hi I'm Michael courier and this is psych
exam review in this video I want to
explain how we're able to perceive death
so we see a world that appears to be
three-dimensional and yet our retinas
are flat surfaces that light is
projected onto so they can really only
see in 2d so how is it that we're able
to feel like the world is in 3d when we
look around alright so these are a
number of different cues that our brain
uses to figure out how much depth there
is in particular situations and how far
away things are from us so the first
group of cues that we'll talk about are
called monocular cues and that just
means one eye so these are cues that
work even if you only use one eye I'll
see you later there's other cues that
involve the use of both eyes so what are
these maanikya ler cues well the first
we have is linear perspective and this
is the idea that parallel lines converge
as they travel away from us into the
distance right so if you look at you
know train tracks appear to join
together in the distance even though of
course they don't actually join together
they stay parallel next we have relative
size now this is the idea that things
that are closer to us appear to be
bigger so if we look at two people if we
look down the sidewalk and we see one
person that the image on the right on
the retina is much larger than the other
person we assume that people are
probably roughly the same size but the
one person is much closer to us than the
other one
next we have texture grading this is the
idea that we can see things more clearly
when they're close to us we can see
textures so if you're looking out and
you can see lots of detail on something
very clearly and crisply that tells you
that it's probably close to you it's not
really far off in the distance because
if it were you wouldn't be able to see
it that clearly next we have
interposition and this is just the idea
that if something blocks the view of
another object then it's probably closer
to you like have my hand here and you
can't see my face anymore well then that
tells you that my hand is closer to you
than my face otherwise it wouldn't block
it and lastly we have the idea of
shading this is a idea that we use the
way that shadows fall in order to tell
us about how close things are to us and
you use this every time
you walk up a flight of stairs or even
just as you walk and you see the slight
shadow cast by a sidewalk you can sort
of automatically calculate without even
thinking about how big of a step you
have to take in order to get up that
curb okay so I thought I would draw a
picture here and try to use each of
these different cues and see how they
would work in an actual picture now this
is obviously is a flat two-dimensional
picture but hopefully it will have a
some sense of depth now I'm not a great
artist so it probably won't be amazing
but we'll get an idea of each of the
cues sort of in practice so let's start
with linear perspective and we'll start
with this simple idea that I mentioned
here all right so here's sort of a
horizon here and then we'll just have
some train tracks which sort of converge
off in the distance right and this
immediately gives us a sense of depth to
our picture we feel like this part of
the picture is farther away than this
part now we'll expand on that by drawing
some people here so here's one person
here and here's another person here now
using this idea of a relative size we
see that this person feels closer to us
right it could be the case that this is
a tiny person floating in the air here
next to this person but that's not
probably how you're gonna perceive it
you're gonna just assume that this
person is about the same size as this
person just farther away and therefore
the image on your retina is much smaller
all right now let's add some texture
gradient so let's imagine that we're
looking at you know some little tree
here or something or actually let's say
rather than a tree let's use sorry
let's do a little plant here be a little
easier for me to draw so if we imagine
you know some plant here that's close to
us right we can see all of the details
we can see individual leaves on there
maybe right we can see the veins on the
leaves I'm not able to draw this in
great amount of detail whereas you know
the plant back here is just kind of a
little bit less detailed and you know
the one over here is just like a green
spot right we can't really see much
detail at all now I've also added some
relative size here but we can we can
imagine that even if this one was a much
larger plant right so it's similar in
size to the other one but we can't see
as much detail and that tells us that
it's farther away whereas this one we
can see all of the fine detail all right
so that's a texture gradient now will
add some interposition here so let's
just have some you know sign here or
something in front of this guy's face
you know
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okay so when we look at this you know
we're going to assume that this sign is
closer to us because it's blocking our
view of this guy here and so that tells
us something about the depth and finally
we'll add some shading so I'll put a
little shading into this picture and
this is just to give you an idea of
where the light is coming from now let's
draw two similar objects here okay now
I'm going to shade these differently
okay so what I'm trying to express here
and this is probably the hardest thing
to try to draw in this is if you were
looking at these objects you hopefully
get the sense that this one is a hole
right this is an indentation right
because of the way the shadow is falling
on this part of it here right we don't
need you see the shadows on tops of
things so that tells you that there's
some depth going down here whereas this
one where this shadow is on the bottom
here tells you that you know this is a
rock or something that you could trip
over that this you know is something
that's sort of sticking out in the world
whereas this is something that's
sticking in all right so that's one way
that we use shading and again as I said
you use this type of thing when you look
at it a flight of stairs right there the
way that the shadows fall on the steps
tell you which part is the step which
part is a sort of vertical part which
part is a horizontal part you can step
on and you do that again unconsciously
