In February 2015 a photo of a striped dress spread across the internet. Some people saw it as white and gold; others were certain it was blue and black. The dress itself was blue and black. Everyone's eyes received the same photo, but their brains made different sense of it. That gap is the difference between sensation and perception.
Sensation vs perception
Sensation is your sense receptors detecting energy, such as light, sound waves or pressure. Perception is your brain organizing and interpreting that information so it means something: a face, a voice, a dress.
The step in between is transduction, turning physical energy into neural signals. Cells in the retina transduce light; hair cells in the inner ear transduce sound vibrations.
A common myth is that we have exactly five senses. Psychologists also count kinesthesis, or proprioception (the movement and position of your body parts), and the vestibular sense (balance and head movement), among others.
Thresholds: how much is enough to notice?
- The absolute threshold is the faintest stimulus you detect 50% of the time.
- The difference threshold, or just noticeable difference, is the smallest change you detect 50% of the time.
- Weber's law says that difference is a constant proportion, not a fixed amount. One extra candle is obvious in a room lit by two candles and invisible in a room lit by two hundred.
Signal detection: the person matters too
Signal detection theory says detection isn't only about the stimulus. Expectations, motivation and fatigue shift where you set your bar. A lifeguard who expects trouble catches more real swimmers in distress (hits) but also raises more false alarms. Someone tired and bored makes more misses. (Signal detection isn't named in the 2024 AP Psychology course description, but it is standard in intro psychology.)
Adaptation and attention
Sensory adaptation is reduced sensitivity to a constant stimulus. The café smell fades after a few minutes because receptors respond less to an unchanging input.
Selective attention lets you focus on a small slice of what you could sense. Colin Cherry's 1953 work on listening to one message among others led to the term cocktail party effect.
The cost shows up in inattentional blindness. In Simons and Chabris's 1999 study, many viewers counting basketball passes failed to notice a person in a gorilla suit walk through the scene. Versions of this finding have been repeated many times. Change blindness is the related failure to notice a change in a scene.
Organizing the world
Gestalt psychologists argued we see wholes, not pieces.
- Figure-ground: an object stands out from its background.
- Grouping: we link things that are close together (proximity) or alike (similarity), and fill in gaps (closure).
For depth, retinal disparity compares the slightly different images from your two eyes. Monocular cues such as relative size, overlap and linear perspective work with one eye.
Perceptual constancy keeps a door looking rectangular as it swings open, even though its image on your retina becomes a trapezoid.
Bottom-up and top-down
Bottom-up processing builds from raw input. Top-down processing uses knowledge and expectation, which is why you can read messy handwriting. A perceptual set is an expectation that tilts what you see, like the face you can't unsee in a cloud once someone points it out.
Where the research is shaky
- Subliminal advertising. In 1957 James Vicary claimed that flashing "Eat popcorn" and "Drink Coca-Cola" during a film boosted sales. In 1962 he admitted the study was a gimmick with too little data to mean anything. Lab studies of subliminal priming find small, short-lived effects at best, and results vary between labs.
- The dress. Researchers have proposed that people made different unconscious assumptions about the lighting in the photo. That explanation has some support, but why individuals differ is still debated.
Common mix-ups
- Sensation vs perception. Detecting the light is sensation; recognizing it as a dress is perception.
- Sensory adaptation vs inattentional blindness. Adaptation is a weaker response to a steady stimulus; inattentional blindness is missing something because attention is elsewhere.
- Absolute vs difference threshold. One asks "is anything there?", the other "did it change?"
Frequently asked questions
What is the difference between sensation and perception?
Sensation is detecting physical energy with your sense organs. Perception is your brain interpreting that input so it has meaning.
What is an example of top-down processing?
Reading a word with missing letters, or seeing a face in a cloud once someone points it out. Your expectations fill in what the raw input doesn't settle.
Does subliminal advertising work?
The famous 1957 popcorn study was admitted to be a gimmick. Lab studies find small, short-lived priming effects at best, nothing like the control the story claimed.
How many senses do humans have?
More than five. Besides sight, hearing, taste, smell and touch, psychologists count senses such as balance and body position.
Try it
Notice the feel of your socks or the chair under you right now. A moment ago you weren't aware of either. Then look at a doorway as you walk past it, and watch your brain keep it a rectangle.