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[MUSIC]. 

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Welcome to our second lecture on visual 
perception. 

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In this lecture we'll be discussing a 
very intriguing property of the visual 

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system, preattentive processing. 
I'd like you to look at this slide and 

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tell me how many five's there are in this 
array of numbers. 

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Do you have the answer? 
Alright, Id like you think about how long 

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it took you to find them. 
And what techniques you used to detect 

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these fives? 
And now I'm going to show you the same 

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array with one small change. 
Now how many fives? 

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All fives are red by the way. 
Notice how easy it was to come to the 

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answer? 
The color made the fives pop out at you. 

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Psychologists have given this property a 
name. 

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Your immediate detection of the red 5's 
is called preattentive. 

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It turns out that certain basic visual 
properties are detected immediately by 

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the low level visual system. 
It means they pop out at you rather than 

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requiring a visual search through all the 
elements. 

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In general if the detection of a target 
takes less than about 200 to 250 

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milliseconds, it qualifies as 
preattentive. 

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The reason is because eye movements take 
at least 200 milliseconds and yet we find 

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that certain processing can be done very 
quickly and this implies low level 

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processing in parallel before the eyes 
even have a chance to move. 

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In general, if a decision takes a fixed 
amount of time, regardless of the number 

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of distractors, it is is considered to be 
preattentive. 

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Now only a limited set of properties can 
be perceived preattentively, that means 

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without the need for focusing attention. 
And you can see why this would be 

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important for designing visualizations. 
What can be perceived immediately? 

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What properties are good discriminators? 
What can mislead viewers? 

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How can you use the ability to process 
information preattentively to strengthen 

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your visualization? 
Let's talk about a couple examples. 

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So looking at these two squares In which 
one is there a red circle, left or right? 

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Pretty obvious right, you could instantly 
see that the red circle is visible in 

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that group of blue circles, it jumps out 
at you. 

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It seems almost ridiculous to think that 
there might be a question. 

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Detection of the red circle in a sea of 
blue circles is preattentive. 

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Now, which square contains the red 
circle, left or right? 

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The form, the curvature is also 
preattentive. 

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The curved circle pops out at you. 
However, you may have noticed that it 

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doesn't seem quite as immediately obvious 
as the different color. 

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Preattentive processing lies on the 
spectrum just like many other such visual 

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features. 
Now, which square contains the red 

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circle? 
Left or right? 

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So that one required serial search. 
So this is interesting. 

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So even though form is pre-attentive and 
color is pre-attentive the conjunction of 

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the two is not. 
It's so the red circle and the sea of red 

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square and blue circle distractors could 
not be perceived pre-attentively. 

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This can be used in practice to save 
lives. 

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Detection of a slanted line in a sea of 
vertical lines is preattentive. 

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In this case, the slanted line in a 
number of cockpit dials is used to 

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indicate systems failure. 
And I'm going to conclude with a list of 

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preattentive visual properties for you to 
consider for more detail. 

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Please see Christopher Healey's article 
on the web, the link is given in the 

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lower right hand corner. 
I hope that you found this lecture 

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interesting and that you're able to use 
this information while you're designing 

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your own visualizations.