fASL Tutorial #3: The N-Back Task

Overview of Working Memory

Working memory refers to the ability to simultaneously hold multiple items in one’s conscious attention. Think of a time when someone told you to write down an event on your calendar: You are given the date, time, and location, and then you need to write it down. Most likely you found yourself repeating those items, either mentally or aloud, as you tried to remember them.

In light of this example, let’s refine our definition: Working memory is the ability to keep certain pieces of information in your awareness while suppressing irrelevant thoughts. These pieces of information - names, dates, numbers - can either be something you just read or heard, or they can be retrieved from longer-term memory; that is, they can be things you learned days or years ago. Working memory is the maintaining of these items in the front of your thoughts and then juggling, manipulating, and rearranging them as needed.

The N-Back Task

As we saw from the above example, working memory is used in all kinds of everyday situations. But how do we tap into it and measure it in a laboratory? The most popular versions of working memory tasks are the spatial memory working task and the letter working memory task. We will focus here on the letter working memory task, as that is the one that was used during the ASL scan you will analyze.

Working memory tasks are usually continuous; that is, the subject will continuously monitor a stream of stimuli - such as letters shown at regular intervals - and keep in mind a rule for whether to respond or not. These rules often take the form of an N-back rule (also referred to as N-back tasks): respond to the letter on the screen if you saw that same letter N letters ago.

To illustrate this, take the simplest rule: The 0-back task. In this case, the subject is told to respond by pushing a button whenever he sees a particular letter, such as X. This requires remembering a rule, but doesn’t require working memory; no items are held in mind, manipulated, or subordinated to other items; all the task requires is recognizing a stimulus, which, with enough training, becomes a reflex.


An example of the 0-back task. In this example, the subject is told to respond whenever he sees the letter X. Perfect accuracy requires responding to the X with a button press, and not responding to any of the other letters.

Using the 0-back task as a template, we can increase the number of items to be held in working memory. In a 1-back task, for example, the subject is required to respond to a letter if it matches the previous letter (i.e., the letter that was shown 1 stimulus ago). In the figure below, the subject would respond to the second and fourth letters that were presented, since they match the letter that was presented on the previous slide. The last letter matches one of the previous letters shown, but since that letter occurred two slides ago, the subject should withhold his response.


As the number of items increases, say in a 4-back task, the task becomes more difficult; more items need to be held in working memory, and more mental comparisons need to be made between what is seen now and what was seen previously:


Next Steps

In the task you will be analyzing, the subjects did both the 1-back and 4-back tasks. We will use ASL to compare differences in cerebral blood flow between them. Click on the Next button to learn how to set up the analysis with fASL.