basic problems in perception - university of...

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CognitivePsychologyCognitivePsychology

University of Idaho © Steffen Werner - 2004Perception.07. 1

Basic Problems in Perception

Lesson II: Perceptionmodule 07




What are general problems of perception?

Sensations and relevant physical dimensions– Which physical events are we sensitive to?

Ambiguity of sensory stimuli– How can we perceive a coherent, meaningful scene if

many parts are uncertain?

Perceptual invariants– What are statistical properties of the perceptual

world that our mind can use to infer structure?

Gestalt psychology and gestalt laws– What are basic laws governing perceived structure?

Perceptual illusions and errors– How can we explain the utility of perception if it also

produces a large number of illusions and errors?



Perception - image of the external world?

The phenomenology of perception– Effortless, clear, few uncertainties– Continuous, stable, image of the external world– Identical across perceivers, people

Perception as a scientific question– How does our mind create percepts (mental

representation of a stimulus) out of the multitude ofsometimes ambiguous information?

– What physical dimension and information are humansensory systems sensitive to?

– How is this information processed to produce stableand persistent percepts?

– How can we recognize objects and scenes?

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From sensation to percept

Different sensory modalities– Vision = light of different wavelengths– Audition = sounds of different frequencies– Smell = chemical / target molecules– Taste = chemical / target molecules– Touch = pressure and vibration– Thermal = infrared (longer) wavelength– Proprioception = internal (muscles, joints, vestibular)

Processing in designated sensory system– Conversion of physical stimulus (e.g., light) to neural

response in receptor and …– … transmission of receptor signal to thalamus and cortical

sensory areas (e.g., visual cortex)– Highly specific processing at multiple levels



Selectivity of senses

Different organisms, different senses– Dog - enhanced sense of smell, reduced vision– Bat - ultrasonic communication & echolocation– Shark - detection of electromagnetic fields

Senses are adapted to the specific behaviors andspecific environment of an organism

The sensed and perceived world thus differsdramatically for different organisms

Human sensory processing is also variable– Extreme cases: blindness, deafness

Color deficiencies in visual perception– Approx. 7% of males red-green deficient (<1% females)– Approx. <1% totally color blind



Can you make out any numbers?

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Ishihara color tables



Sense and Sensibility

As different organisms are sensitive to differentphysical stimuli, people’s senses differ in theirsensitivity to available physical stimulation

More examples– Sensitivity to high acoustic frequencies drops over

the lifetime of an individual. As a consequence,mature adults have more trouble following aconversation in a noisy environment.

– One approach to overcoming sensory disabilitiesuses sensory prosthetics like a cochlear implant foraudition (fairly common) or an artificial retina forvision (very rare)

– Technology enables humans to extend their sensoryarsenal to otherwise inaccessible physical dimensions



Sensation vs. Perception

The distal stimulus– Refers to the physical object in the environment that

is giving rise to sensory changes; stable

The proximal stimulus– The resulting sensory stimulation at the receptors– Can vary greatly (e.g., the size of an object’s image

on the retina increases with decreasing distance)

Sensation– Deals with the sensory processing of physical

stimulation

Perception– Deals with the interpretation of sensory stimulation

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Seeing what is not there…

Perception does not just reflect the direct resultof sensory processing

Processing of visual information leads to anorganization and interpretation of stimuli– Perception organizes and integrates sensory events– Perceptual constancies– Grouping of perceptual objects and events– Semantic interpretation of perceptions

Final stages of processing produce the percept ofa unified, meaningful, three-dimensional scene

Let’s look at some examples …



Your blind spot

One of the best examples that you are activelyadding to the sensory events in your perception

Are there any holes in your perception of the world?– Look around you– Did you find any?

Each eye has one area where the optical nerve exitsthe retina where no visual information can beregistered

Even though we cannot see things in that spot(blind spot), our brains fill in something that makessense



The Penrose triangle

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What is odd about the Penrose triangle?

Why do we perceive an impossible 3D-object?– Each corner is possible– The connection of the corners lead to inconsistencies

Why do we perceive only one 3D-object?– For any line drawing, there are an infinite number of

three dimensional objects that could produce theselines in a two-dimensional projection

Why don’t we perceive a possible 3D-object?– There are an infinite number of possible objects that

viewed from one viewpoint would lead to this image

We will encounter more examples of the selectiveperceptions - injecting expectations and bias intothe interpretation of physical stimulation



Interpreting brightness and illumination



Seeing occluders that aren’t there

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Perceptual invariants

Many ambiguities and illusions are resolved under“ecological” perceptual conditions - adding depth,time & movement, and multimodality

Unlike the previous focus on perceptual “error”,perceptual invariants describe characteristics ofthe environment that carry important informationabout the structure of the external world

Examples– Optical flow during observer movement– Doppler-effect (pitch change) for moving objects

We will encounter this view again in module 08when talking about James Gibson’s theory ofdirect (ecological) perception



Organization of visual perception

The law of prägnanz– Visual elements are organized to achieve the

simplest and most parsimonious interpretation of thedistal stimulus

Gestalt laws of grouping– Figure-ground segmentation– Grouping by proximity– Grouping by similarity– Grouping by continuity– Grouping by common fate– Grouping by symmetry



Gestalt laws of proximity and similarity

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Continuity, common fate & symmetry



Grouping vs. good continuation



Grouping vs. good continuation

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Perceptual constancies

Visual perceptions are built across many saccadesand from different view points

Retinal images differ dramatically,yet the percept doesn’t

Types of perceptual constancies– Size constancy– Shape constancy– Brightness constancy– Color constancy– … etc.



Depth perception and size constancy

Size constancy– The retinal projection of an object changes with

distance, but the object is perceived the same

How do we perceive depth?– Monocular depth cues

• Depth cues that are used in 2D depictions of space• texture gradients, relative size, interposition, linear

perspective, aerial perspective, height in the pictureplane, motion parallax

• Accommodation - muscle contractions

– Binocular depth cues• Binocular convergence• Binocular disparity



Depth illusions: The Ponzo illusion

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Shape illusions



What illusions teach us about perception

Human perception has evolved to enable us tointeract with the external world

Perceptual information often is ambiguous, andperception uses heuristics and assumptions aboutthe physical world to make sense of it

Illusions are examples of situations whichhighlight the underlying processing assumptionsby producing odd results

In each illusion we can find basic principles ofperception at work that are well-adapted and thatproduce useable results in most situations

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