|InfoVis.net>Magazine>message nº 179||Published 2006-02-12|
|También disponible en Español|
The digital magazine of InfoVis.net
It sometimes happens that certain elements of a graphic representation, maybe a colour or an icon "pop out". We detect them instantaneously.
These phenomena, whose visual identification is performed in a very short time lapse (typically between 200 and 250 miliseconds or less) are called pre-attentive since they occur without the intervention of consciousness. There's no need to focus on the search task. Even when they are hidden among many other objects they are identified immediately.
As a familiar example, the previous paragraphs and the following ones have some words in bold text. This is a way of highlighting text, quite typical of the web, where users have very little time to read long paragraphs in the screen, in an environment where you jump to another page very easily.
The goal of this common practice is providing the key ideas of the text. Since illumination is processed pre-attentively, our sight identifies the sentences in bold face first so that, when properly constructed, you can read the essentials of the text synthesized in a few lines.
In order to know whether a phenomenon is pre-attentive or not the procedure usually consists of measuring the time needed to identify the target in a set of other objects called "distractors". Pre-attentive processes are basically independent of the number of distractors placed. It doesn't matter how many of them are present, the time to identify the target is constant and typically below 1/4 of a second. On the contrary, if we get larger times when more distractors are added, the phenomenon is not pre-attentively processed.
According to Colin Ware in his book Information Visualization: Perception for Design the list of features that are pre-attentively processed can be grouped into four basic categories: Colour, Form, Movement and Spatial Localization. Let's look at them more in detail.
Both Hue (difference between elements) and intensity are processed pre-attentively. Some examples follow:
Again the best way to understand is by looking at some examples:
If we focus on what all these phenomena have in common we see that there's always a clear difference between the whole and the part. The brain is able to discriminate certain dfferences in a pre-conscious way. You don't need to focus your attention or be conscious of the meaning or implications for these phenomena to "pop out".
On the other hand the conjunction of several pre-attentive elements can reduce its effect and limit its pre-attentive processing. As the variety (not the quantity) of the distractors increases the search time can increase. In other words, the best case is when you have all the distractors equal. There are several theories that try to explain how pre-attentive processing works. You can consult, among others, the whole article Perception in Visualization by Christopher Healey
Needless to say that a proper use of the pre-attentive phenomena is fundamental for the generation of good user interfaces and graphics. Still today we can find many websites full of animated gifs that compete for our attention overloading our cognitive capabilities. Making this knowledge a good practice is our responsibility as professionals of information visualisation.
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