INTRO TO PSYCHOLOGY
Set #3 (Lectures 7, 8, 9)
1 Lecture 7
September 22 2009
Perception and mental representations:
Today I want to talk about perception. At first, this issue of perception might begin with the
question: why do we bother studying perception? Perception is the means by which we link to
external world to our inner mental world. If it sounds jargon, its a bunch of language that comes
from cognitive psychology. And it traces its origins back to William James and the idea is that
there is this external world out there and we take it in with our senses.
Lets set monist and dualist arguments apart and move forward. The idea is that, what were
building up in our minds, in our brains, is some representation of that world. Were representing
the external world, internally. These internal representations allow us to interact with the world.
You expect that when you buy chocolate ice cream from Ben and Jerrys, its going to taste like
it did last time. And so youve developed an internal representation for the taste of the ice cream
and the sub-extent that relies on memory, but the idea is this mental representation allows you
to detect whether something in the world is changed or not. If you buy an ice cream and if its a
sour awful taste that you dont remember being there before, you might think that its spoiled.
Its the mental representation that allows you to think that.
We have mental representations about things like gravity. When you see something thrown, you
expect it to fall eventually. We have mental representations about other people, what we expect
them to do. It allows to recognize them either through their voice or through their face or their
behaviors. A person could tell you a story about somebody at a party without naming them and
you might be able to say Oh, that sounds like Sasha. Only she would do that. So youve got a
mental representation of your friend.
The fundamental idea here is perception is necessary for us to do mental representations. And
that might not be obvious at first, but it is John Lock, the British philosopher, who stated that
everything we know, we know through our senses. We cannot know anything else. His challenge
was to try to imagine a smell that youve never smelled before. This is called Locks challenge.
Lock argued that you cannot do this. We can imagine conjunction of things. So, if I ask you to
imagine peanut butter and pickle ice cream, you can probably imagine it because you can
imagine peanut butter and you can imagine pickles and you can imagine ice cream. And you
might even have had peanut butter ice cream, so you can imagine that conjunction, but thats
different than something completely new. If I ask you to imagine a flavor that youve never
tasted, Lock predicts that youll be unable to do it and nobody has shown him to be wrong yet.
This underlines the importance of our senses: they are the gateway to external world. Its true
that you can have thoughts, desires and ideas that are entirely in your own mind. You can
2 imagine things that youve never done, but generally speaking these are based on real things that
youve sensed with your body, real experiences which youve had.
Another fundamental principle of perception that may not be obvious is that perception is
constructive. Im going to try to persuade you into this because its not intuitive for most
people. By constructive, what I mean is that if you look out across the world, it seems like the
world is jut there and you simply see it. You see, interpret colors, shapes and it seems as though
your eyes just deliberate it to you. This is an illusion. You can imagine seeing something when
your eyes are closed. Those things feel as thought theyre projected on some kind of screen
inside your brain. Thats also an illusion.
There is special purpose processing devices in the brain that process things like the color of an
object and they are in the separate locations in the brain. Weve seen patients whove suffered
from damage to one system. You can take such patients and hold a green apple and a red apple
and they could tell you that there is something red and there is something green, but they dont
know where the red is and where the green is. Theyve got a disconnection between the color
part and the location of shape parts of the brain. Some people lose color vision entirely. Some of
the people lose the part of a brain, which processes color, location or movement. Your brain is
using parallel processing to put together and it presents you world and all happens so quickly
that you dont realize. But the brain really is constructing this real world for you. It does in each
of the senses in slightly different ways.
Wavelengths and vision:
We tend to think that our senses give us an unbiased view of the world, but this isnt really true.
In terms of the visual world in color, you could ask what color are light waves? It turns out they
have no color. Newton understood this. Light waves present themselves to you with different
frequencies. This is not obvious and it is counter intuitive, but this is the way it is. The light
wave itself has no color. If you see a red apple, its because those light waves that are reflected
off the surface of the apple are of particular neural wavelengths that your brain interprets as
Similarly, when molecules are vibrating, theyre not actually making a sound. Its a chain of
electro-chemical events that occur after these molecules in your eardrum and they cause you to
feel a sense of itch and a sense of a timber of the instrument.
Your brain is constructing all of these. If you want to be accurate about it, there are light waves
of varying frequencies. There are sound waves of varying frequencies. But as soon as you talk
about color or pitch, youre talking about things that are not in the physical world. Theyre
entirely in the mental world and they are constructed by the brain.
3 The same is true with loudness. Something is only loud when you hear it. There is no such thing
as loudness of the world. Loudness itself is in the brain. You recall Berkeleys question: if a tree
falls in the forest, it doesnt make sound. It may excite the molecules, but if there isnt a brain
there to interpret the vibration, there is no sound and theres no loudness.
This is the threshold that was very carefully acquired
from human subjects in an experiment looking at
different wavelengths of light. I filled in the colors that
are associated with those wavelengths just because you
may not have memorized it. What you need to know
for the midterm, which is that the 400 nanometers of
wavelength is at the bluish violet end and the 700
nanometers is at the reddish end. And as the
wavelengths get shorter, the colors move from red to orange to green to violet.
These thresholds are very carefully obtained by human subjects who saw lights projected at a
constant illumination in terms of how much energy the light has been putting up, measured with
a photon-meter. It turns out, although the lights were emitting exactly the same energy, human
perception was that they were not emitting the same energy. In fact the sensitivity curve shows
you that people are more sensitive to green light than they are to red or violet light. What that
means is that a green light with an equal luminance appears brighter than a red light or violet
There are some odd things here. The wavelength as you can see is varying in a strictly linear
fashion: the distance between 400 and 500 is the same with the distance between 500 and 600 on
this chart that Ive made, but you dont see a continuous change. Its not a gradual change where
each new nanometer gives you a slightly different color. Perception therefore is categorical: we
see things in categories.
Youll notice also some colors are represented across a larger wavelength than others. Yellow is
rather narrow compared to green. The other thing we know is that when we ask people to tell us
which colors are most similar, in general people say that red is more similar to violet than it is to
yellow. Theres this weird effect where the things are at the opposite end seem more alike to
each other than they do to things in the middle leading what we call a color circle. Your mind
is imposing a kind of two dimensional structure on this one dimensional attribute of the world.
The world presents you with one dimensional variation wavelength and your brain perceives it in
two dimensions. We see the same thing in music.
Lets repeat what were the biases of the visual world. Its linear, its not continuous, its
categorical and its 2 dimensional in the mind whereas its reflecting only 1 dimensional change