surface and yet the appearance of the drop was enough to prevent them from venturing further. 

Non-human infants

Chicks, kids and lambs:

never hopped or stepped on to the 'deep side' even at one day old.

If a kid or lamb was placed on the deep side it froze in a posture of defence.

Rats: depended upon their whiskets to navigate, rather than using visual cues. This is explains why the hooded rats were equally content to explore either side as long as they could feel the glass with their whiskers. However when the centre voard was placed higher so that the glass surface was out of reach of their whiskers, they nearly always descended from the centre board onto the shallow side (95%-100% of the time)

Kittens: At four weeks old showed preference for the shallow side and froze when placed on the 'deep side' or circled back at the centreboard. However the kittens that had been reared in darkness for their first 27 days of life crawled onto the shallow and deep side equally. When placed on the deep side they demonstrated similar behaviour to when they were placed on the shallow side and they did not 'freeze' like the normal kittens

Turtles: 

preferred the shallow side. It was expected that they might select the deep side because it looked like the surface of water but in fact 76% crawled towards the shallow side.

Conclusions

Gibson and walk concluded that most human infants were avle to discrimnate depth as soon as they could crawl. As some of the human infanrs backed onto the 'cliff-side' accidently when moving around the 'centre board' it was concluded that human infanrs should not be left close to a cliff edge despite being aple to percieve depth.

Non-human infants- Gibson and walk concluded that depth perception varied between different species and this lent support to the argument that depth perception was adaptive and therefore dependent upon the needs of the species. They also concluded that 'a seeing animal will be able to discriminate depth' wehrn it is able to move by itself but they were unsure what role different cues had on depth perception.