Understanding the Limits of Short-Term Memory

In truth, we don't see with our eyes; we see with our brains. Our eyes are the sensory mechanisms through which light enters and is translated by neurons into electrical impulses that are passed on to and around in our brains, but our brains are where perceptionthe process of making sense of what our eyes registeractually occurs.

Our eyes do not register everything that is visible in the world around us, but only what lies within their span of perception. Only a portion of what our eyes sense becomes an object of focus. Only through focus does what we see become more than a vague sense. Only a fraction of what we focus on becomes the object of attention or conscious thought. Finally, only a little bit of what we attend to gets stored away for future use. Without these limits and filters, perception would overwhelm our brains.

Our memories store information starting the moment we see something, continuing as we consciously process the information, and finally accumulating over years in a permanent (or nearly so) storage area where information remains ready for use if ever needed againthat is, until access to that information eventually begins to atrophy.

Memory comes in three fundamental types:

• Iconic memory (a.k.a. the visual sensory register)
• Short-term memory (a.k.a. working memory)
• Long-term memory

Iconic memory is a lot like the visual memory buffer of a computer: a place where images are briefly held until they can be moved to random access memory (RAM), where they reside while being processed by the CPU. Even though what goes on in iconic memory is preconscious, a certain type of processingknown as preattentive processing occurs nonetheless. Certain attributes of what we see are recognized during preattentive processing at an extraordinarily high speed, which results in certain things standing out and particular sets of objects being grouped together, all without conscious thought. Preattentive processing plays a powerful role in visual perception, and we can intentionally design our dashboards to take advantage of this if we understand a bit about it.

Short-term memory is where information resides during conscious processing. The most important things to know about short-term memory are:

• It is temporary.
• A portion of it is dedicated to visual information.
• It has a limited storage capacity.

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Note: Information remains in short-term memory from a few seconds to as long as a few hours if periodically rehearsed; then it is flushed. If rehearsed in a particular way, information is moved from short-term memory to long-term memory, where it is stored more permanently for later recall. When information is recalled from long-term memory, it is temporarily moved once again into short-term memory, where it is processed.

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We can store only three to nine chunks of visual information at a time in short-term memory. When its capacity is full, for something new to be brought into short-term memory, something that's already there must either be moved into long-term memory or simply removed altogether (that is, forgotten). What constitutes a "chunk" of visual information varies depending on the nature of the objects we are seeing, aspects of their design, and our familiarity with them. For instance, individual numbers on a dashboard are stored as discrete chunks, but a well-designed graphical pattern, such as the pattern formed by one or more lines in a line graph, can represent a great deal of information as a single chunk. This is one of the great advantages of graphs (when used appropriately and skillfully designed) over text. Dashboards should be designed in a way that supports optimal chunking together of information so that it can be perceived and understood most efficiently, in big visual gulps.

The limited capacity of short-term memory is also the reason why information that belongs together should never be fragmented into multiple dashboards, and scrolling shouldn't be required to see it all. Once the information is no longer visible, unless it is one of the few chunks of information stored in short-term memory, it is no longer available. If you scroll or page back to see it again, you then lose access to what you were most recently viewing. As long as everything you need remains within eye span on a single dashboard, however, you can rapidly exchange information in and out of short-term memory at lightning speed.