What is Depth Perception?Depth perception refers to our ability to comprehend that some objects are more distant than others. 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!

The images received by our retinas are two dimensional, yet somehow our brain turns this 2D image into a 3D image. Depth can be perceived by comparing the image received from both eyes and using the disparity between the images as a gauge of depth (the easiest way to demonstrate this by holding a pen at arms length, and bringing it closer to your eyes while simultaneously switching between left and right eye). This is known as a binocular cue as it involves both eyes.

However, most depth perception is based on monocular cues (from one eye). These including relative size (things further away are smaller), texture gradient (closer objects have more detail) and motion parallax (as we move, things closer to use move across our field of vision faster than those things far away – imagine

Gibson and Walk (1960) The Visual Cliff

Field of psychology: Developmental

1. CONTEXT AND AIMS

Context

Remember Nature/Nurture? How could this apply to vision?

Recap: What is the nervous system? _________________________________________________________________ ______________________________________________________________________________________________________________________________________________________________________________________________

Nature:

People who hold this point of view are called:

Nurture:

People who hold this point of view are called:

Nature and Nurture:

People who hold this point of view are called:

When we are born the nervous system has all appropriate components, however it is immature – it is about half the size of an adult. The optic nerve (which leads from the eye to the brain), for example is obviously shorter than it will be when it is adult sized, but it is also narrower as it does not have the necessary myelin sheath that ensures good transmission of information.

With regards to depth perception, _____________________________ assume that it is an innate characteristic. It does not have to be learned, and is present from birth, although it may need time to develop fully. ________________________ on the other hand would assume that depth perception is acquired after we start to become mobile. Infants would learn to perceive depth through experience. _________________________ assume that depth perception is a product of the developing visual system (for example, the myelin sheath around the optic nerve is fully developed at 4 months) and experience (from the moment we are born, we are exposed to stimuli such as shadows and depth cues).

EvolutionEvolution is the process whereby organisms have adapted to their environment over millions of years. Traits are said to be adaptive if they lead to increased survival and reproduction of an organism. This means that the gene which causes this behaviour will be passed on to the next generation.

Most human infants have limited mobility for the first six months of their lives; whereas many animals are able to demonstrate movement from the time they are born (for example _____________________________________________________________). These animals are described as precocial meaning that the young are relatively mature and mobile from birth.

If depth perception is innate, we would expect it to be apparent by the time a young animal is mobile, because this would be adaptive. A young animal which does not have this ability at this critical time is less likely to survive, and therefore an innate ability to perceive depth would be adaptive.

What did Lashley and Russell (1934) find? What does this suggest about depth perception?

Why did Gibson and Walk criticise this experiment?

Aims

Gibson and walk aimed to investigate if the ability to perceive depth is learned through experience or whether it is a part of a child’s “original endowment”.

They argue that if depth perception is innate, then it should be apparent by the time infants are able to move independently.

If it is a learned ability, mobile infants may not be able to perceive depth

As humans are not mobile until about 6 months old, only using infants would mean that the results would be inconclusive. For example, a child who could perceive depth at 6 months may have always been able to do so (nativist), or may have learned to do so in the 6 months since birth (empiricist).

Gibson and Walk therefore decided to also test the depth perception of a range of non-human animals such kids (baby goats), lambs, rats, turtles, chicks, and kittens. They would be able to investigate if cliff avoidance behaviours (depth perception) were evident from the time such young animals were mobile (which for some animals, is directly after birth). This would then provide more evidence about whether depth perception is innate.

2. PROCEDURES

Number and make-up of participants

Also, a range of non human animals were used: Chicks, lambs and kids who were one day old Kittens mobile at four weeks Kittens reared in the dark Rats mobile at four weeks Rats reared in the dark Pigs, dogs and turtles were also tested

Research method used Laboratory experiment

Experimental design Repeated measures

Independent variable Depth of cliff

Dependant variable Whether or not the infants/animals would move onto the cliff

Apparatus:Gibson and Walk invented the “Visual Cliff” a simple set up that allowed them to investigate depth perception in babies and other young animals. It allowed them to control optical, auditory and tactile stimuli, and was also safe for the participants.

