Chapter five on
sensation and perception.
If you’ll go to slide number two, you’ll see some of the broad issues that
we wanta talk about in this chapter. Keep in mind that most of what we
experience is constructive internally in our heads. The world, the universe
provides us with certain forms of energy and then that energy has to be
converted to something that the nervous system can work with, so in the
visual system electromagnetic radiation has to be converted to neuro energy
and the auditory system, vibrations there have to be converted to neuro
energy and so on. That process is that going from the external energy to
neuro energy or something that is, changed the neuro firing rate, this is
called transduction. That’s an important term and I want you to keep that in
mind as we go through because a lot of what we’ll be talking about is
transduction in the various sensory systems.
If you go to slide three, we’ll start with vision, by the way, and we’ll
spend most of our time on the visual and the auditory systems because that’s
where most of the research has been done. We won’t ignore the olfactory
system and the gustatory system, the tactal senses and so on, but a lot less
research has been spent on those systems and so the amount of time we spend
really reflects the amount of knowledge we have in these areas and yes, it
is perfectly true that you can live without either and have a very rich life
without either of the visual or the auditory systems working for you; Helen
Keller being the prime and best known example of that. But I think most of
us would agree that if you have one of those two systems or both of those
systems, you will rely primarily on those compared to the other systems that
you have, so again we’ll spend more time on that.
If you go to slide three, you’ll see, again, the overview of what we’ll talk
about with the visual system and the visual transduction. We’ll talk about
electromagnetic radiation, conversation of that to neuro energy. Adapting to
light and dark, how we get the sensory information, how we convert it to
firing rates and how we then produce stable interpretations of that input.
If you go to slide four, we’ll talk about visual transduction specifically.
One of the questions I’ll ask in each of the sensory systems is what is out
there and by out there, I mean what is the energy that’s provided by the
universe. In the visual system, what’s out there that we use is
electromagnetic radiation. The human visual system is sensitive to a narrow,
narrow band of wave length of the electromagnetic spectrum. From about 400
to 750 nanometers and there’ll be a picture on slide five in a second, but I
want you to see the wave length, of course, they have wave-like properties
and by that we're talking about the intensity which corresponds to sort of
the height, although I think of a sign wave, sort of an s laid on it’s side,
and the height from one crest to the cross, which is the terms used for the
top and the bottom of the wave is a measure of the intensity of the
experience, wave length is the distance from where the s begins to where it
ends, so that’s one length of the wave and a wave length generally produces
the experience of hue or color. The third quality is purity which really
refers to the extent to which they’re, the quality gray or black is present
in the experience. If you think about it, it’s very important that the
nervous system encode grays, of course, more saturation information because
it is the gray or shadowing of most of our visual experiences that gives us
information about texture. So for example a carpet has a lot of grays in it
because there’s a lot of shadowing in it because the tufts of the carpet and
that’s important to us. It’s important to us to know as we look at a
building up close, if we can see the lines of the brick and that tells us
something about the nature of that article, whether it’s a single unit or
multiple units have been put together and so on, so texture information is
very useful and so the nervous system does encode texture information. Light
enters through the cornea and we’ll see that in slide six.
So let’s go to slide five first, which; and on slide five you’ll see figure
5.1 and there you can see the electromagnetic spectrum from the very large
wave lengths to very small wave lengths and then you can see that chunk of
the spectrum that the visual system is sensitive to. It really sort of
wedges in there in that little arrowhead which you’ll see points to where
the visual spectrum fits into the larger electromagnetic spectrum. Again,
about 400 nanometers to 700, actually closer to 750 nanometers. This figure
shows 400 to 700 and that’s fine as a rule of thumb and it also shows you
again what I mentioned before, though, what wave length is and what the
amplitude or intensity of the wave is. This shows you amplitude measured at
from the trough of the wave to the mid-point, and that’s fine. If you wanta
go from the trough of the wave to the crest of the wave or trough to the
middle point, one is mathematically derivable from the other so it doesn’t
matter too much which one of those you use. Wave length is also related to
frequency because remember the wave is moving and so as one length of the
wave, which you’ll see there in the figure on the right side, passes a unit
in time or how many of those wave lengths pass a unit in time past that
specific point in the unit of time like wave lengths per second you get a
measure of frequency. As you can see that the old nursery rhyme was correct;
red, orange, yellow, green, blue, indigo, violet or red, orange, yellow,
green, blue, indigo, purple, whatever. And that is exactly how our
experience measures off against these wave lengths. Again, it’s a very
narrow band, there are a lot of other wave lengths, of electromagnetic
radiation that produces other kinds of phenomenon; x-rays, you’ll see
television, radio waves, microwaves, and so on, so this very narrow band of
wave lengths that is bouncing around in our atmosphere produces chemical
changes in the retina and those are what we use to produce the visual
experience. I could ask you whether as you look around the room that you’re
sitting in right now, whether there’s light in that room and technically the
answer is no. What’s in that room is radiation, all within this very narrow
band of wave lengths and those wave lengths produce chemical changes on the
retina and you produce the light inside your head, so light experiences in
your head out there because radiation is bouncing around off surfaces and
off individuals and so on and that is something that you use, your brain
uses to produce light. But the light is actually in your head, it’s not out
there in the world.
