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Chapter 7 - Perception Slide 1
Chapter 7
Perception
Chapter 7 - Perception Slide 2
Chapter 7 - Perception Slide 3
Sensation Vs. Perception, Again!
Chapter 7 - Perception Slide 4
Brain Regions & Visual Perception
Primary visual cortex is made up of a large number of “modules”which contain a large number of nerve cells that all respond todifferent aspects of the same part of the retina … termed thevisual field of those nerve cells.
The retina is not evenly represented but, instead, more primarycortex is devoted to images at or near the fovea.
Some nerve cells in a module respond only to lines of certainorientations, others respond only to motion, others to colour, etc.
Thus, primary cortex codes the basic features of the image it receives.
Chapter 7 - Perception Slide 5
Brain Regions & Visual Perception - 2
Secondary visual cortex regions (i.e., association cortex) is responsible for higher level visual processes as revealed byvarious types of brain injury:
Damage to primary visual cortex - often results in “blind spots”but no problems with object recognition.
Damage to one part of association cortex can lead to an inabilityto see colour altogether, a problem termed achromatopsia.
Damage to a slightly different part of visual association cortexcan result in an inability to perceive motion.
Chapter 7 - Perception Slide 6
Visual Agnosia and Prosopagnosia
Perhaps the most interesting deficits occur when the parietalregion of visual association cortex is damaged.
Sometimes damage here leads to an inability to identify objectsdespite “normal” visual acuity - visual agnosia.
Other times, the damage results in an inability to recognize faces,even those of very familiar people - prosopagnosia.
These inabilities can occur despite a lack of problems with othercomplex visual tasks such as reading.
Chapter 7 - Perception Slide 7
Basic Issues - Figure vs. Ground
Figure vs. Ground
One of the most basic issues invisual perception concerns how welook at some scene of an image andfigure out what is figure (the objectof interest) and what is ground(the context the figure occurs in).
Edges and countours are usuallycritical in this respect … and usuallyprovide good information
Chapter 7 - Perception Slide 8
Figure vs. Ground - Ambiguous Images
Chapter 7 - Perception Slide 9
Basic Issues - Perspective
An issue related to figure/groundis the following. Sometimesvisual scenes are somewhat ambiguous, and can be scene indifferent ways … what are the cues that allow us to see oneperspective over another?
Is the picture on the left a pictureof a young stylish woman, or ofan old woman?
Chapter 7 - Perception Slide 10
Other Reversible Figures
The classic reversible Neckercude and some more stylishversions
Chapter 7 - Perception Slide 11
Gestalt Laws of Grouping
According to a group of German Psychologists called GestaltPsychologists, the primary purpose of the visual system is therecognition of objects from basic visual elements.
The objects are seen as more than a sum of the parts, and the critical problem facing the visual system is how to group theelements to form objects.
Several principles, or laws, are used by the visual system todo this grouping
Chapter 7 - Perception Slide 12
Demonstration of the Importance of Objects over Elements
When elements are arranged in groups that define an object,we tend to see the object and not the elements.
This object-superiority can be demonstrated in Stroop-like experiments that use stimuli such as:
FFFFFFFFFFFFFFFFFFFFFF
EEEEEEEEEEEEEEEEEEEE
Global interferes substantiallywith local decisions, but thereis much less interference oflocal on global decisions
vs.
Chapter 7 - Perception Slide 13
Law of Proximity
Things that are relatively close to one another tend to begrouped together
Chapter 7 - Perception Slide 14
The Law of Similarity
Items that look similar will be seen as parts of the same form
Chapter 7 - Perception Slide 15
The Law of Good Continuation
The tendency to perceive unseen parts of a patterns as continuing in a predictable and simple manner.
Chapter 7 - Perception Slide 16
The Law of Closure
Often an object is partly occluded by other objects in our environment, and the visual system must fill in themissing information
Chapter 7 - Perception Slide 17
A Related Phenomenon - Illusory Contours
Chapter 7 - Perception Slide 18
The Law of Common Fate
Finally, elements of visual perception that move togetherare seen as forming a common object.
This law is best imagined in terms of those animals yousee on nature shows that seem to perfectly blend intotheir background, until they move. Then suddenly theyappear visible.
As an example though ...
Chapter 7 - Perception Slide 19
Common Fate Example - 1
Chapter 7 - Perception Slide 20
Common Fate Example - 2
Chapter 7 - Perception Slide 21
Common Fate Example - 3
Chapter 7 - Perception Slide 22
Common Fate Example - 4
Chapter 7 - Perception Slide 23
Perception of Form - Summary
Thus, a number of laws help us to perceive form … that is,to figure out what the objects are, and how to interpret themdespite actual sensations.
