AP Psychology 2.1: Perception

In 1.6 Sensation, your senses turned light and sound into neural messages. Perception is what the brain does with those messages: it organizes and interprets them, so you experience a face, a word or a song.

Two kinds of information feed it. In bottom-up processing, perception relies mainly on the incoming sensory information, built up detail by detail, as when you study the lines of a symbol you have never seen to work out what it is. In top-down processing, your prior expectations lead, as when you read a friend's messy handwriting with ease because you know what she usually writes about. Most perception mixes the two.

Some influences come from inside you. A schema is a mental framework of what you know about something, such as what a kitchen usually holds. A perceptual set is a readiness to perceive something in one particular way, often left by what you just saw or were told. Hear that a song hides a secret message, and you may start to hear one.

Other influences come from outside. The context around a stimulus changes how it is read: the same shape can read as a letter among letters and as a number among numbers. Your experiences, including the cultural experiences and expectations you grew up with, filter perception too. A smell that means a favorite dish to someone raised on it can seem simply strange to someone who was not.

Your senses take in far more than you can perceive at once, and attention picks what gets through. It is an interaction of sensation and perception: the stimulus has to be sensed, and attention decides whether it is perceived. Both outside and inside processes steer it. A sudden bang or a flashing light pulls attention from outside. Your goals, needs and interests pull it from inside, which is why someone shopping for a bike starts noticing bikes everywhere.

Selective attention is focusing on some information while filtering out the rest. The filter is not sealed, as the cocktail party effect shows. In a loud room you can follow one conversation and tune out the others, yet you notice at once when someone across the room says your name or mentions a topic you care about.

When attention is elsewhere, people can miss what is right in front of them. In inattentional blindness, a person fails to notice something in plain view because their attention is on something else, like a driver searching for a house number who never sees a neighbor waving from a porch. In change blindness, a person fails to notice that something in the scene has changed, such as a poster swapped on a wall between two glances. In both cases the light reached the eyes. Without attention, the thing or the change was never perceived.

Gestalt psychologists argued that the brain organizes sensations into whole forms, and they proposed principles for how it does so.

• Figure and ground: you separate an object, the figure, from its surroundings, the ground.
• Proximity: things that are near each other are grouped together.
• Similarity: things that look alike, in color, shape or size, are grouped together.
• Closure: you fill in gaps to perceive a complete whole.

These principles usually help, and they can also mislead. Camouflage works because an animal that matches its background is hard to pull out as a figure.

The brain also keeps the world stable. Perceptual constancy maintains the perception of an object even when its image in the visual field changes.

• Size constancy: as a friend walks away, her image on your retinas shrinks, yet she does not seem to shrink.
• Shape constancy: a dinner plate seen from the side casts an oval image, yet you perceive it as round.
• Color constancy: a yellow banana still looks yellow under the dim light of a lamp.

Constancy can mislead as well. If cues trick you into judging an object to be farther away than it is, size constancy makes you perceive it as larger than it is.

Your retinas are flat, yet you see a world with depth. Binocular depth cues use the images from both eyes. Your eyes sit a few centimeters apart, so each gets a slightly different view. Retinal disparity is the difference between the images projecting onto the two retinas, and the nearer an object, the larger the difference. Convergence is the brain merging the two retinal images into one; for a near object, your eyes turn inward to do it. The more your eyes have to turn in, the nearer the brain judges the object to be.

Monocular depth cues need only one eye, and they give the illusion of depth on flat surfaces such as photos and paintings. Five of them matter here.

• Relative clarity: hazy or blurry objects seem farther away than crisp ones.
• Relative size: of two objects you assume are about the same size, the one with the smaller image seems farther.
• Texture gradient: a surface looks coarse and detailed up close, and finer and denser far away.
• Linear perspective: parallel lines seem to come together in the distance.
• Interposition: an object that blocks part of another seems nearer.

You can also see motion where there is none. In apparent movement, still images or lights in slightly different places, shown one after another in quick succession, are perceived as one thing moving. A movie is a series of still pictures.