Figure and Ground
This fish is the closest, most active component of the above painting. The fish is the figure.
The plants make up the less immediately significant component of the painting. The plants are the ground.
It is impossible to perceive every object in our world individually. Because of this, we group together our sensations.
For example, the letter "T" in the above picture is much more easily perceived as a "T" than as a series of lines, due to the lines' closeness and similarity.
For that matter, every object you perceive on this screen is only made of small colored pixels.
Depth perception is the visual ability to determine the distances of objects.
These require the use of two eyes.
The brain determines distance, in part, from retinal disparity. In other words, the difference between the image of an object which appears in each eye trigonometrically enables the calculation of distance.
If you cover one eye, you will lose the ability to use binocular cues.
These require only one eye to determine distance.
If we perceive an object, we recognize it as the same in all surroundings.
The color, shape, depth, and size of the four above objects are all different. Yet, we perceive all four pictures as doors.
Kant said that perceptual abilities are inborn, and that all people can perceive certain sensations the same way.
Modern research does not support his hypothesis.
Locke said that people learn to perceive (remember his "Tabula Rasa," or "Blank Slate" idea).
Locke's idea is supported by modern research. In an experimental setting, researchers put goggles on kittens soon after birth. Later, when the goggles were removed, the kittens had difficulty in moving and functioning. The same phenomenon occurs with humans, blind at birth, whose vision is later restored.
Gibson and Walk
Further supporting Locke's hypothesis, Gibson and Walk conducted the visual cliff experiment.
Until a certain age, a child will fail to perceive the depth of the "cliff," and walk over the glass. Once the child develops depth perception, he will remain at the edge of the cliff, fearful of the height.
This experiment shows that humans develop depth perception with age.