PSYC 1000Lecture 21

Selective Attention: Stroop

Change BlindnessChange Blindness: People fail to notice pronounced changes inperceptual stimuli, evenintroduction of completely different person into conversation

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Perceptual Grouping• How do we determine what parts of a

scene go together?

• Gestalt Psychology

– Perspective that psychological phenomena could be understood only when viewed as organized, structured wholes and not when broken down into primitive perceptual elements

– The whole is often different from the sum of its parts

Perceptual Grouping

1. Law of proximity– We group together the nearest (most proximal) elements

2. Law of similarity– We group together the most similar elements

3. Law of good continuation– We experience lines as continuous even when they are

interrupted

4. Law of closure– We fill in small gaps to experience objects as wholes

5. Law of common fate– We group together objects that are moving in the same

direction

Gestalt principle: Proximity

• Proximity one of most important factors for perceptual organization

• Here elements grouped as columns, dots, and rows depending on spacing (proximity)

Gestalt principle: Similarity

• Things that look similar tend to be grouped together• The “dots” image now appears to be rows• What makes images “similar” is empirical issue

– In circular image task is to identify odd quadrant of circle, easier for leftmost of two images: distinctiveness?

Gestalt principle: Continuity

• Left we see straight line figure as two separate lines crossing at rather than other configurations, such as “kissing beaks”

• Right, proximity, similarity and continuity interact to yield same image structure

Gestalt Principles: Closure

Parts of occluded objects are completed by visual system

The Law of Common Fate

• Elements that move together seen as forming common object– Animals may blend into background until

they move. Suddenly visible.

Common Fate Example - 1

Common Fate Example - 2

Common Fate Example - 3

Common Fate Example - 4

Spatial & Temporal Integration

• We perceive entire scenes using multiple fixations on our visual world …

• Key principle to understanding our surroundings:

– You must combine information from fixations of different spatial locations (spatial integration) at different moments in time (temporal integration)

Motion Perception• Motion perception requires you to make

comparisons across glimpses of your world

• Phi phenomenon

– A movement illusion that occurs when two stationary spots of light in different positions alternately turn on and off and appear to be one light moving back and forth

• E.g., advertising signs

Motion Illusion

Focus on dot, move head forward and backward

Depth Perception

• Binocular cues– Require use of two eyes– Retinal Disparity

• Images from the two eyes differ • Closer object, larger the

disparity (right)

– Convergence• Neuromuscular cue• Two eyes move inward for near

objects

Tilt-Shift lens

Depth Perception

• Monocular Cues

– Relative size: Smaller image more distant

– Interposition: Closer object blocks distant object

– Relative clarity: Hazy object seen as more distant

– Texture: Coarse close, Fine distant

Depth Perception: Relative Size

Depth Perception: Interposition

Depth Perception: Clarity

Clarity and Perceived Distance

Depth Perception: Texture•Texture gradient

–Stimuli that are nearer have coarser texture.

Depth Perception

• More Monocular Cues

– Relative height: Higher objects seen as more distant

– Relative motion (motion parallax): Closer objects seem to move faster

– Linear perspective: Parallel lines converge with distance

– Light and Shadow: Closer objects appear brighter

The Ames Room

Ames Room: Size-Distance Relation

Size/Distance Illusion•Small figure at front is same size as figure at back, as are two arrows. Front figures appear much smaller. If figures dragged from back to front, would appear to shrink.

Size & Ponzo Illusions

Monocular Cues: Shading

•Major light sources tend to come from above, so shading is another source of depth information. Objects shaded lighter on top seen as “sticking out towards us”.•Here, crater (top) becomes mound (bottom) when picture turned upside down.

Perceptual Constancies• Size constancy

– Ability to perceive the true size of an object despite variations in the size of its retinal images

• Shape constancy– Ability to perceive an object’s shape correctly even

when the object is slanted away from you changing the shape of the retinal image

• Lightness constancy– Tendency to perceive whiteness, greyness, and

blackness as constant across changing levels of illumination