Depth, constancy, illusions and culture

The image on the retina is flat (two-dimensional), yet we see a world with depth and distance. We use two kinds of cues (information):

Monocular cues (also called psychological or pictorial cues) work with one eye; artists use them to show depth on paper:

• Relative size: of two similar objects, the one that looks smaller seems farther away.
• Interposition (overlap): an object that partly covers another is seen as nearer.
• Linear perspective: parallel lines seem to come closer together in the distance, like the two edges of a long straight highway.
• Aerial perspective: distant objects look hazy and bluish because of dust and moisture in the air, like faraway hills.
• Light and shade: highlights and shadows tell us about an object's shape and distance.
• Relative height: objects placed higher in the visual field (closer to the horizon) are seen as farther away.
• Texture gradient: the texture of a surface looks finer and denser as it gets farther away, as with a field of pebbles.
• Motion parallax: when we move, near objects seem to rush past quickly and distant ones move slowly; things very far away seem to move with us.

Binocular cues (also called physiological cues) need both eyes:

• Retinal (binocular) disparity: each eye sees a slightly different image because the eyes are about 6.5 cm apart; the brain combines them to judge depth. Close one eye, then the other, while looking at a lamp-post: it seems to jump.
• Convergence: as an object comes nearer, the eyes turn inwards to keep it in focus; the brain reads that muscle effort as distance.
• Accommodation: the ciliary muscles change the shape of the lens to focus near or far objects, much like a phone camera shifting focus.

Perceptual constancies keep the world stable even when the image on the retina changes:

• Size constancy: a friend walking away does not seem to shrink, though the retinal image gets smaller.
• Shape constancy: a door swinging open is still seen as a rectangle, though the image becomes a trapezium.
• Brightness constancy: a white shirt looks white both in bright sunlight and in a dim room, because we judge the light it reflects compared with its surroundings.

Illusions are misperceptions: the senses give correct information but we interpret it wrongly. Some are universal (almost everyone sees them), others are personal (one person misreads a sign in their own way).

• Muller-Lyer illusion: two equal lines look different in length because one has arrowheads pointing in and the other out.
• Vertical-horizontal illusion: a vertical line looks longer than an equal horizontal line.
• Apparent movement (phi phenomenon): lights switched on and off one after another at the right speed seem to move, as in the running lights around a shop sign.

Culture and perception. Experience trains perception. Segall, Campbell and Herskovits found that people from different environments differ in how strongly they see illusions such as Muller-Lyer. Hudson found that African participants with little exposure to pictures had difficulty seeing depth in drawings. Sinha and Mishra studied the perception of pictures among groups in India and found that familiarity with pictures shaped how they were understood.