Slide 1Chapter 7 - Perception Chapter 7 Perception

Slide 2Chapter 7 - Perception

Slide 3Chapter 7 - Perception Sensation Vs. Perception, Again!

Slide 4Chapter 7 - Perception 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 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.

Slide 5Chapter 7 - Perception Brain Regions & Visual Perception - 2 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.

Slide 6Chapter 7 - Perception Visual Agnosia and Prosopagnosia 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.

Slide 7Chapter 7 - Perception Basic Issues - Figure vs. Ground 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

Slide 8Chapter 7 - Perception Figure vs. Ground - Ambiguous Images

Slide 9Chapter 7 - Perception Basic Issues - Perspective 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?

Slide 10Chapter 7 - Perception Other Reversible Figures The classic reversible Necker cude and some more stylish versions

Slide 11Chapter 7 - Perception Gestalt Laws of Grouping 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

Slide 12Chapter 7 - Perception 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: FFFFFFF FF FFFF FF FFFFFFF EEEEEE EE EEEE EE EEEEEE Global interferes substantially with local decisions, but there is much less interference of local on global decisions vs.

Slide 13Chapter 7 - Perception Law of Proximity Things that are relatively close to one another tend to be grouped together

Slide 14Chapter 7 - Perception The Law of Similarity Items that look similar will be seen as parts of the same form

Slide 15Chapter 7 - Perception The Law of Good Continuation The tendency to perceive unseen parts of a patterns as continuing in a predictable and simple manner.

Slide 16Chapter 7 - Perception The Law of Closure Often an object is partly occluded by other objects in our environment, and the visual system must fill in the missing information

Slide 17Chapter 7 - Perception A Related Phenomenon - Illusory Contours

Slide 18Chapter 7 - Perception The Law of Common Fate 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...

Slide 19Chapter 7 - Perception Common Fate Example - 1

Slide 20Chapter 7 - Perception Common Fate Example - 2

Slide 21Chapter 7 - Perception Common Fate Example - 3

Slide 22Chapter 7 - Perception Common Fate Example - 4

Slide 23Chapter 7 - Perception 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 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

Slide 24Chapter 7 - Perception Figuring out what the objects are 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.

Slide 25Chapter 7 - Perception Templates and Prototypes 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.

Slide 26Chapter 7 - Perception Evidence Supporting Prototypes 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

Slide 27Chapter 7 - Perception Recognition via Distinctive Features 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

Slide 28Chapter 7 - Perception Evidence Supporting Distinctive Features Let’s play spot the Z!

Slide 29Chapter 7 - Perception Evidence Supporting Distinctive Features Let’s play spot the Z! OBCCGDOOPDDGQQCCPOCGD OPRGPOCBGQRQSSUOPCSRUP QCDBPOSCURPOPPCDBZPODQ POQSGOPQCBBCGPOQDUOPQ OPQDCBGSOSPQSRCBDOPQSC

Slide 30Chapter 7 - Perception Evidence Supporting Distinctive Features Let’s play spot the Z! KLEFIXKNMLMVXWIYLKMNX IKLWNMVXAILKHNMVTEFILM IMKLNVXWAVNMKLIYZFENM IMKLNHXVEYIFKLMNVWTYIL XVNMKLIYWTNMLKMFWENM

Slide 31Chapter 7 - Perception 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 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

Slide 32Chapter 7 - Perception Help from Context 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

Slide 33Chapter 7 - Perception Perception of Objects - Summary 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

Slide 34Chapter 7 - Perception Answer to Question Someone ed 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.

Slide 35Chapter 7 - Perception Spatial Information 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.

Slide 36Chapter 7 - Perception Depth Perception 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.

Slide 37Chapter 7 - Perception Binocular Cues - Convergence 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

Slide 38Chapter 7 - Perception Binocular Cues - Retinal Disparity 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?

Slide 39Chapter 7 - Perception Monocular Cues - Interposition 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.

Slide 40Chapter 7 - Perception Monocular Cues - Perspective 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.

Slide 41Chapter 7 - Perception 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 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.

Slide 42Chapter 7 - Perception 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 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).

Slide 43Chapter 7 - Perception Monocular Cues - Motion Parallax 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.

Slide 44Chapter 7 - Perception Depth Perception - Summary So, we use a number of sources to infer depth that fall under two general headings: Binocular Cues Convergence Retinal Disparity Monocular Cues InterpositionPerspective TextureHaze ShadingProximity to Horizon Motion Parallax Steve, address the question concerning the nature/nurture of depth perception

Slide 45Chapter 7 - Perception Constancies of Visual Perception 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.

Slide 46Chapter 7 - Perception Brightness Constancy 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.

Slide 47Chapter 7 - Perception Form Constancy 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.

Slide 48Chapter 7 - Perception Perception of Motion 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.