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[MUSIC]. 
In the previous lectures, we've been 

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discussing data and visual mappings. 
Now we're going to talk about the other 

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end of the pipe line. 
How that information is processed by 

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human brain. 
This is the first of three lectures on 

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visual perception. 
70% of our body's sense receptors reside 

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in our eyes. 
Our very language resonates with the 

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importance of our eyes. 
Metaphors to describe understanding often 

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refer to vision. 
I see, insight or illumination. 

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Colin Ware stated in 2004. 
They eye and the visual cortex of the 

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brain form a massively parallel processor 
that provides the highest bandwidth 

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channel into human cognitive centers. 
In these three lectures we're going to 

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describe how it's important to understand 
how visual perception works in order to 

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effectively design visualization. 
The first key principle to keep in mind 

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is that the eye is not a camera. 
A better metaphor for vision was given by 

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Christopher Healey in 1995. 
It's a dynamic and on going construction 

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project. 
It's important to keep this in mind as 

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you go thru the next three lectures. 
Because when you go design a 

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visualization, if you think of the eye as 
a camera, it can lead you to some eno, 

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erroneous conclusions. 
Attention is selective. 

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We, we all know that. 
it's filtered to a high degree. 

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And it's really cognitive processes that 
transform the visual information into 

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something that we can comprehend. 
The field of psychophysics is concerned 

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with establishing quantitative 
relationships between physical 

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stimulation and perceptual events. 
So we're going to be talking about some 

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of the important discoveries made in this 
field in the next three lectures. 

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Let's think a little bit about how to use 
perceptual properties. 

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Information visualization should cause 
what is meaningful to stand out. 

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So what do you see in this image. 
Can you see the 2 spots that differ from 

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their surroundings? 
Why do you see them? 

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We're going to answer that question in 
the next few slides and lectures. 

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Some important differences between eyes 
and cameras. 

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Cameras have very good optics. 
They have a single focus, they maintain a 

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white balance and a single exposure. 
And they also do what's known as a full 

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image capture. 
In another words, all the photons coming 

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into the eye are captured with equal 
importance and at the same time. 

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The eyes on the other hand have 
relatively poor optics. 

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They're constantly scanning. 
This is known as saccades, the shifting 

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of your eyes to various positions There 
constantly adjusting focus, there 

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constantly adapting, both the white 
balance and the exposure. 

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What's really going on is mental 
reconstruction of the image, sort of, as 

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we'll find out it's not even a complete 
image thats reconstructed. 

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So here's one example. 
Visual perception really isn't just 

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camera work. 
Looking at this checker board it's pretty 

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clear that square A is darker than square 
B, right? 

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Turns out if you gray out the image, you 
see the truth. 

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Square A and square B have identical 
pixel values. 

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As a matter of fact when I first saw this 
image I didn't believe it. 

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I actually put the image in Photoshop and 
used the Eyedropper tool in order to find 

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out what the RGB values were of those two 
squares. 

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And yes indeed they are identical. 
So you're welcome to do the same you can 

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take a screen capture and test it out 
yourself. 

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But it's pretty interesting. 
The human visual system is so dependent 

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on context and upon reconstructing the 
image that we see these two squares, 

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which are identical as quite different. 
Color is relative as are many other of 

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our perceptions. 
If you look at this slide, doesn't it 

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look like the x on the left hand side is 
gray and the one on the right hand side 

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is yellow? 
But if you go look at were they connect 

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at the center bottom of the slide you'll 
see that they're identical. 

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And that's again because the eye 
perceives differences. 

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It does not perceive actual pixel values. 
So in conclusion I hope you've taken away 

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from this lecture that the eye is not a 
camera. 

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In the next 2 lectures, we're going to 
discuss more about how understanding how 

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the visual perception can help us develop 
more effective visualizations