If you go to figure 5.2 on slide six, you’ll see just a little bit of a
picture of an eye and so it’s good to remember learn a little bit about the
anatomy of the eye, you can see where the retina is, that’s where the photo
receptors are the cells that give a response to the radiation. Of course the
light comes through the pupil and it, the image does flip over and it hits
the retina; it’s upside down, but don’t worry, your brain will prepare that
as it gets back to the occipital cortex. The two chambers of fluid; the
aqueous humor and the vitreous humor, the lens has some flexibility to it
because it has to change shape because objects in front of us are closer to
us or farther away, and so the lens has to accommodate and that’s an
important term, has to accommodate to the distance of objects from us. It
does that by thinning out a little bit as objects are distant from us and
thickening a bit as objects get closer and you’ll see at the bottom and top
of the lens in this figure, the huge cross section, so remember in fact it
makes, this diagram makes it appear as if there are muscles at the top of
the lens and muscles at the bottom of the lens and that those muscles sort
of tug on the lens to make it thinner. Actually because it’s a cross
section, what it doesn’t seem to capture is the fact that these muscles,
which are called ciliary muscles or ciliary body they actually surround the
lens, so they actually wrap around the lens, so actually what happens is
when those muscles are flexed, the lens gets fatter, it sort of squeezes
around the edges of the lens and pooches it out and makes it a little bit
thicker and that’s why as you look at objects, you feel the muscle straining
the eye, objects in the distance as you sort of stare out at the horizon,
your eyes feel very relaxed and that’s because there’s no tension. The
phobia just a little bit of an indentation, almost dead center in the
retina, is very rich in photo receptor cells, particularly cones, but it
doesn’t do anything, it’s not a structure that has it’s own function, it’s
just a geographic reference point. The signals all have to be gathered and
they all have to leave the eyeball and at that point there are no receptors
and that’s the blind spot. Now the brain does not make you experience that,
it says I’m just keeping terribly distracted, got these little blanks in
your visual field, so it’s just sort of fills in that blank spot,
perceptually, so it takes the blank spot and fills in with sort of the kind
of stuff that’s around the blank spot, the blind spot. And then of course
the optic nerve leaves the orbit.
If you go to slide seven, first of all you’ll see on the figure, figure 5.3,
that’s an actual photograph of rods and cones; these are the receptors in
the retina and you can see that the rods kind of look like rods and the
cones kind of look like cones, so that works out nicely. The light, again,
your textbook will always; and I sometimes do to, we tend to refer to the
radiation as light because it will become light, but really what’s more
technically correct is the radiation hits the retina and the light is
produced later, but sometimes as a shorthand we will say as the light hits
the retina and that’s fine. But rods and cones are receptive to the wave
lengths of radiation that we showed you on previous slides, so rods; there
are many more rods than cones. The cones are located more towards the
phobia, toward the center of the retina and gets less frequent as you move
toward the periphery. The rods are more frequent for the periphery and less
frequent for the phobia or the center of the retina. The rods work pretty
well in low levels of intensity, that is when it’s kind of dark. Cones need
a fair amount of intensity, it needs to have a fair amount of light, in
terms of intensity for the cones to work. The rods will not pick up wave
length information, the rods will not give you any color information, the
cones do. The cones also can pick up finer detail, so we have essentially
two different systems on the retina. This rod system works pretty well when
the levels of illumination are low and this cone system which requires a
fair amount of illumination to work, but will give you finer detail and also
color information. How they do that. Well surrounding the cones and rods and
inside there are these pigments, these are chemicals that respond to the
radiation. There are things like this in life, in other aspects of life,
film for example, camera film is an example of chemicals that will react to
the radiation provided by the universe. Polaroid film in particular.
