Object and Scene Perception

Sensations

Elementary processes occur in response to stimulation - what happens in retinas and transduction of image

Perceptions

Conscious awareness of objects and scenes.

Gestaltism

Conscious awareness is more than the sum of elementary sensations; it can have characteristics not present in elementary sensations (part hallucinations). evidence = apparent motion and illusory contours

Apparent Motion

An observer sees two stationary dots flashed in succession and perceives motion, although each dot is stationary - conscious awareness has a character (motion) not present in elementary sensations (as both are stationary)

Illusory Contours

Contours seen in locations where there are no physical contours

perceptual organisation

The process by which our brain organizes and interprets sensory information to form meaningful patterns and objects. By grouping and segregation

Segregation

The process by which parts of a scene are perceptually separated to form separate wholes.

Good Continuation

Aligned contours are grouped together to form a single object e.g. connect correct parts of 2 computer cables that cross over

Prägnanz

Grouping occurs to make the resultant figure as simple as possible = assembly into nameable figure

e.g. WWF logo looks like a panda and not random splotches

Similarity

The more similar objects are, the more likely they will be grouped together.

Proximity

The closer the elements are, the more likely they are to be grouped together.

Common Fate

Things that are moving in the same way are grouped together.

Common Region

Elements within the same region of space tend to group together.

Uniform Connectedness

Connected regions with the same visual characteristics tend to group together.

Figures

Objects distinct from the background. More likely to be seen as a figure if

• in front of rest of image

• at bottom of image

• convex

• recognisable

Gist Perception

Overall impression of what a scene is about.

why is object perception hard

• stimulus on retina is ambiguous

• objects can be hidden or blurred

• objects look different from different viewpoints and in different poses

in humans these problems fixed - reasons proposed by 2 contradicting schools of thought: Structuralism and Gestaltism

structuralism

Edward Titchener - distinguished between sensations and perceptions - claims sensations combine to form perceptions = conscious awareness (perceptions) is sum of these elementary sensations and contains nothing that was not already present in sensations

grouping

process by which parts of an image are perceptually bound together to form a perceptual whole. governed by 5 principles

• good continuation

• Pragnanz

• similarity

• proximity

• common fate

2 added later: common region and uniform connectedness

Potter 1976

studied gist perception - in each trial the observer was cued with a particular scene description then saw 16 randomly chosen scenes each for 250 ms. Then asked if any scenes fitted the description = near 100% accuracy

Fei-Fei et al 2007

investigated minimum scene exposure time needed to perceive scene’s gist. Observer presented with single scene followed by mask and then asked to describe scene = the longer the stimulus presentation time the more detailed and accurate the description =start to perceive aspects of scene at about 27 ms but perceptions not very detailed = accurate perception by 250 ms