Set #2 (Lectures 5 & 6)
Lectures 7 & 8 will be emailed to you (in order for you to have enough time to study them).
January 19 , 2010
Tomorrow the syllabus will be updated with the TA/student assignments.
If things are unclear to you, post question in the midterm discussion forums.
All midterms will be held in class.
All material in the textbook and in the class will be in exams. Names will not be the focus of the
CHAPTER THREE: PERCEPTION (Continued)
What is vision? There are three widespread myths about vision.
1. Vision gives a faithful record of our surroundings. We looked at the Fraser Spiral and some of
the Escher drawings. If vision was a faithful record, like a camera taking snapshots of our world
and thats how we see, if we opened our eyes (surgically), we would be able to see a whole
bunch of little pictures. However, this is not the case. (See colour insert #5 in book.) There are
different parts of our visual system: the signal from the eyes goes through different parts of the
brain reaching the primary visual cortex. This slice is like slicing your head at eye level. One
thing that you see here is that we have cross-over. The left visual field, which is not just your left
eye but the left part of both of your eyes (the red). If we trace the red, it all ends up in the right
visual cortex. The left visual field is in the right visual cortex, and vice versa. This is
contralateral representation. What also happens is that the images get into our eyes, and the first
thing that happens is that the three-dimensional image is flattened. Once that information goes
through the brain structures, it gets completely decomposed. The primary visual cortex area only
has information about luminance, lines, and very basic visual information. There are no complete
pictures or people, only basic information about edges and contours. This contrasts the idea that vision is like a photograph. It is not. Visual information gets completely decomposed and then
recomposed by the brain again.
2. Vision is a passive process (it just happens). We have seen that in Eschers drawings. We think
that opening our eyes is effortless: everything from the environment just hits us and that is just
what happens. Then we are able to see the inconsistencies of Eschers examples. We not only see
this (a picture done by Escher with a building) as a three dimensional figure, but we actually
dont even notice the local inconsistencies that this is a contorted building. We dont notice these
things and there are two major things that put limitation on our vision:
(a) Acuity: only in our foveal (central, a few degrees of visual angle (one
degree is your thumb at arms length)) vision can we see very fine details.
Our peripheral vision cant see very well; it is kind of blurry.
(b) Attention: where our attention is guided determines what we are
looking at and what we see. This is why two people can look at the exact
same thing and see two different things.
3. Our eye sees. Can you see without your eyes? Can you have images without your eyes? The
question here is what comes first: is it seeing or perception? When we examine how we see, the
physiology of eyes and how it happens, our vision is really bad. The first thing is that our eyes
have blind spots in them (you can see the demonstration of this in our book because it involves
moving the book back and forth: Fig 3.19). Both of our eyes have two blind spots. (a picture of
the blind spots located in the eye): This is the space where blind spots occur, where the optic
nerve leaves the eye is where we have blind spots. We dont see spots, its mostly continuous.
Another thing is that we have large blood vessels running through our eyes, our cones, but we
dont see that too: that actually obstructs our vision, we mostly dont see that. We have this clear perception. Our eyes are also not stationary, they move all the time. They move in saccadic,
short movements but also longer movements when you track from center to periphery: that
creates a smear. Every eye movement does this. Despite that we still see a continuous picture; we
still see a picture with no blotchiness, blind spots, or smears. Finally, this picture is completely
distorted at the cortical level, because we have a topographic representation of our visual field.
Our visual field is left-right reversed and top-down reversed as well. We have disproportionately
a lot of cortex responsible for processing information in the fovea, and less of the cortex devoted
to the processing of peripheral information.
(A picture of the brains visual field cut into four sectors): This is the visual field. The left, and
right. Those are the quadrants. This is if you open your brain up and see the two hemispheres. The
representations of 1, the upper left quadrant is actually outside and on the bottom in the right
hemisphere: it is reversed. It gets disproportionately more information than quadrant nine. The
images that we get are quite distorted; that is one of the arguments against the myth that the eyes
The second one is that we can actually see without our eyes. How many people have classic
migraines? How many with auras? These people know that visual auras happen in absence of visual
input; it is seeing without information from the environment. Classic migraines are headaches
accompanies by visual sensation that precedes the migraine by a half an hour or so. That visual
sensation comes directly out of the brain.
Another one is your dreams. When we dream, REM dream, we see vivid pictures in absence of
visual input. While we need our eyes, the perception that we are getting is constructed in our brain.
Anything without external input: ex. when you close your eyes, remember your home. How many
windows does your living room have? You close your eyes and walk through it mentally, get to the