Slide 1Chapter 7 - Perception Chapter 7 Perception.

Chapter 7 - Perception Slide 1

Chapter 7

Perception


Sensation Vs. Perception, Again!


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.


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.


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.


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


Figure vs. Ground - Ambiguous Images


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?


Other Reversible Figures

The classic reversible Neckercude and some more stylishversions


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


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.


Law of Proximity

Things that are relatively close to one another tend to begrouped together


The Law of Similarity

Items that look similar will be seen as parts of the same form


The Law of Good Continuation

The tendency to perceive unseen parts of a patterns as continuing in a predictable and simple manner.


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


A Related Phenomenon - Illusory Contours


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 ...


Common Fate Example - 1


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


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.


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.


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


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


Evidence Supporting Distinctive Features

Let’s play spot the Z!




OBCCGDOOPDDGQQCCPOCGDOPRGPOCBGQRQSSUOPCSRUPQCDBPOSCURPOPPCDBZPODQPOQSGOPQCBBCGPOQDUOPQOPQDCBGSOSPQSRCBDOPQSC




KLEFIXKNMLMVXWIYLKMNXIKLWNMVXAILKHNMVTEFILMIMKLNVXWAVNMKLIYZFENMIMKLNHXVEYIFKLMNVWTYILXVNMKLIYWTNMLKMFWENM


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


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


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


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.


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.


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.


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


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?


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.


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.


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.


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).


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.


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


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.


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.


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.


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.

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