The Visual Cliff consisted of a large glass sheet that 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 cues that one side was ‘shallow’ and the other was ‘deep’ (the retinal image of more distant patterns is smaller and this information provides depth cues). Procedure:The infant (or young animal) was placed on the centre board that lay between the shallow and deep sides, and then encouraged (by the infant’s mother in humans) to move across the shallow and deep sides in order to observe whether they would crawl over the drop.

Controls:A number of experiments were carried out before the main study. The aim of these was to identify and eradicate extraneous variables. For example, it is possible that reflections from the glass affected participants’ behaviour, so they lit the patterned surfaces from below the glass.

Variations:As well as testing depth perception in a general way, Gibson and Walk altered the variables in the study so that they could isolate and asses the different depth cues

Relative Size: the size/spacing of the pattern would indicate depth. On the shallow side the squares would appear larger than on the deep side. To remove the depth cue of relative size, the pattern on the deep side was increased so that the retinal image for both sides was identical.

Motion Parallax: the patterned elements on the shallow side move more rapidly across the field of vision when an animal or an infant moves their head than on the deep side. To remove the depth cue of motion parallax the patterned material was placed directly under the glass on both sides, but the pattern size was adjusted so that the squares on the shallow side were bigger, giving the illusion of depth.

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

They also adapted the apparatus so that _______________________________________________________________ ____________________________________________________________________________________________________________________________________________________________________________________________________

3. FINDINGS AND CONCLUSIONS

Findings

Human infantsQuantitative data

Number who did not move from the centreboardNumber who crawled onto deep side

Number who crawled onto shallow side

Qualitative dataWhen the mother called to the infant from the deep side, many crawled away to the shallow side, others cried because they could not get to their mothers without crossing the deep side. The infants patted the glass with their hands so that they knew that there was a solid surface, and yet the appearance of a drop was enough to prevent them from venturing any further.

Chicks, kids and lambs Quantitative data

Number who walked onto deep side

Number who walked onto shallow side

Qualitative dataIf a lamb or a chick was placed on to the deep side it froze into a posture of self defence; front legs rigid, back legs limp. Even if this procedure was repeated, the animals did not adapt but froze each time.

RatsRats depend upon their whiskers to navigate rather than their visual cues. This explains why rats were equally happy to explore either side of the board, so long as they could feel the glass with their whiskers. However, when the centre board was placed higher so that the glass surface was out of the reach of their whiskers, they nearly always descended from the centre board to the shallow side (95-100% of the time). When motion parallax was the only cue, rats showed a preference for the shallow side. When relative size was the only cue, they showed no preference

KittensFour week old kittens raised normally showed a preference for the shallow side and when placed on the deep side they froze (similarly to the chicks, kids and lambs) or circled back onto the centre board.

However, four week old kittens reared in the dark from birth crawled onto the shallow and deep sides equally. When placed on the deep side, they did not freeze, but acted the same as they did on the shallow side. After this initial research, these kittens were kept in ‘normal’ lighting conditions. They were tested daily on the visual cliff and by the end of 1 week the ‘dark reared’ kittens demonstrated similar behaviours to kittens who had been reared in the light i.e. almost unanimous preference for shallow side.

Turtles:Quantitative data

Percentage who crawled onto deep side

Percentage who crawled onto shallow side

When the pattern was changed to uniform grey _______________________________________________________ _______________________________________________________________________________________________

Conclusions

HumansWhat does this research suggest about human depth perception?

Is the data from human infants conclusive enough to determine nature or nurture?

Non Human AnimalsGenerally, the findings from non-human animals fit with the life history and natural environment of the animal.

Chicks, lambs, kids and normally raised kittens all showed good depth perception. This is because ______________ ______________________________________________________________________________________________________________________________________________________________________________________________

Rats also showed good depth perception, but relied more on their whiskers to navigate because _______________ ______________________________________________________________________________________________________________________________________________________________________________________________

The turtles had the poorest depth perception. This may be because _______________________________________ ______________________________________________________________________________________________________________________________________________________________________________________________

Does the research above support a nativist or empiricist position on depth perception? Why?

