In order to count the 3s in the table of digits it is only necessary
to scan the red ones. This is because color is pre-attentively
processed.
The typical experiment that is conducted to find out if something
is pre-attentively processed involves measuring the response time
to find a target in a set of "distractors". For example
finding the 3s in set of other numbers. If processing is pre-attentive,
the time taken to find the target should be indepenedent of the
number of distractors. Thus, if time to find the target is plotted
against number of distractors the result should be a horizontal
line.
Some aspects of shape are also processed pre-attentively, but
not all, otherwise we could find the 3s in the table at the top.
The 0s can be pre-attentively discriminated from the vertical
bars.
|| ||| | 0 | |
| || || 0 || |
|| 0 || | || |
| | || | |
Most conjunction searches are not pre-attentive.
Thus if the task is to find red|s where the
distractors can consist of black |s and red
0s then the processing time does depend
on the number of distractors. We cannot see combinations of features
pre-attentively, only simple features.
Note that this gives us a way of defining what perceptually simple
features are.
In displaying information it is often useful to be able to show
things "at a glance" which is why pre-attentive processing
is so useful. If you want people to be able to identify some mark
on a map as being of type 'x', instantaneously then it should
be differentiated from all other marks in a pre-attentive way.