Remember that Polaroid cameras that you don’t see as many of these days as
you used to, but Polaroid cameras basically blank film and when it’s exposed
to the radiation, colors and figures and shapes emerge and the eye does
that. The difference is the photo pigments which bleach, that’s the term
that’s used when the radiation is present, this bleaching process occurs
over and over and over again very quickly.
Now if you go to slide eight, you will see this very strange shaped curve.
Look at the heavy line, it has this sort of scallop shape so it comes down a
little bit and then it seems to flatten out and then it comes down a little
bit farther. This is what people, this curve comes from people’s reports of
what they experience as they go from a lighted area to a darkened area, it’s
called dark adaptation and what you can see is two different kinds of
adaptation. You see cone adaptation occurs more quickly but it levels off,
that is your cones will adjust to dark more quickly but they cease adjusting
at a certain point and you only get so much of a return of experience. Rod
vision takes a little bit longer to adapt to darkness, but it adapts more
completely and so its really the second curve there and that’s what somebody
sort of notices is a single scallop curve is actually two curves, a cone
curve and a rod curve, but to you as a subjective experiencer, just seems
like well it seems to be getting, my eyes seem to be adjusting and then they
quit, getting better for a moment and then they start getting better again
and you go to a darkened theater, you may start to notice that. That just
making the point again to see if there are two different systems. You know
if we knew this before, people could sort of microscopically look at the two
different sets of receptors and discover that they actually look different
from each other. Somebody was able to produce these curves and say you know
there might be two different systems, you know, one system seems to adapt a
little bit more quickly but it doesn’t adapt as well, the cone system, and
the other takes a little bit longer to adapt to dark, but adapts a little
bit better and so again rod vision remember works better when the levels of
light are less and its not quite as bright.
If you go to slide nine, we’ll take a closer look at the retina and figure
5.5 shows a cross section. You can see the rods and cones. This is
interesting because it’s counter intuitive. We tend to think you wouldn’t
design a system like this but then it kind of emerges over time and what you
have is rods and cones in the back of the retina and so the radiation
filters through the other layers of cells to the rods and cones. The rods
and cones react to the radiation. The chemicals surrounding the rods and
cones bleach and then the part of the rod or the cone that is neuron changes
rate of firing and then that firing rate is sent forward, it is sent back so
if the light comes from left to right in figure 5.5, and that is the
radiation, travels through the other layers itself until it gets to the rods
and cones. The rods and cones are half neuron and half chemicals, photo
pigment chemicals and these photo pigment chemicals react to the radiation
then the half of the rod or the cone that is electrical, is neuron, changes
firing rate, that firing rate is sent forward, that is right to left in the
figure, goes through several layers of cells, the bipolar cells, the
ganglion cells and then there’s two kinds of cells; horizontal and amacrine
cells that actually pass not from right to left but up and down in that
figure, which is very useful because it helps produce things like lateral
inhibition, some color mixing, so for example if you look at one of the
rods, then if you look at the rods and cones on the right, you’ll see that
some will send signals directly to like a bipolar cell which will then send
it directly to the ganglion cells. But others will hit one of these sort of
lateral cells sort of these horizontal or amacrine cells and those will send
signals not toward the surface but from top to bottom and that’s very useful
because for example lets say if you are a rod and you want to send your
signal on, but you want to shut down the adjacent signals, what we have is a
very thin line that’s sitting right on top of you and for that line to be
experienced as something sharp and with acuity what you wanta do is not only
send your signal very clearly on the central nervous system, but you wanta
inhibit the signals that might spill over on your right and on your left and
so what you get is some lateral inhibition as its called or you get mixed in
the signal. Let me give you a sort of a metaphor. Let’s say you’re standing
on the stage with two friends and you wanta stand out from those two
friends, but one of the things you can do is take a curtain in each hand and
cover your two friends, so now only you can be seen by the audience and
that’s sort of what these horizontal and amacrine cells do that produce sort
of lateral effects. In the case of this figure, they produce message signals
from top to bottom that modify the major signal which is being sent from
right to left through the surface of the retina.
