Most computer users have learned that running too many programs at the same time can slow down or even crash the machine. We work around these limitations by closing programs when we aren't using them.
Just like computers, human brains have a limited amount of processing power (as further discussed in our course on The Human Mind and Usability). When the amount of information coming in exceeds our ability to handle it, our performance suffers. We may take longer to understand information, miss important details, or even get overwhelmed and abandon the task.
In the field of user experience, we use the following definition: the cognitive load imposed by a user interface is the amount of mental resources that is required to operate the system. Informally, you can think of mental resources as "brain power" — more formally, we're talking about slots in working memory.
The term cognitive load was originally coined by psychologists to describe the mental effort required to learn new information. Though web browsing is a much more casual activity than formal education, cognitive load is still important: users must 'learn' how to use a site's navigation, layout, and transactional forms. And even when the site is fairly familiar, users must still carry around the information that is relevant to their goal. For instance, when planning a vacation, the users’ cognitive load includes interface-related knowledge and specific vacation-related constraints that they may have (such as price and timeframe).
When a computer can't handle our processing demands, we can simply upgrade to a newer, more powerful machine. But to date there's no way to increase the actual processing power of our brains. Instead, designers must understand and accommodate these limits.
Intrinsic vs. Extraneous Cognitive Load
There's no way to eliminate cognitive load entirely—in fact, even if this was possible, it wouldn't be desirable. After all, people visit websites to get information. They've come to find out something about your product, organization, or content; most likely it's something they didn't already know. Intrinsic cognitive load is the effort of absorbing that new information and of keeping track of their own goals.
Designers should, however, strive to eliminate, or at least minimize, extraneous cognitive load: processing that takes up mental resources, but doesn't actually help users understand the content (for example, different font styles that don’t convey any unique meaning).
Minimizing Cognitive Load
User attention is a precious resource, and should be allocated accordingly. Many of our top usability guidelines—from chunking content to optimizing response times—are aimed at minimizing cognitive load. In addition to these basics, there are 3 more tips for minimizing cognitive load:
Avoid visual clutter: redundant links, irrelevant images, and meaningless typography flourishes slow users down. (Note that meaningful links, images, and typography are valuable design elements; it is only when overused that these backfire and actually impair usability.)
Build on existing mental models: People already have mental models about how websites work, based on their past experiences visiting other sites. When you use labels and layouts that they've encountered on other websites, you reduce the amount of learning they need to do on your site.
Offload tasks: Look for anything in your design that requires users to read text, remember information, or make a decision. Then look for alternatives: can you show a picture, re-display previously entered information, or set a smart default? You won't be able to shift all tasks away from users, but every task you eliminate leaves more mental resources for the decisions that truly are essential.