Perception Notes

Cognition - Unit 2

Unit 2 focuses on important introductory psychology concepts.
These concepts are heavily represented on the AP Psychology test.
Concepts can help improve study and learning effectiveness.

Perception

Perception: Process of understanding and interpreting sensations.
Psychophysics: Study of the interaction between sensations and our experience of them; aims to uncover the rules our minds use to interpret sensations.

Learning Objectives

Thresholds
Perceptual theories
Principles of visual perception
Effects of culture on perception
Extrasensory perception

Key Terms

Placebo effect
Just-noticeable difference
Weber's law
Figure-ground relationship
Binocular depth cues
Linear perspective
Gestalt psychology
Relative size cue
Proximity
Interposition cue
Top-down processing
Similarity
Schemata
Schema
Perceptual set
Bottom-up processing
Continuity
Closure
Visual cliff
Monocular depth cues
Texture gradient
Retinal disparity
Binocular disparity
Convergence

Thresholds

Absolute threshold: Smallest amount of stimulus we can detect.
- Vision example: A single candle flame about 30 miles (48 km) away on a perfectly clear, dark night.
- Technical definition: Minimal amount of stimulus detectable 50% of the time to account for individual variation and interference.
Subliminal stimuli: Stimuli below our absolute threshold.
- Companies claim subliminal messages can change behavior, but psychological research doesn't support this.
- Truly subliminal messages wouldn't affect behavior because we wouldn't perceive them.
- Faint messages might subtly affect behavior (e.g., choosing a word from a list), but complex messages aren't effective.
- Behavior change from such media is likely due to the placebo effect.
Difference threshold (just-noticeable difference): Smallest amount of change needed in a stimulus before we detect a change.
- Computed by Weber's law: Change needed is proportional to the original intensity of the stimulus.
- More intense the stimulus, the more it needs to change to notice a difference.
- $\frac{\Delta I}{I} = k$ , where $\Delta I$ is the minimal change in intensity needed to perceive a just noticeable difference, $I$ is the intensity of the stimulus, and $k$ is the constant ratio.
- Each sense has a different constant.
  - Hearing constant: 5% (100-decibel tone needs to increase to 105 decibels to be noticeably louder).
  - Vision constant: 8% (8 candles need to be added to 100 candles to look brighter).

Perceptual Theories

Psychologists use several theories to describe how we perceive the world.
These perceptual theories are not competing with one another, each theory describes different examples or parts of perception. Sometimes a single example of the interpretation of sensation needs to be explained using all of the following theories.

Signal Detection Theory

Investigates the effects of distractions and interference on perception; predicts what we'll perceive among competing stimuli.
- Considers motivation and expectations (response criteria or receiver operating characteristics).
- Example: More likely to smell rhubarb pie if hungry and like rhubarb.
Explains perceptual mistakes:
- False positive: Perceiving a stimulus that isn't there.
  - Example: Waving at a stranger thinking they are a friend.
- False negative: Not perceiving a stimulus that is present.
  - Example: Not noticing directions at the top of a test.
- Seriousness of errors can alter perception (e.g., false negative is more serious for a surgeon).

Top-Down Processing

Perceiving by filling in gaps using context and background knowledge (schemata).
Schemata (schema): Mental representations of how we expect the world to be; influence perception.
Perceptual set: Predisposition to perceiving something in a certain way, created by schemata.
Example: Seeing images in clouds; backmasking concerns in the 1970s where parents perceived threatening messages in backward music due to expectations.

Bottom-Up Processing

Also called feature analysis.
Using only the features of an object to build a complete perception.
Starts with individual characteristics and builds up to the final perception.
Feature detectors in the visual cortex perceive basic features like lines, curves, and motion.
We constantly use both bottom-up and top-down processing.
Top-down is faster but more prone to error; bottom-up is slower but more accurate.

Principles of Visual Perception

Numerous rules exist; some basic rules are important for the AP Psychology exam.

Figure-Ground Relationship

Deciding what part of an image is the figure and what is the background.
Optical illusions play with this rule, such as the vase/face profile illusion.

Gestalt Rules

Gestalt psychologists described principles for how we perceive groups of objects.
Images are perceived as groups, not isolated elements; this process is thought to be innate.
Factors influencing grouping:
- Proximity: Objects close together are perceived as belonging to the same group.
- Similarity: Similar objects are perceived as belonging to the same group.
- Continuity: Objects arranged in a continuous line or curve are perceived as belonging to the same group.
- Closure: Objects that make up a recognizable image are perceived as a group even with gaps; influenced by top-down processing.

Constancy

Ability to maintain a constant perception of an object despite changes in angle of vision, light, etc.
- Size constancy: We know objects don't grow or shrink as they move closer or farther away.
- Shape constancy: We know the shape of an object remains constant even when viewed from different angles (e.g., a coffee mug top viewed elliptically is known to be circular).
- Brightness constancy: We perceive objects as having a constant color even as the light changes (e.g., a brick wall is seen as brick red even in fading daylight).

Perceived Motion

Brains detect speed of images across retinas and take our own movement into account.
Brains can perceive objects as moving when they are not:
- Stroboscopic effect: Still pictures presented in series appear to be moving (movies, flip books).
- Phi phenomenon: Series of lights turned on and off appear as one moving light (movie marquees, holiday lights).
- Autokinetic effect: Spot of light in a dark room appears to move.

Depth Cues

Essential for perceiving the world as three-dimensional.
Visual cliff experiment: Eleanor Gibson's experiment to determine when infants can perceive depth; depth perception develops around three months old.

Monocular Cues

Depth cues that do not depend on having two eyes; used by artists to imply depth.
- Linear perspective: Converging lines imply distance (railroad tracks).
- Relative size cue: Closer objects appear larger (boxcars closer to the viewer are drawn larger).
- Interposition cue: Objects blocking the view are closer (water tower blocking part of the train).
- Texture gradient: Details in texture are visible up close but not far away (rocks close to the viewer are drawn in detail).
- Shadowing: Shading implies light source and position of objects.

Binocular Cues

Depth cues that depend on having two eyes.
- Retinal disparity (binocular disparity): Each eye views an object from a slightly different angle; the brain interprets the difference (disparity) to gauge distance; closer objects have more disparity.
- Convergence: Eyes move toward each other to focus on closer objects; brain receives feedback from eye muscles and knows that the more the eyes converge, the closer the object is.

Effects of Culture on Perception

Cross-cultural research shows some perceptual rules are learned, not innate.
Cultures that don't use monocular depth cues in art don't see depth in pictures using those cues.
Optical illusions are not perceived the same way across cultures.
Muller-Lyer illusion: Lines with different arrowheads appear to be different lengths; people from non-carpentered cultures (without many right angles) are not fooled by it.
Basic perceptual sets are learned from our culture.

Extrasensory Perception

Claiming to perceive sensations