Perception

Mini Review

We’ve learned the key principles that govern how the brain works and which parts of the brain do what. But how does information get from the world to the brain? And how does the brain turn into something meaningful?

Poll: What Do You See?

Poll Results

Sensation and the Sense Organs

• Sensation: The process by which our sensory systems receive raw physical data from the environment.
• Major sense organs = eyes, ears, nose, tongue, and skin
  • Ex: Light reflecting off a mirror lands on the retina.
  • Ex: A ambulance siren reaches the eardrum.
  • Ex: Chemicals from hot sauce activate receptors on the tongue.
• These signals carry no meaning with them! They are raw data from the world that must be interpreted in the brain.

Perception Turns Sensory Data Into Meaning

• Perception: The process by which our brain interprets raw sensory data and gives it meaning.
• You never experience the raw signals. The first thing you are aware of is already the perceived result.

The Same Sensory Data, Two Different Perceptions

Perception: Coffee

Object Recognition Happens in a Fraction of a Second

• Object recognition: Matching the pattern arriving from the senses to something already stored, so it is identified as a particular kind of thing.
  • Ex: You identify a chair as a chair from an angle you have never seen before, in a room you have never been in, in well under a second.
• Nothing about the light hitting your retina says “chair.” The category is supplied from memory.
• Recognition runs automatically, which is why you cannot choose to look at a familiar word and not read it.

Perceptual Constancy Keeps Objects Stable

• Perceptual constancy: Continuing to perceive an object as the same size, shape, and color even though the raw sensory data about it changes.
  • Ex: A door stays rectangular in your perception whether it is shut or standing half open, though its shape on your retina is a changing trapezoid.
  • Ex: A white shirt looks white indoors and outdoors, though the light reaching your eye is completely different.
• The stable object is the construction. The changing data is what actually arrived.

When Perception Is Confidently Wrong

• The same process that usually gets it right produces confident errors under the wrong conditions.
• Illusions are not tricks played on your eyes. They are your normal interpretation rules applied to input those rules were not built for.
• You cannot talk yourself out of it. Knowing the answer does not change what you see, which shows the process is automatic.
• This is why “I know what I saw” is a weaker claim than it sounds.

The Checker Shadow Illusion

• Squares A and B are printed in the same gray. Your visual system corrects for the shadow, so B looks lighter.

Image: Edward Adelson, via Wikimedia Commons, CC BY-SA 4.0.

The Amount of Sensory Data the Brain Receives

• At any given moment, your sensory systems are bombarding the brain with millions of signals. Most of these never reach your conscious awareness.
• Something has to decide which signals go further, given our Limited Capacity to process information.

Attention

• Attention: The process by which some sensory input is selected for further processing and the rest is left out.

• Attention directs our limited resources toward part of the incoming data.

  • Ex: Following one conversation at a family reunion while other conversations continue nearby.
  • Ex: A referee tracking one player and missing a foul at the other end of the court.
  • Ex: Reading one line of a text message while the lines above it stay in view but are unread.

• The signals that are not selected still arrived at the sense organs. They were sensed but never perceived.

• The dichotic listening studies (1953): Participants wore headphones playing a different message in each ear and repeated one message out loud.

  • Finding: Participants reported almost nothing from the ignored ear, but they did notice their own name in it.

Divided Attention

• Divided attention: Splitting limited processing resources across more than one task at a time, which lowers performance on both.
  • Ex: Texting while driving.
  • Ex: Cooking from a recipe while helping a child with homework.
• The cell phone driving study (2001): Participants drove in a simulator while holding a conversation on a phone, either handheld or hands-free.
  • Finding: They missed about twice as many traffic signals and reacted more slowly, and hands-free was no better than handheld.
• Tasks that run automatically share resources more easily than tasks that require control.
  • Ex: Walking and talking at the same time is easy, but reading and talking at the same time is not.

Inattentional Blindness

• Inattentional blindness: The failure to notice something in plain view because attention was directed elsewhere.
  • Ex: Looking right at the peanut butter while searching the cabinet for it.
  • Ex: A driver checks for cars, pulls out, and hits a motorcycle that was in plain view.
• The basketball counting study (1999): Viewers counted passes between players in white shirts while a person in a gorilla suit walked through the middle of the scene. Watch it.
  • Finding: About half of viewers reported never seeing the gorilla.
• The gorilla reached their eyes for the whole clip. Attention was directed elsewhere, so it was not processed.
• Not noticing something is evidence about where a person’s attention was, rather than evidence about what was there.

Change Blindness

• Change blindness: Failing to detect a change in a scene when attention is interrupted while the change happens.
  • Ex: A glass is full in one shot of a TV scene and half empty in the next, and most viewers never notice.
  • Ex: You look down at your phone during a conversation, and when you look up you do not notice that your friend took off their hat.
• The door study (1998): A stranger asked a passerby for directions, two workers carried a door between them, and a different person continued the conversation.
  • Finding: About half of the passersby did not report noticing that the person had changed.
• NOTE: Don’t confuse this with inattentional blindness! Inattentional blindness is missing something that was there the whole time. Change blindness is missing that something became different.