2.1 Perception

Bottom-up processing starts with the sensory details and builds up to a whole. When you look at an unfamiliar symbol, you have to piece together its lines and curves. Top-down processing starts with what you already know and expect, and uses that to interpret the details. It's why you can read a sentence with a few letters missing.

Internal factors shape top-down processing. A schema is a mental framework for organizing information, such as your idea of what a classroom contains. A perceptual set is a readiness to perceive something in a particular way. If you've just been told a cloud looks like a dog, you're far more likely to see a dog.

External factors matter too. Context changes what you see: the same ambiguous shape is read as the letter B among letters and as the number 13 among numbers. Experience and culture also filter perception. For example, people raised in different environments can differ in how strongly they fall for certain visual illusions.

Gestalt psychologists argued that the whole is more than the sum of its parts: your brain automatically organizes pieces into meaningful wholes. The course focuses on four principles.

• Figure-ground: you separate what you're focusing on (the figure) from its background (the ground). In the famous vase-and-faces image, the figure and ground can flip.
• Proximity: you group things that are close together. Six dots arranged in three pairs look like three groups, not six dots.
• Similarity: you group things that look alike. In a grid of circles and squares, you see rows or columns of matching shapes.
• Closure: you fill in gaps to see a complete object. A circle with a small break still looks like a circle.

Attention links sensation and perception: you can only process a small part of what your senses take in. Selective attention means focusing on one thing while ignoring others. The cocktail party effect shows that unattended information still gets some processing: in a loud room, you'll notice if someone across the room says your name.

Because attention is limited, you can miss things right in front of you. Inattentional blindness is failing to notice a visible object when your attention is elsewhere; in a well-known study, many viewers counting basketball passes didn't notice a person in a gorilla suit walking through the scene. Change blindness is failing to notice a change in your environment. In one field study, many people giving directions to a stranger didn't notice when a different person took the stranger's place after a door was carried between them.

Binocular cues need both eyes. Retinal disparity is the difference between the images on your two retinas; the bigger the difference, the closer the object. Convergence is how much your eyes turn inward to focus on something; more inward turning signals a closer object.

Monocular cues work with one eye, and artists use them to create depth on flat surfaces. The exam tests only these five:

• Relative size: if two objects are assumed to be the same size, the one with the smaller image seems farther away.
• Interposition: an object that blocks part of another is seen as closer.
• Relative clarity: hazy, blurry objects seem farther away than sharp, clear ones.
• Texture gradient: a surface's texture looks coarse up close and finer and denser with distance.
• Linear perspective: parallel lines, like railroad tracks, seem to come together in the distance.

Perceptual constancy lets you see an object as unchanging even when the image on your retina changes. With size constancy, a friend walking away doesn't seem to shrink. With shape constancy, a door swinging open still looks rectangular, even though its image becomes a trapezoid. With color and lightness constancy, a white shirt looks white in sunlight and in shade.

Apparent movement is seeing motion where there isn't any. In the phi phenomenon, lights blinking on and off in sequence, like on a theater sign, seem to move. Stroboscopic movement is how a rapid series of still images, as in a flipbook or film, looks like smooth motion.