[MUSIC]. 
Welcome to our second lecture on visual 
perception. 
In this lecture we'll be discussing a 
very intriguing property of the visual 
system, preattentive processing. 
I'd like you to look at this slide and 
tell me how many five's there are in this 
array of numbers. 
Do you have the answer? 
Alright, Id like you think about how long 
it took you to find them. 
And what techniques you used to detect 
these fives? 
And now I'm going to show you the same 
array with one small change. 
Now how many fives? 
All fives are red by the way. 
Notice how easy it was to come to the 
answer? 
The color made the fives pop out at you. 
Psychologists have given this property a 
name. 
Your immediate detection of the red 5's 
is called preattentive. 
It turns out that certain basic visual 
properties are detected immediately by 
the low level visual system. 
It means they pop out at you rather than 
requiring a visual search through all the 
elements. 
In general if the detection of a target 
takes less than about 200 to 250 
milliseconds, it qualifies as 
preattentive. 
The reason is because eye movements take 
at least 200 milliseconds and yet we find 
that certain processing can be done very 
quickly and this implies low level 
processing in parallel before the eyes 
even have a chance to move. 
In general, if a decision takes a fixed 
amount of time, regardless of the number 
of distractors, it is is considered to be 
preattentive. 
Now only a limited set of properties can 
be perceived preattentively, that means 
without the need for focusing attention. 
And you can see why this would be 
important for designing visualizations. 
What can be perceived immediately? 
What properties are good discriminators? 
What can mislead viewers? 
How can you use the ability to process 
information preattentively to strengthen 
your visualization? 
Let's talk about a couple examples. 
So looking at these two squares In which 
one is there a red circle, left or right? 
Pretty obvious right, you could instantly 
see that the red circle is visible in 
that group of blue circles, it jumps out 
at you. 
It seems almost ridiculous to think that 
there might be a question. 
Detection of the red circle in a sea of 
blue circles is preattentive. 
Now, which square contains the red 
circle, left or right? 
The form, the curvature is also 
preattentive. 
The curved circle pops out at you. 
However, you may have noticed that it 
doesn't seem quite as immediately obvious 
as the different color. 
Preattentive processing lies on the 
spectrum just like many other such visual 
features. 
Now, which square contains the red 
circle? 
Left or right? 
So that one required serial search. 
So this is interesting. 
So even though form is pre-attentive and 
color is pre-attentive the conjunction of 
the two is not. 
It's so the red circle and the sea of red 
square and blue circle distractors could 
not be perceived pre-attentively. 
This can be used in practice to save 
lives. 
Detection of a slanted line in a sea of 
vertical lines is preattentive. 
In this case, the slanted line in a 
number of cockpit dials is used to 
indicate systems failure. 
And I'm going to conclude with a list of 
preattentive visual properties for you to 
consider for more detail. 
Please see Christopher Healey's article 
on the web, the link is given in the 
lower right hand corner. 
I hope that you found this lecture 
interesting and that you're able to use 
this information while you're designing 
your own visualizations.