Bottom-up vs. Top Down Processing Video Lecture | Compiler Design - Computer Science Engineering (CSE)

Voiceover: Let's look at
the difference between
top-down and bottom-up processing.
So, what is bottom-up processing?
Bottom-up processing basically
begins with the stimulus,
so let's imagine that
we're looking at something,
or let's say I'm looking at a banana.
The banana is sitting there and
it influences what we perceive.
So, stimulus influences what
we perceive, our perception.
So, if I know absolutely
nothing about something,
then the stimulus or whatever
it is I'm looking at,
yet I don't know anything
about, I've never seen it,
I don't have any preconceived
cognitive constructs
about what it is I'm looking at.
The stimulus basically is
influencing my perception.
So, for example let's imagine that
I'm looking at a cockpit of a plane.
I'm not a pilot, so I'm not really
too familiar with everything and
everything kinda looks fairly confusing.
So, basically all the different stimuli,
so this stimulus, a bunch of gauges,
and this rudder-looking thing,
I'm basically looking at all the different
little parts of something
that is new and novel to me,
and trying to kind of comprehend
what it is I'm looking at.
So, this is bottom-up.
This is when you start
with no preconceived idea
of what it is that you're looking at,
and allow the stimulus to influence
your perception of what it
is that you're looking at.
So, bottom-up processing is data-driven,
and your perception of what
it is that you're looking at
directs your cognitive
awareness of the object.
So, in contrast, top-down processing
basically uses your background knowledge,
so uses your background knowledge
to influence perception.
So, let's look at this example over here.
So, what we're actually
seeing are a bunch of circles,
they are just a bunch of circles and then
inside the circle there are
a couple of lines drawn.
So, we are looking at this set of circles,
these white circles with
lines drawn inside of them.
We are creating this cube.
We're basically taking
these lines and then
putting them together in
order to create a cube.
Even though the stimulus itself,
which is the circles with the lines,
actually doesn't draw a cube because
there are these black spaces over here,
and there is absolutely
nothing in the black spaces,
but our brains are basically
taking this information
and using our knowledge of cubes
and what they're supposed to look like,
we're recreating a cube despite a lack of
a cube actually being
present in the image.
So, that's top-down processing.
It's using your background information,
your background knowledge, your learning,
your expectations, in order to influence
what it is that we're perceiving.
So, in other words, it's theory-driven.
We look at this and we assume that
they're trying to represent a cube,
even though one's not
actually drawn there,
and we're using that theory in order to
shape our cognitive understanding
of what it is that we're looking at.
So, our perception, our behavior
is influenced by our expectations,
which is top-down processing.
So, we're using what's
already in our heads
in order to perceive what
it is that we're looking at,
whereas in bottom-up
processing we're using
the stimulus itself in order
to drive our perception.
So, another good example of top-down
processing would be "Where's Waldo?"
So, in "Where's Waldo?", we have a
mental idea of what we're trying to do,
which is to find Waldo amidst this
really jumbled mess of a picture.
So, if we were using bottom-up processing
in order to look at this we would just
be seeing a whole bunch of little people,
and we wouldn't really be goal-driven,
we wouldn't be trying to do anything,
but with top-down
processing we have a goal,
and we're able to look
through here to find Waldo.