Topic 2.1 Notes – Perception

What Perception Is

Perception is the brain’s organization and interpretation of sensory input. Sensation is earlier. It means detecting stimulus energy, like light waves or sound waves. Perception is what those signals mean to you.

That matters because perception is constructive. Your brain doesn’t just copy the world. It builds an interpretation, which means perception can be accurate or mistaken.

Bottom-up processing

Bottom-up processing begins with sensory details from the environment.

• You take in features like edges, color, brightness, and shape.
• The brain builds upward from those parts to a whole object.
• This is strongest when the stimulus is new, clear, or unfamiliar.

Top-down processing

Top-down processing uses what you already know.

• It relies on prior knowledge, expectations, context, and experience.
• It is strongest when the input is ambiguous, incomplete, or brief.

The B/13 example is classic: the same exact symbol is perceived differently because context pushes your expectation.

Filters That Shape Perception

Perception gets filtered by what’s inside you and what’s around you.

Internal filters

• Schemas are mental frameworks for categories, objects, events, or situations.
  • They help you interpret things fast by filling in likely meaning.
  • A restaurant schema includes menus, ordering, eating, paying, and leaving.
  • Schemas save time, but they can also distort perception by making you notice what fits expectations.
• Perceptual sets are a readiness to perceive something in a certain way.
  • They’re shaped by expectations, emotion, motivation, and recent experience.
  • If a stimulus is unclear, a perceptual set can bias what you think you saw or heard.

External filters

• Context changes meaning. The B/13 symbol proves that surrounding cues matter.
• Experience changes what you can organize quickly. A trained musician sees notes and rhythm in sheet music. A beginner sees marks.
• Culture shapes attention and interpretation through learned meanings, gestures, symbols, reading direction, and spatial habits.

Gestalt principles of perceptual organization

Gestalt psychology says we naturally organize sensory input into meaningful wholes. The Rubin vase illusion is a classic example of figure-ground organization because your perception can flip between a vase and two faces.

Rubin vase illusion

• Closure means you fill in missing gaps to see a complete object.
• Figure-ground means you separate the object of attention from its background.
• Proximity means you group nearby items together.
• Similarity means you group items that look alike.

Attention and What We Miss

Attention is focusing awareness on selected stimuli. It connects sensation and perception because lots of things may be detected, but only some get full processing.

• Selective attention lets you focus on one task or stimulus while filtering others out.
• The cocktail party effect happens when personally important information, especially your name, grabs your attention even in a noisy setting.
• Attention is limited, so inattention can make you miss visible information.

Two terms get tested a lot here:

• Perceptual blindness means visible information goes unnoticed because attention is elsewhere.
• Change blindness means you fail to notice a change in the environment, often when attention is occupied or viewing is interrupted.

Depth Cues

Your retinas get a 2D image, but you perceive a 3D world because of depth cues.

Binocular depth cues

These use both eyes.

• Retinal disparity means each eye gets a slightly different image. Greater disparity usually means the object is closer.
• Convergence means your eyes turn inward more for nearby objects. The muscle tension helps signal distance.

Monocular depth cues

These work with one eye and often create depth on flat surfaces. The railroad tracks below are a classic example of linear perspective, where parallel lines seem to meet in the distance.

Linear perspective

Only these listed monocular cues are in AP scope for this topic.

Perceptual Constancy and Apparent Movement

Perceptual constancy means you see objects as stable even when the retinal image changes.

• Size constancy keeps an object the same perceived size as distance changes.
• Shape constancy keeps an object the same shape across angle changes, like an opening door still seeming rectangular.
• Brightness constancy keeps lightness fairly stable across lighting conditions.
• Color constancy keeps familiar colors fairly stable under different illumination.

Apparent movement is motion you perceive even when nothing is moving continuously.

• Film and animation use rapid still images.
• The phi phenomenon happens when adjacent stationary lights flash in sequence and appear to move.