Immediate and Delayed Memory Tasks (IMT/DMT)

The IMT/DMT (Dougherty, Marsh, & Mathias, 2002) is a continuous performance test that was designed to assess impulsive behavior. The Immediate Memory Task involves comparison of consecutively presented number and responding in those instances when the current number matches the number immediately before it (called a correct detection). The Delayed Memory Task similarly involves responding to matching numbers, but the numbers to be compared a separated by a filler sequence (e.g. 12345).

Like all continuous performance tests, the IMT/DMT involves selectively responding target stimuli and avoiding responses to non-target stimuli that are presented in rapid sequence. For many years, continuous performance tests were primarily analyzed in terms of success in identifying target stimuli, which have been interpreted as measuring sustained attention. The characteristics of these test were selected to maximize sensitivity to individual differences in target responding and typically rates of non-target responding were low.

The IMT/DMT was created to test the notion that the non-target responding could be a sensitive measure impulsivity. The parameters of this new continuous performance test were modified so that certain types of non-target responding could be interpreted within a definition of impulsive behavior. The innovation of the IMT/DMT is the inclusion of catch stimuli that are very similar to targets: they match on all but one digit with the target sequence. This is in contrast with other non-target stimuli that are random and share very little overlap with the target. The concept here is that these very similar, catch stimuli, require a long period of information processing in order to resolve as different from the target. More impulsive individuals will respond to these catch trials more often, because they emit the behavior prior to the completion of information processing. This is called by the authors Response Initiation Impulsivity. Another perspective has been to refer to this type of impulsivity as Rapid Response Impulsivity.

Inclusion of the catch trials and the timing of the stimulus delivery produce a response range that is broad and normally distributed. Additionally, the rates of impulsive responding) were sufficiently high that the test (1) avoids floor effects among higher functioning populations; and (2) is sensitive to detection of not just increases, but also decreases, in impulsive responding in response to an intervention.

IMT/DMT instrument reference:
    Immediate and delayed memory tasks: 
    a computerized behavioral measure of memory, attention, and impulsivity.
 
    Dougherty DM, Marsh DM, Mathias CW (2002)
    Behavioral Research Methods: Instruments and Computers, 34, 391-398.   PubMed ID 12395555

For more information, the authors maintain a website for the IMT/DMT that can be accessed at:
http://www.nrlc-group.net/software/IMT_DMT.php