Perception Perception = the process of understanding & interpreting sensations. What are the rules our minds use to interpret sensations?

Bottom-Up vs. Top-Down When analyzing how we move between sensation and perception, we describe two types of processing: Bottom-Up Processing Senses analyze the stimulus (Bottom) Analysis of the stimulus begins with the sense receptors and works up to the level of the brain and mind Top-Down Processing Brain – higher level thinking (Top) Information processing guided by higher-level mental processes as we construct perceptions, drawing on our experience and expectations.

Bottom-Up Processing Also called “feature analysis”. We use only the features on the object itself to build a perception (instead of relying on experience). Takes longer that top-down but is more accurate.

Top-Down Processing We perceive by filling the gaps in what we sense. I _ant ch_co_ate ic_ cr_am. Based on our experiences. If you see many old men in glasses, you are more apt to process a picture of an old man (even when you may be in error).

Top-Down Processing example: Aoccdrnig to rscheearch at Cmabrigde Uinervtisy, it deosn’t mttaer in what oredr the ltteers in a word are; the olny iprmoetnt tihng is that the frist and lsat ltteer be at the rghit pclae. The rset can be a toatl mses and you can still raed it wouthit a porbelm. This is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the word as a wlohe.

Principles of Visual Perception Figure Ground Relationship Our first perceptual decision is what image is the figure and what is the background? The organization of the visual field into objects (figures) that stand out from their surroundings (ground)

Figure-Ground Reversal When figure-ground relationships are ambiguous our perceptions tend to be unstable, shifting back & forth.

Perceptual Set A mental predisposition. What you see in the center picture is influenced by flanking pictures. Preview Question 23: How do our expectations, contexts, and emotions influence our perceptions? From Shepard, 1990.

Is the cat going up or down?

Gestalt Rules Gestalt means “an organized whole” These psychologists emphasize our tendency to integrate pieces of information into meaningful wholes The whole is greater than the sum of its parts

Grouping The perceptual tendency to organize stimuli into groups that we understand Gestalt psychologists focused on how we GROUP objects together. We innately look at things in groups and not as isolated elements.

Grouping Proximity (objects that are close together are likely to be perceived as being part of same group) Similarity (objects similar in appearance are perceived as being part of same group) Continuity (objects that form a continuous form are perceived as same group) Closure (like top-down processing…we fill gaps in if we can recognize it)

Perceptual Consistency Objects change in our eyes constantly as we or they move….but we are able to maintain the original form Perceiving objects as unchanging Without Perceptual constancy, we would be constantly re-interpreting TYPES: Shape Constancy Size Constancy Brightness Constancy Next Slide

Shape Constancy Everybody has seen a plate shaped in the form of a circle. When we see that same plate from an angle, however, it looks more like an ellipse. Shape constancy allows us to perceive that plate as still being a circle even though the angle from which we view it appears to distort the shape.

Size Constancy Refers to our ability to see objects as maintaining the same size even when our distance from them makes things appear larger or smaller. We keep a constant size in mind for familiar objects We know that objects don’t grow & shrink in size as they move closer or farther away

Brightness/Color Constancy DAY Refers to our ability to recognize that color remains the same regardless of how it looks under different levels of light.  Without color constancy, we would be constantly re-interpreting color NIGHT

Perception of Motion The visual perception of movement is based on change of position relative to other objects WHAT ABOUT ILLUSIONS OF MOVEMENT??? The Autokinetic Effect: The tendency to perceive a stationary point of light in a dark room as moving Research shows that if people stare at a white spotlight in a dark room, it appears to move.

Perceived Motion 2. Stroboscopic effect (flip book effect) This is what makes motion pictures possible Involves the presentation of a rapid progression of images of stationary objects Each frame differs slightly from the one preceding it Showing frames in rapid succession creates the illusion of movement

Perceived Motion Ex/ news headlines that wrap around a building Ex/ scoreboard graphics at a baseball game Ex/ NY Stock Exchange ticker 3. Phi Phenomenon An illusion of movement created when two or more adjacent lights blink on and off in succession. A row of lights is switched on & then off. As the first row is switched off, a second row is switched on, and so on for dozens or hundreds or rows

Depth Perception Eleanor Gibson and her Visual Cliff Experiment. If you are old enough to crawl, you are old enough to see depth perception. Most agree by 3 months We see depth by using two cues that researchers have put in two categories: Monocular Cues Binocular Cues

Visual Cliff

Monocular Cues Tricks that we use to judge depth perception You need only one eye to use them Artists use these same cues to imply depth in their drawings

Monocular Cues 1. Relative Size: If two objects are similar in size, the one that looks smaller is farther away

Monocular Cues 2. Interposition: AKA: Overlapping Objects that occlude (block) other objects tend to be perceived as closer.

Monocular Cues 3. Linear Perspective: Parallel lines, such as railroad tracks, appear to converge in the distance. The more the lines converge, the greater their perceived distance.

Monocular Cues 4. Light and Shadow: Nearby objects reflect more light into our eyes than more distant objects. Shading can imply where a light source is and thus imply depth and position. Dimmer objects appear farther away because they reflect less light.

Binocular Cues We need both of our eyes to use these cues Retinal Disparity: Each of our eyes see any object from a slightly different angle The brain gets both images If an object is far away the images will be similar If an object is close there will be more disparity between the images 2. Convergence: As an object moves closer to us our eyes must move toward each other to keep focused The brain receives feedback from our eye muscles & knows that the more your eyes converge, the closer the object must be

VISUAL ILLUSIONS Sometimes these same rules fool us. The distant monster (below, left) and the top red bar (below, right) appear bigger because of distance cues. PONZO ILLUSION

VISUAL ILLUSIONS Sometimes these same rules fool us. Both girls in the room are of similar height. However, we perceive them to be of different heights as they stand in the two corners of the room. Both photos from S. Schwartzenberg/ The Exploratorium

Ames Room