without even thinking about it you just
see the stairs and you immediately know
how to walk up them okay so those are
all of the monocular cues there's a
couple other cues actually these next
two are also manaat' hiller but these
involve motion these are motion based
cubes
so there's two main ways that we use
motion to tell about depth the first of
these is something called motion
parallax and the idea of motion parallax
is that things appear to move at
different speeds based on how far away
from us they are so sort of a great way
to demonstrate this is just to imagine
that you're looking out you know a train
window here let's go ahead and give
ourselves this same white background
here and so I'm looking out this is a
train window and as I'm looking out the
window here I'll see that you know that
the tree that's really close to the
train tracks here right the trees are
gonna whip by the window really quickly
right so they're gonna travel a large
distance in a short amount of time
right yeah they're whipping by but
things that are farther away so now in
the background here we'll draw some
mountains or something so the mountains
back here in the background they're
moving by the window but they move much
more slowly right in the same amount of
time of that tree whoops by the mountain
barely moves and then you know the the
Sun in the sky up here you know I mean
it's barely even perceptible movement
over that same amount of time right so
the Sun is going to be sort of making
its way across the window over the
course of like you know an hour or two
or something whereas you know the trees
are whipping by you know every second
and the mountains are there for maybe
half an hour and eventually you know you
sort of pass by them so that's the idea
of motion parallax that we can judge the
distance from us based on how quickly
something appears to move past our field
of vision and the second motion based
cue that we have is called optic flow
and the idea of optic flow is that the
way that things move on our retina tells
us about their their distance from us so
if I throw a ball to you the ball is you
know initially small so it's going to
incorporate relative size as well but as
it moves towards you it's expanding in
all directions equally right it's
growing and at a uniform rate in all
directions as it's moving closer to you
and it's the sort of rate of this growth
the way that it moves on your retina
that the way that it expands on your
retina tells you about how quickly it's
approaching you and how far away it is
all right so when it's very small and
far away you know it's not growing as
much and as it gets closer you know as
it's about to hit you in the face here
it's getting very large very quickly and
that's going to tell you about the depth
all right so those are our motion based
cues and the last cues that we have are
the ones that involve the fact that we
have two eyes so we say these are
binocular cues right to us by an ocular
all right so there's two real main
binocular cues that we use the first of
these is disparity this is called
binocular disparity or it can also be
called retinal disparity and this is the
idea that we see two versions of the
world and then we combine them into one
right because we have two eyes so we've
seen two different views of the world
slightly different angles and so the way
that these two views differ helps us to
judge depth so how can you demonstrate
this well if you hold an object really
close to your face right and you view it
with just your left eye and then just
your right eye you see that it sort of
jumps back and forth it's it's on the
far right side of your visual field for
your left eye but on your right eye it's
on the far left side of visual field
it's in very different places and that
tells you that it's very close to you
whereas if you hold it at arm's length
and you do the same thing it moves a
little bit when you switch eyes but not
that much and if you put it on the other
side of the room you look at something
that's very far away and you switch eyes
you see that the object doesn't really
move at all
and that tells you that it's farther
away so the more different these views
are that tells us that the objects are
closer to us and this is how 3d movies
work they present two versions of the
film to us one to each eye and then they
sort of manipulate how different certain
objects are in those two views and that
gives them this appearance of death now
in the old days they did this with red
and blue colored lenses so the red would
filter out one version of the movie the
blue would filter out another version
the screen would be shown both and
that's why if you looked at it without
glasses it looks weird and the colors
are kind of off and everything's a
little fuzzy that's because to overlap
two versions of the movie on the screen
and the glasses separate them so that
each eye only sees one version and then
your brain combines those into a
apparently 3d version of the film and
this is the same way that modern 3d
movies work as well they don't use the
colored lenses anymore that use
polarized lenses now to separate the two
versions of the film but it's the same
principle we have to send two different
versions of the movie one to each eye so
if you only have one eye there's no way
that you can enjoy those kinds of 3d
movies okay and the last cue that we
have is called convergence and so this
is the idea that we have two eyes and
that we have to move them differently
depending on whether something is close
to us or far away from us so what do I
mean by that well let's imagine these
are my two eyes here and if I'm looking
at something that's really close to me I
have to point my eyes at this very
narrow angle I have to actually move the
muscles around my eyes and sort of point
the mean like this right to look at my
finger and so that tells me that the
thing I'm looking at if I have to angle
my eyes that way it's very close to me
whereas if I'm looking at the same
object but my eyes are pointing mostly
straight ahead you know the angle is
different the point where they converge
means that they're at different angles
that tells me that the object I'm
looking at is farther away from me all
right so that's a final cue that we use
to help judge depth
okay so I hope you found this helpful if
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watching