If you go to slide ten, you’ll see where the message travels next. Of course
it leaves the orbit through the optic nerve and then it splits into a very
complex kind of a split actually at the optic cyasm and you see that on the
next slide, actually but it, the message splits so as I mentioned I think in
the previous chapter, we talking about in biology, that the left visual
field of both eyes goes to the right hemisphere, the right visual field of
both eyes goes to the left hemisphere and I said then that we’d talk a
little bit about the wiring that makes that possible and now we’re gonna
talk a little bit about that wiring, but lets, and I’ll show you that in the
slide, but let me say that once this sort of splitting of the signal occurs,
the message goes back to the occipital cortex, which is in the back of your
head, and from there messages travel to multiple places in the brain; some
are sent to the lateral nucleus in the superior colliculus and some of that
involves the perceptual experience, some of it involves the motor control of
the eye itself because whatever you see stimulates the eyes to move, either
stay still or move, so for example as you look at an object that’s moving,
the eyes have to move with that object and so the perception itself is
partly guiding the eye movement and so you have to send some signal, not
only to the processing areas of the image but also to the motor areas of the
brain to control eye movement. So some will go against the frontal lobe to
the, and some to the parietal lobe, you get multiple messages being sent
because once the visual image is constructed, they have to be coordinated
with other things that you know in the memory system and other aspects of
input that you may be receiving from other sensors. There are featured
detectors on the occipital cortex that are very interesting and again you’ll
see a slide on that. There seems to be receptors, for example, or receiving
sites in the occipital cortex that are sensitive to very basic shapes. There
are some that for example move, respond to a stationary object, others that
respond to that same shape stationary object or same shaped object but only
when it’s moving and not stationary, such receptors that respond to curves,
some that only respond to curves when they’re moving, some that respond to
pinpoint, some that respond only when those pinpoints are moving but not
when they’re stationary and so on and that you’ve got receptor areas or
receiving areas in the occipital cortex that are sensitive to very basic
shapes, you have the beginning of perception, the beginning of simple shapes
that can be then converted to more complex shapes and configurations.
Let’s go to slide eleven and figure 5.6 and you can see that cross over, so
look at the left visual field and the right visual field and you can see how
the left visual field of both eyes, if you follow the pathways, you can see
the left visual field of both eyes will end up in the right hemisphere and
the right visual field of both eyes will end up in the left hemisphere and
you can see the optic cyasm of where the signal from both eyes sort of cross
over and go to the opposite hemisphere. So it’s a very complex system, it’s
not as simple as the left eye goes to the right side and the right eye goes
to the left side, that would not allow something like stereovision, it’s
only this kind of cross over that allows the eyes to work in tandem as a
stereovision system.
Go to slide twelve and you’ll see figure 5.7, you’ll see this poor little
monkey is actually having future detectors stimulated and there are ways,
actually the original experiments were done with cats, but what you do is
move simple shapes in front of a critter or in front of a human and you’ll
find you can actually measure the electrical activity of the receiving area
beyond the occipital cortex and you can look at, in this case it shows a
vertical line that might move, that might when it’s not present, some
receptors will fire, when it is present they’ll stop firing but some other
set of receptors will fire and if you actually change the orientation of
that line, say horizontal, then suddenly you’ll find that neither of the two
previous receptors will fire but the third set will fire, but then some
fourth set responds only when it’s diagonal, but won’t respond in any other
configuration and maybe a fifth set that only responds if its moving in one
of those previous configurations and so you get movement information, you
get shape information, you have the building blocks for more complex
perceptions.
If you go to slide thirteen, there’s just a little bit of a summary of what
I just said, but look at what a face is, a face is basically a configuration
of various shapes and simple shapes that are put together in a complex way
and so you can get the beginning of facial recognition, which is a good
example of more complex kinds of recognition. It turns out, however, that
it’s also a case of faces who have their own particular system in addition
to the feature detectors that are picking up the basics of the face, it
turns out that there may also be built into us a sensitivity to facial
information which is very useful, I think of course that would be something
that you would design into a system like a human processing system because
we get so much information from the faces of each other and of course we are
social beings, so it does make sense to have a dedicated area of the brain
that simply gathers a lot of the complex feature information and sends it to
a particular configurations that look like faces.