Once again, these laws are:
The law of proximityThe law of similarityThe law of good continuationThe law of closure, andThe law of common fate
Chapter 7 - Perception Slide 24
Figuring out what the objects are
The Gestalt principles help us to understand how wefigure out what the objects are, and how to interpret them.
However, they do not explain how we figure out what an object is once we realize it is an object.
Chapter 7 - Perception Slide 25
Templates and Prototypes
One idea about how we might recognize objects centers aroundthe notion of templates … the reverse cookie-cutter idea.
Such templates seem unlikely given the extremely huge numberof them we would need to recognize all the objects we knowfrom all the orientations we know them in.
However, a fuzzy template idea called prototypes may work.The notion here is that we might have some idea about whata typical version of some object might look like, then we compare experienced objects to these prototypes and except thebest match.
Chapter 7 - Perception Slide 26
Evidence Supporting Prototypes
If subjects are asked to categorize items as being birds ornot, they can correctly categorize more typical birds fasterthan they can categorize less typical birds.
Imagine a penguinin this box (or a
picture of a penguin at least)
Slowest RT Middle RT Fastest RT
Chapter 7 - Perception Slide 27
Recognition via Distinctive Features
Another view is that we recognize objects via distinctivefeatures that define those objects.
For example, consider these examples of the letter Z, what dothey have in common?
Z Z Z Z Z Z
The answer, two horizontal lines and one diagonal line. Perhaps it is the presence of these features that define an objectas being a Z
Chapter 7 - Perception Slide 28
Evidence Supporting Distinctive Features
Let’s play spot the Z!
Chapter 7 - Perception Slide 29
Evidence Supporting Distinctive Features
Let’s play spot the Z!
OBCCGDOOPDDGQQCCPOCGDOPRGPOCBGQRQSSUOPCSRUPQCDBPOSCURPOPPCDBZPODQPOQSGOPQCBBCGPOQDUOPQOPQDCBGSOSPQSRCBDOPQSC
Chapter 7 - Perception Slide 30
Evidence Supporting Distinctive Features
Let’s play spot the Z!
KLEFIXKNMLMVXWIYLKMNXIKLWNMVXAILKHNMVTEFILMIMKLNVXWAVNMKLIYZFENMIMKLNHXVEYIFKLMNVWTYILXVNMKLIYWTNMLKMFWENM
Chapter 7 - Perception Slide 31
Distinctive Features and Real World Objects
Features seem like a natural way to think about how we identify letters, but what about real world objects?
Beiderman (1987, 1990) suggested that a similar logic couldbe applied to real world objects, except they need to be thought of as being composed of geons instead features.
What the heck are geons? - basic three dimensional shapes:
Steve, show figure 7.17at this point
Chapter 7 - Perception Slide 32
Help from Context
In addition to the “bottom-up” recognition achieved by theanalysis of features, context also provides a “top-down” wayto bias the system for perceiving some items over others.
I have already shown you severalexamples of this including the oneto the right here.
Studies with tachistoscopes showthat this bias works two ways …as in the bread versus mailboxexample of Figure 7.22
Chapter 7 - Perception Slide 33
Perception of Objects - Summary
Thus, a number of things may be crucial in our abilityto figure out what things are, including:
The use of prototypesAnalysis of featuresContextual Support
Chapter 7 - Perception Slide 34
Answer to E-mail Question
Someone e-mailed me a question asking what it means whensomeone’s eye jitters … apparently some cultures have somebizarre theories about this.
As the following demo shows,all our eyes jitter a certain amount all the time, we justdon’t tend to notice it.
It is possible for amount of jitter to vary across people ortime.
Chapter 7 - Perception Slide 35
Spatial Information
OK, so now we figured out what sensations are objects, andwhat those objects are … the next problem is figuring outwhere the objects are in space.
In fact, after leaving the primary visual cortex, visual information seems to travel along two fairly-distinct pathways,one pathway is devoted to figuring out what things are, the other is devoted to figuring out where they are.
There are many cases in the neuropsychology literature of patients that can perform tasks based on one of these sourcesof information, but cannot do tasks based on the other.
Chapter 7 - Perception Slide 36
Depth Perception
In order for us to figure out where an objects is, we need someway of judging depth in our visual environment.
