Learners will be introduced to the problems that vision faces, using perception as a guide. The course will consider how what we see is generated by the visual system, what the central problem for vision is, and what visual perception indicates about how the brain works. The evidence will be drawn from neuroscience, psychology, the history of vision science and what philosophy has contributed. Although the discussions will be informed by visual system anatomy and physiology, the focus is on perception. We see the physical world in a strange way, and goal is to understand why.
Seeing Object Size
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From the course by Duke University
Visual Perception and the Brain
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From the lesson
Seeing Space
Meet the Instructors
- Dale PurvesM.D.
Duke Institute for Brain Sciences
So we've talked now about line lengths and their perception and
angles and their perception and the discrepancy that exists between
the physical measurements of lines and angles and the way we see them.
What we haven't talked about yet is object size.
So certainly object size is related to intervals, line lengths and angles.
They're all in some sense about size.
But, considering size in its own right has been a long standing
domain of psychology and psychophysics and there
are a whole bunch of different phenomena that I want to describe to you now.
And show how the same explanation that we've been using to
get around the inverse problem with respect to line lengths and
angles applies to single sizes of objects as well.
So, let's first look at
the classical size contrast effects as they're called.
And I've represented seven of them here, A through G.
And all of these have names, I'm not going to belabor you with the names of
these, although we will talk in more detail about one of them.
Let me explain what's going on here.
So in each case what people are asked to do,
what you as the observer is asked to do or
a psychophysical subject is to compare the perception
of the light gray circle In two different context.
In this context versus this context.
In this context versus this context and so on for
the rest of the seven that are illustrated here.
And in each of these cases, the two central circles
in light gray are physically exactly the same.
The phonology that people have been interested over the decades.
And again these examples go back to the 19th century,
the names that were attached to them are generally names from long ago.
In each case the light gray circles are seen as somewhat different,
despite their physical identity.
Again, this is now sort of a familiar sequence.
So look, for example, at this first one,
which we're going to talk about in more detail.
And compare your perception of the size of this circle to this circle on the right.
They look a little bit different.
The circle on the left looks a little bit larger than the circle on the right.
And in each of these different contexts,
the size of the two circles look a little bit different.
Sometimes the one on the left looks larger than the one on the right,
sometimes the one on the right looks larger than the left.
In every case the context that's being provided by the surrounding
circles or circle is changing your perception.
Of the size relationship between two identical circles,
the light gray circles in the center.
So, again, one sort of has to ask what's going on, and
one issue is, well, these aren't very big effects.
Maybe we shouldn't worry about them too much.
And as in the case that I've shown you before,
luminance and color, line lengths and so on.
They can actually be very big discrepancies and often are.
So let me show you an example that I [COUGH] invite you to
take to look at in more detail and play with yourselves.
This is on the purveslab.net website that I've referred to a bunch of times.
And it allows you to interactively play with this difference
in apparent size of two figures that are identical.
And I think here you'll be amazed, when you go to the website and
do this, to see how remarkably different the appearance of size of these
two gold figures can be as a function of their placement in the context.
Whether this figure is seen in the foreground or
the same figure in the background.
So let me make sure you understand what's going on here.
This figure is exactly the same size as
this figure even though they look very different in size.
And when you move this figure to the background as I'm drawing here,
you'll see when you interactively do this.
Emphasize changes.
And the difference is not trivial.
It's about 20%.
Depending on the context and the setup that you use,
the differences in the apparent size of objects,
the discrepancy between the real measured size of this figure.
And you can take a run, show yourself right away in this picture that these two
figures are the same physical size even though they look different,
this one looking much smaller than this one.
So I invite you to go and play with that, move the figures back and forth,
test how much the discrepancy is for you.
It will be on the outer 15 or 20% I think.
And convince yourself that these are not trivial effects.
These examples that I've shown you here are not trivial effects,
they can be very large and as before in the case of luminance and color and
line lengths and angles, they demand some kind of explanation.
Why are we seeing these different gray circles for example?
We're going to go figure that our identical,
why are we seeing them as different sizes in different contexts?
Sometimes bigger, sometimes smaller.
There's a whole raft of phenomenology here that demands some kind of explanation.
And each one of these examples is a challenge to anybody who
wants to say well, yeah I understand why we see sizes as being different.
These are very subtle and
can be compelling, but they are subtle in this case.
And they need some kind of explanation and the question is what should that be.
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