N-back

The n-back task is a Continuous Performance Task that is commonly used in neuroimaging to stimulate brain activity in test subjects. It was introduced by Wayne Kirchner in 1958.

The task
The subject is presented with a sequence of stimuli, and the task consists of indicating when the current stimulus matches the one from n steps earlier in the sequence. The load factor n can be adjusted to make the task more or less difficult.

For example, an auditory three-back test could consist of the experimenter reading the following list of letters to the test subject:
 * T L H C H S C C Q L C K L H C Q T R R K C H R

The subject is supposed to indicate when the letters marked in bold are read, because those correspond to the letters that were read three steps earlier.

Dual n-back
The dual-task n-back task was proposed by Susanne Jaeggi et al. in 2003. In this variation, two independent sequences are presented simultaneously, typically using different types of stimuli, such as one auditory and one visual.

A 2008 research paper claimed that practicing a dual n-back task can increase fluid intelligence (Gf), as measured in several different standard tests. This resulted in some attention from popular media, including an article in Wired. However, a subsequent criticism of the paper's methodology suggested questionable validity and a lack of uniformity in the tests used to evaluate the control and test groups. For example, the progressive nature of Raven's Advanced Progressive Matrices (APM) test may have been compromised by modifications of time restrictions (10 minutes were allowed to complete a normally 45-minute test). The authors of the original paper later addressed this criticism by citing research indicating that scores in timed administrations of the APM are predictive of scores in untimed administrations.

The 2008 study was replicated in 2010 with results indicating that practising single n-back may be almost equal to dual n-back in increasing the score on tests measuring Gf.