Perception

How your mind understands sensory information

Sensation vs. Perception• Sensation – to construct the outside world inside our heads

we must detect physical energy from the environment and then encode it as neural signals. – Bottom-up processing—Data-driven where sensory information

travels from receptors to the brain. – assembling a jig-saw puzzle without the picture

• Perception - The process of integrating, organizing, and interpreting sensory information– Top-down processing— Draw upon our knowledge, experiences &

expectations to arrive at meaning. Also called conceptually driven processing.

– using the picture to assemble the jig-saw puzzle

• Sensation and Perception blend into one continuous process, progressing upward from specialized detector cells and downward from our assumptions.

Selective Attention• At any moment we focus our awareness on only a limited aspect of

all that we are capable of experiencing. • Cocktail party effect – the ability to attend selectively to only one

voice among many.• True for visual attentiveness – Neisser study with woman &

umbrella - Examples of Change Blindness.• True for auditory attentiveness – Wilson experiment where person

listens to two separate conversations (one in each ear). Can only listen to one at a time.

• Unattended stimuli can have subtle effects. If someone says your name at a party, your perceptual system may bring that voice to consciousness.

• We are constantly filtering sensory info and inferring perceptions in ways that make sense to us.

• Look at Necker Cube on oncoming slide – because attention is selective, you can see only one interpretation at a time.

Gestalt

• Founded by Max Wertheimer, it emphasizes that we view things as a unified whole or figure rather than in isolated bits and pieces

• Given a cluster of sensations, the human mind organizes them into a Gestalt

• Gestalt psychologists stressed that the whole is greater than the sum of its parts.

• By breaking experiences into their basic parts, something important is lost.

A Gestalt Figure & Selective AttentionWhat do you see: circles with white lines, or a cube?

If you stare at the cube, you may notice that it reverses location, moving the tiny X in the center from the front edge to the back. At times the cube may seem to float in front of the page, with circles behind it; other times the circles may become holes in the page through which the cube appears, as though it were floating behind the page. Because attention is selective, you see only one interpretation at a time. For an online demo click below: Necker Cube

Temple or Tunnel?

To transform sensory information into meaningful perceptions our perceptual

processes help us organize info so we can answer 3 questions:

1. What is it?2. How far away is it?3. Where is it going?

Organizational Principles

What is It? Perception of Shape• We primarily rely on shape to identify things in our

environment.

• Dax Experiment showed this in 3-year-olds. As long as the object had the same shape as the original dax, the children identified it as a dax.

How Far Away Is It? Figure-Ground Relationships

Figure and GroundGestalt Psychologists also

thought that an important part of our perception was the organization of a scene in to its:

Figure—the object of interest

Ground —the background

Pictures have reversible figure-ground

Different neurons in the brain fire for shapes that are figure than do for shapes that are ground.

Another Figure-Ground ExampleDo you see musicians or old people?

M.C. Escher

What’s the figure in this picture?Having trouble? Click HERE or HERE

IllusionsAn object’s background can change and make us perceive things that are not really present.

Click on the video clip below. Are the lines slanted or straight?

The Café Wall Illusion

Zollner Effect

Are these squares straight? Straight lines appear to bend if they intersect with or are seen against a background of curved lines. Your eyes and brain are working together to try to make the straight lines fit the background pattern.

Spiral Square Case

Are these squares bent? The curves of the spiral in the background make the square seem bent. Ground has a role in how we perceive the figure.

Are the letters tilted?

“Now, squint your eyes to see it straight”- Laura Kooistra

Organizational Principles:

Grouping Principles

Law of Pragnanz (Simplicity)

• When several perceptual organizations are possible, the simplest and most stable shape will be perceived

• What do you see?•You probably perceived this image as that of three overlapping squares rather than as two six-sided objects and one four-sided object.

Grouping

• Organizing the figure information into meaningful forms.

• The perceptual tendency to organize stimuli into understandable groups

• Several principles of grouping include:– Similarity– Proximity– Closure– Continuity (Good Continuation)

Grouping - Similarity

• The tendency to place items that look similar into a group

We perceive objects of similar size, shape, or color as a unit or

a figure

Grouping - Proximity

• The tendency to place objects that are physically close to each other in a group

Our tendency perceive objects that are close to one another as a unit or a figure. We see three sets of two line or two groups of three people.

Grouping – Closure

• The tendency to look at the whole by filling in gaps in a perceptual field

Our tendency to fill in the gaps or contours in an

incomplete image.

Completing or Connecting the object

Grouping – Continuity

• We perceive lines as smooth continuous patterns rather than discontinuous ones.

• Once an object appears to move in a particular direction, your brain assumes that the movement continues unchanged.

• Also known as Law of Good Continuation

Our tendency to group lines that appear to follow in the

same direction as a single unit or figure. Seeing the line as continuous and the curves as

continuous.

Connectedness

• When they are uniformed and linked, we perceive spots, lines, or areas as a single unit.

These Grouping Principles Can Lead us Astray…

• You probably perceive this doghouse as a gestalt—a whole (though impossible) structure. Actually, your brain imposes this sense of wholeness on the picture. As the photo on page 225 shows, gestalt grouping principles such as closure and continuity are at work here.

Impossible Figures Revealed

It’s a matter of perspective!