If you go to slide fourteen, we’ll talk a little bit about color vision. In
our history, the history of psychology, history of physiology, there have
been multiple theories about how the visual system works and how in
particular the color vision works. People know a little bit about color and
always have. Artists have been using colors for millennia and as such some
of our early theories about how we process color actually are biased a
little bit by artists hues and what its like to mix paint and so on, it
turns out that the visual system actually works more like mixing lights,
it’s more of an additive system than a subtractive system, but in trying to
imagine how the color mixing system works, we’ve had a couple of different
theoretical approaches in particular a man named Hermann von Helmholtz
suggested that we have by then in sort of the late 1800s, middle to the end
of the 1800s, he suggested that what sort of knew that we had cones by then,
people had an understanding that there might be a cone system but he
suggested maybe we have three cone types, you know, blue, green and red, and
that these cones respond and then you get; the red cones respond to red and
the green cones respond to green and so on and then that information sort of
mixed somewhere in the back of the central nervous system. An intellectual
rival of his at the time, Herring, suggested that that couldn’t possibly be
true because how do you explain things like negative after-images, which for
example if you stare at red for a long period of time, then you look away at
a white surface, you’ll see a little bit of green. If you stare at blue a
long time, you look away and you’ll see a little bit of yellow and if you
stare at black for a long time, you’ll see white. If you stare at white for
a long time and you look at a dark surface, it’ll appear to be dark. It’ll
appear to be dark, I’m sorry, if you stare at, for example if you get a
quick glimpse of a bright light, you’ll notice that you get a dark spot in
your visual field for a while. And so he said the actually what you have is
cone types that actually are responsive to two different wave lengths or two
different colors, so a red/green cone and a blue/yellow cone and then a
black/white cone to pick up texture information. Well it turns out they’re
both right and they’re both wrong. It’s a little more complex than that.
There are three types of cones in the retina, but they respond maximally to
different ranges of wave length, so you’ve got, for example, one set of
cones that respond very quickly and remember what did not know is that the
information in the firing rates of neurons, not in whether they fire or not,
but how fast they’re firing and so what you have is a cone for example that
responds most rapidly to pure blue, but if something becomes a little less
blue, if they respond a little bit less. But then a green type, another type
cone response primarily to the green wave length and close to green as we
get; and another one primarily the red a little bit farther from red and so
on, so you do have three cone types as Helmholtz suggested but think of all
three cone types responding to every wave length just at different rates and
so you get a very complex set of numbers, three numbers that could vary
essentially infinitely for every wave length that you process and so what
you’re getting is a code and that’s what’s being sent to the color
processing area of the occipital cortex. It takes the coded information a
wave length and then produces the color experience for you in your head and
color blindness for example, is usually because one of the cone types is
missing or impaired and so I won’t go into the complexity of the wiring, but
blue, red yellow, I mean red green color blindness is the most common of the
color blindness, blue yellow is the less common and there are people out
there who are achromatic, who only get the black white information that is
essential gray.
There is, if you go to slide fifteen, you know, if Herring was right, too,
there is an opponent process, that is if he was right, if you do get a
bright flash of light in your eye, there will be a dark sort of after effect
from that. If you do stare at green, you will get a reddish after effect and
so we, you do have to explain that, having decided Helmholtz is right about
the cone types or very close to right and what we’ve decided is that a lot
of this sort of motor processing takes place in once the signals left the
retina or maybe it begins as early as the amacrine and sort of horizontal
cells. Maybe it begins at the retinal level but the actual motor processing
occurs on the way back to the occipital cortex.
If you look at, and you see a little bit more on slide fifteen, but if
you’ll jump over to slide sixteen, you’ll see this strange young person on
figure 5.8 and again this is something you can do on your own, but you just
stare at her for a while and give yourself quite a few seconds of staring at
her, then look away to a white surface. You’ll see that, you’ll get an after
image of her that will make the point about negative after images and I’ll
let you come back and do that because I wanta go on to slide number
seventeen and on slide number seventeen you’ll see another figure and again
stare at the little white center of this figure, if you stare at it and
stare at it and stare at it for quite a long time and then again look away
to a white surface and what you’ll see is these colors should produce after
images that are actually closer to real colors of the flag.
I’ll leave that for you to do on your own time because I wanta shift over to
slide number eighteen and figure 5.10, you’ll see some samples of Ishihara
plates and Ishihara plates are used to test color blindness and what you
should be able to do if you have good color vision, is in three of those
three plates, you should see a number and that number is, I won’t go into it
because I want you to feel like you’re testing yourself, but if you only see
it in 2, then you might wanta have your vision checked for color blindness.
If you only see it in 1, you certainly wanta get your; if you see no numbers
at all, you really do wanta get your eyes checked for color blindness. If
you actually do see a number in panel 4, I’d be very surprised because there
is no number there. That’s to pick up what are sometimes called false
positive responses and so it’s a control plate, designed to see whether or
not somebody’s picked you will see numbers, no matter what you show them,
but you should only see a number in 3 of the 4 panels.
