Perceiving Objects and Scenes

what makes scene perception difficult

stimulus on the retina is ambiguous, objects can be hidden or blurred, objects look different from different viewpoints and in different poses

stimulus on the retina is ambiguous

lines at different positions can create the same retinal image, making it difficult for the brain to determine which object it is looking at

theories that propose why humans succeed in perceiving scenes

structuralism, gestaltism

structuralism

-proposed by Edward Titchener

-claims that sensations combine to form perceptions

gestaltism

-directly contradicts structuralism

-claims that conscious awareness can have characteristics not present in any of the elementary sensations (we fill in details when perceiving)

evidence for gestaltism

apparent motion, illusory contours

apparent motion

the perception of movement from a stationary image

illusory contours

when we perceive forms in locations with no physical contours

according to gestaltism, how are humans able to perceive objects and scenes?

through perceptual organisation - humans are able to make sense of a visual image because they can perceptually organise it into constituent objects

how is perceptual organisation achieved

by the processes of grouping and segregation

grouping

the process by which parts of an image are perceptually bound together to form a perceptual whole (perception of an object)

segregation

the process by which parts of a scene are perceptually segregated to form separate wholes (perception of separate objects)

5 principles of grouping

good continuation, Pragnanz, similarity, proximity, common fate

2 additional principles of grouping

common region, uniform connectedness

good continuation

aligned (or nearly aligned) contours are grouped together to form a single object

Pragnanz

groupings occur to make the resultant figure as simple as possible

similarity

similar objects are grouped together

proximity

objects that are closer together tend to group together

common fate

things that are moving the same way are grouped together

common region

elements that are within the same region of space tend to group together

uniform connectedness

connected regions with the same visual characteristics tend to group together

figural properties

regions are more likely to be perceived as the figure if:

-they are in front of the rest of the image

-they are at the bottoms of the image

-they are convex

-they are recognisable

gist perception

the overall impression of a scene acquired from viewing rapid flashes of a scene

who studied gist perceptions and how

-Potter (1976)

-observer was cued with a particular scene description, then was presented with 16 randomly chosen scenes for 250ms each, then was asked if any of the scenes fitted the description - observers were at near 100% accuracy

who investigated the minimum scene exposure time needed to perceive a scene's gist and how?

-Fei Fei

-observers were presented with a single scene, followed by a mask, and were then asked to describe what they saw

results of Fei Fei's experiment

-the longer observers view a scene, the more detailed gist they extract

-27ms is enough to perceive some (not detailed) gists, 67ms is enough for most things, 250ms is very accurate, 500ms is everything

define sensations

elementary processes that occur in response to stimulation

define perceptions

conscious awareness of objects and scenes