Chapter 7 - Perception
Perception
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Primary visual cortex is made up of a large number of “modules” which contain a large number of nerve cells that all respond to different aspects of the same part of the retina … termed the visual field of those nerve cells.
The retina is not evenly represented but, instead, more primary cortex is devoted to images at or near the fovea.
Some nerve cells in a module respond only to lines of certain orientations, others respond only to motion, others to colour, etc.
Thus, primary cortex codes the basic features of the image it receives.
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Secondary visual cortex regions (i.e., association cortex) is responsible for higher level visual processes as revealed by various 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 inability to see colour altogether, a problem termed achromatopsia.
Damage to a slightly different part of visual association cortex can result in an inability to perceive motion.
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Perhaps the most interesting deficits occur when the parietal region of visual association cortex is damaged.
Sometimes damage here leads to an inability to identify objects despite “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 other complex visual tasks such as reading.
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Figure vs. Ground
One of the most basic issues in visual perception concerns how we look at some scene of an image and figure out what is figure (the object of interest) and what is ground
(the context the figure occurs in).
Edges and countours are usually critical in this respect … and usually provide good information
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An issue related to figure/ground is the following. Sometimes visual scenes are somewhat ambiguous, and can be scene in different ways … what are the cues that allow us to see one perspective over another?
Is the picture on the left a picture of a young stylish woman, or of an old woman?
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The classic reversible Necker cude and some more stylish versions
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According to a group of German Psychologists called Gestalt
Psychologists, the primary purpose of the visual system is the recognition 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 the elements to form objects.
Several principles, or laws, are used by the visual system to do this grouping
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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:
FFFFFFF
FF
FFFF
FF
FFFFFFF
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EEEEEE
EE
EEEE
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EEEEEE
Global interferes substantially with local decisions, but there is much less interference of local on global decisions
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Things that are relatively close to one another tend to be grouped together
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Items that look similar will be seen as parts of the same form
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The tendency to perceive unseen parts of a patterns as continuing in a predictable and simple manner.
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Often an object is partly occluded by other objects in our environment, and the visual system must fill in the missing information
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Finally, elements of visual perception that move together are seen as forming a common object.
This law is best imagined in terms of those animals you see on nature shows that seem to perfectly blend into their background, until they move. Then suddenly they appear visible.
As an example though ...
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Thus, a number of laws help us to perceive form … that is, to figure out what the objects are, and how to interpret them despite actual sensations.
Once again, these laws are:
The law of proximity
The law of similarity
The law of good continuation
The law of closure, and
The law of common fate
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The Gestalt principles help us to understand how we figure 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.
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One idea about how we might recognize objects centers around the notion of templates … the reverse cookie-cutter idea.
Such templates seem unlikely given the extremely huge number of them we would need to recognize all the objects we know from 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 what a typical version of some object might look like, then we compare experienced objects to these prototypes and except the best match.
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If subjects are asked to categorize items as being birds or not, they can correctly categorize more typical birds faster than they can categorize less typical birds.
Imagine a penguin in this box (or a picture of a penguin at least)
Slowest RT Middle RT Fastest RT
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Another view is that we recognize objects via distinctive features that define those objects.
For example, consider these examples of the letter Z, what do they 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 object as being a Z
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Let’s play spot the Z!
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Evidence Supporting Distinctive Features
Let’s play spot the Z!
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Evidence Supporting Distinctive Features
Let’s play spot the Z!
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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 could be 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 7.17
at this point
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In addition to the “bottom-up” recognition achieved by the analysis of features, context also provides a “top-down” way to bias the system for perceiving some items over others.
I have already shown you several examples of this including the one to the right here.
Studies with tachistoscopes show that this bias works two ways … as in the bread versus mailbox example of Figure 7.22
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Thus, a number of things may be crucial in our ability to figure out what things are, including:
The use of prototypes
Analysis of features
Contextual Support
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Someone e-mailed me a question asking what it means when someone’s eye jitters … apparently some cultures have some bizarre theories about this.
As the following demo shows, all our eyes jitter a certain amount all the time, we just don’t tend to notice it.
It is possible for amount of jitter to vary across people or time.
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OK, so now we figured out what sensations are objects, and what those objects are … the next problem is figuring out where 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 sources of information, but cannot do tasks based on the other.
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In order for us to figure out where an objects is, we need some way of judging depth in our visual environment.
In turns out that we use a fairly large range of cues to help in our perception of depth, and the fall under two general classes.
Some depth information can only be obtained when both eyes are viewing the world … these types of information are termed binocular cues to depth.
In contrast, monocular cues can be obtained using only one eye.
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Because the two eyes converge on an object when we are viewing it, the brain can use the angle of convergence as a cue to how far away that object is. For example:
The larger the angle, the nearer the object
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Whenever we are not focusing on an object, the image of that object falls on different points of the two retinas.
The amount of disparity (difference) between the two retinal images can be used as a cue for distance.
Try holding up two fingers one in front of the other. Focus on the 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 two images you see as the back finger moves?
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When one object partially occludes our view of a second object, we assume that the first object is closer to us that the second.
For example:
Vs.
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Things appear to get smaller as they recede into the distance even though we know they are not actually getting smaller.
Given this, if the general size of some object in the scene is known, the size of the retinal image cast by that object can be used to judge its distance from us.
This can sometimes lead to neat illusions such as the one to the right.
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We live in a world where our major light sources tend to come from above. As a result, shading has come to be another source of depth information such that objects that are shaded lighter on the top are seen as “sticking out towards us”.
Steve, show Figure 7.32, and turn it around.
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As illustrated in Figure 7.30, closer objects tend to have a courser texture that do far away objects (primarily due to perspective). 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 therefore use haze to infer distance.
The horizon also provides a cue to depth as we know it is far away. Thus, objects closer to the line of the horizon are perceived as being further away (Figure 7.33).
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As we move in our environment, objects closer to us appear to move more relative to their background than do objects far from us.
For example, as I move around the front of the class, the position of the students close to me relative to some point at 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 can be used to judge its distance.
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So, we use a number of sources to infer depth that fall under two general headings:
Binocular Cues
Convergence
Retinal Disparity
Steve, address the question concerning the nature/nurture of depth perception
Monocular Cues
Interposition
Texture
Shading
Motion Parallax
Perspective
Haze
Proximity to Horizon
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As we move around our visual world is constantly changing.
Objects cast different images as we move around them, lighting conditions change, the retinal size of objects change as we move towards and away from them, etc.
However, we do not notice all this. Instead, we form a fairly stable perception wherein we do not suddenly see everything as completely 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 brightness constancy and form constancy.
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A piece of white paper looks white to us whether we see it in sunlight 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.
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When we approach and move around an object, we do not see it getting larger and changing shape although the retinal image is indeed getting larger and changing shape.
This seems to be a somewhat “top-down” effect. We know how large and the typical shape certain objects have. When the size is smaller we assume the object is far from us. When the shape is different, we assume we are not viewing the object straight on.
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We quickly and easily detect motion in our environments.
The text book describes some of the studies of phenomenon relevant to motion perception.
I am not going to discuss these in class. Rather, I leave this end part of the chapter for you.
Hopefully, it will move you.
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