In turns out that we use a fairly large range of cues to helpin our perception of depth, and the fall under two generalclasses.
Some depth information can only be obtained when both eyes are viewing the world … these types of information are termedbinocular cues to depth.
In contrast, monocular cues can be obtained using only one eye.
Chapter 7 - Perception Slide 37
Binocular Cues - Convergence
Because the two eyes converge on an object when we areviewing it, the brain can use the angle of convergence as acue to how far away that object is. For example:
The larger the angle, thenearer the object
Chapter 7 - Perception Slide 38
Binocular Cues - Retinal Disparity
Whenever we are not focusing on an object, the image of thatobject falls on different points of the two retinas.
The amount of disparity (difference) between the two retinalimages can be used as a cue for distance.
Try holding up two fingers one in front of the other. Focus onthe front one (you should now see two images of the back one).Now move the back one away from, then back towards you, while still focusing on the front one. What happens to the twoimages you see as the back finger moves?
Chapter 7 - Perception Slide 39
Monocular Cues - Interposition
When one object partially occludes our view of a secondobject, we assume that the first object is closer to us thatthe second.
For example:
Vs.
Chapter 7 - Perception Slide 40
Monocular Cues - Perspective
Things appear to get smaller as they recede into the distanceeven though we know they are not actually getting smaller.
Given this, if the general size ofsome object in the scene is known,the size of the retinal image castby that object can be used to judgeits distance from us.
This can sometimes lead to neat illusions such as the one tothe right.
Chapter 7 - Perception Slide 41
Monocular Cues - Shading
We live in a world where our major light sources tend to come from above. As a result, shading has come to be anothersource of depth information such that objects that are shadedlighter on the top are seen as “sticking out towards us”.
Steve, show Figure 7.32, and turn it around.
Chapter 7 - Perception Slide 42
Monocular Cues - Texture, Haze & Horizons
As illustrated in Figure 7.30, closer objects tend to have a courser texture that do far away objects (primarily due toperspective). Thus, texture can be used as a cue to depth.
Also, further away subjects tend to be hazier than close objects. As illustrated in Figure 7.31, we can thereforeuse haze to infer distance.
The horizon also provides a cue to depth as we know it isfar away. Thus, objects closer to the line of the horizon areperceived as being further away (Figure 7.33).
Chapter 7 - Perception Slide 43
Monocular Cues - Motion Parallax
As we move in our environment, objects closer to us appearto move more relative to their background than do objectsfar from us.
For example, as I move around the front of the class, the position of the students close to me relative to some pointat the back of the class moves much more that does the position of the student at the back of the class.
Thus, the amount of motion parallax an object produces canbe used to judge its distance.
Chapter 7 - Perception Slide 44
Depth Perception - Summary
So, we use a number of sources to infer depth that fallunder two general headings:
Binocular CuesConvergenceRetinal Disparity
Monocular CuesInterposition PerspectiveTexture HazeShading Proximity to HorizonMotion Parallax
Steve, address the question concerningthe nature/nurture of depth perception
Chapter 7 - Perception Slide 45
Constancies of Visual Perception
As we move around our visual world is constantly changing.Objects cast different images as we move around them, lightingconditions change, the retinal size of objects change as we movetowards and away from them, etc.
However, we do not notice all this. Instead, we form a fairlystable perception wherein we do not suddenly see everything ascompletely different when a cloud goes in front of the sun.
Our ability to see a stable percept is due to certain constancies in visual perception, the two most prominent being brightnessconstancy and form constancy.
Chapter 7 - Perception Slide 46
Brightness Constancy
A piece of white paper looks white to us whether we see it insunlight or shadow.
We do not view the brightness of some area in absolute terms,but rather we view it relative to the brightness of the context
Thus, when a cloud passes over, everything gets less bright and,as a result, nothing really seems to.
Chapter 7 - Perception Slide 47
Form Constancy
When we approach and move around an object, we do not seeit getting larger and changing shape although the retinal imageis indeed getting larger and changing shape.
This seems to be a somewhat “top-down” effect. We know howlarge and the typical shape certain objects have. When the sizeis smaller we assume the object is far from us. When the shapeis different, we assume we are not viewing the object straight on.
Chapter 7 - Perception Slide 48
Perception of Motion
We quickly and easily detect motion in our environments.
The text book describes some of the studies of phenomenonrelevant to motion perception.
I am not going to discuss these in class. Rather, I leave thisend part of the chapter for you.
Hopefully, it will move you.