However, the dark reared kittens showed a lack of depth perception. Depth perception for these kittens took about a week to mature. This suggests that while depth perception may be innate, it needs visual input to reach maturation.

All of the species tested could perceive depth by the time that they were mobile. This fits

with the assumption that depth perception is ____________________________.

Eleanor J Gibson

“The survival of a species requires that its members develop discrimination of depth by the time they take up independent locomotion, whether it be at one day (the chick and goat) four weeks (the rat and cat) or 16-14 months (the human infant). That such a vital capacity does not depend on possibly fatal accidents of learning in the lives of individuals is consistent with evolutionary theory.”

We can also conclude from the experiments on rats and turtles that motion parallax is an innate cue for depth perception, but that relative size is not. This is because both rats and turtles were not able to judge depth when motion parallax was removed.

The main conclusion from Gibson and Walk is that depth perception is innate and can be summed up as follows

4. EVALUATING THE METHODOLOGY (pg 114)Method: Strengths and weaknesses of the lab study method used by Gibson and Walk

Reliability : How replicable is this study? Why?

Validity: Can we assess whether depth perception is innate or learned with the infants used by Gibson and Walk given that they were aged 6-14 months?

Adolph and Berger (2006) argue that the infants learned very quickly that if they did slip from the centre board they would not fall. How would this affect the validity of the visual cliff?

How much ecological validity does this study have? Why?

Sampling: The sample used consisted of children available to researchers at Cornell University. Is this a representative sample? What might be unique about this sample, and how might that effect the findings?

Ethical issues: What ethical issues are raised by using human infants?

What ethical issues are raised by the use of animals?

Any other evaluation points

5. CRITICALLY ASSESS WITH REFERENCE TO ALTERNATIVE EVIDENCE

A limiting Factor of Gibson and Walk’s original experiment was that the Visual Cliff required the infants to move across the two sides of the apparatus. This obviously meant that it was impossible to test infants who had not yet learned to crawl.

Schwartz et al (1973) got around this by placing infants directly on to the deep and shallow sides of the cliff and measuring their heart rate. Presumably, if they sensed the drop their heart rate would increase. He tested infants aged between 5 and 9 months, and found that the five month olds generally showed no change in heart rate, whereas those aged nine months did.

What does this suggest about Gibson and Walk’s original claim that depth perception is innate?

Can we be certain that the five month olds had not perceived the depth? How else could we explain these results?

Bower (1970) found evidence that infants as young as six days old have some depth perception. They were shown two objects; a large disc that approached to within 20 cm of them, and a smaller disc which came up to 8cm of them. If the infants had no depth perception, their response to a large disc stopping further away should be the same as a small disc stopping closer because they both create the same retinal image. However the infants were so upset by the smaller closer disc (as they could tell that it was approaching their face) that the experiment had to be ended early.

Sorce (1985) performed a variation on the visual cliff experiment. What did he do and find?

This develops Gibson and Walk’s research, as it shows that while depth perception may very well be innate, we still need to learn through the environment and from upbringing how to respond to depth. The ability to judge depth may be present, but our reaction to it may be learned. This could explain the results of Schwartz et al (1973) above.

Witherington et al (2005) noticed that even the youngest infants in the original footage of the visual cliff experiment braced themselves before touching the deep end. He repeated the original study with two groups of infants. Ten infants were experienced at crawling, but were not yet walking. Ten infants had just begun to walk. It was this second group of infants who were the most wary of the visual cliff. Witherington argued that new learning has to take place when the world is viewed from a new perspective (when walking)

Another limit of Gibson and Walk’s study was that while it suggests that infants have developed depth perception, it does not tell us what cues they use to perceive depth (see box on first page).

Select another piece of research from pg 115 and summarise its findings and conclusions.

Use three colours to highlight research that supports, contradicts, or develops Gibson and Walk’s research.