We are continuing to develop both the expectancy AX-CPT task created by Cohen and colleagues as well as the DPX, which is an adaptation of the expectancy AX task that uses pairs of simple dot patterns rather than letter pairs as stimuli (Jones, Sponheim, & MacDonald, 2010; A. W. MacDonald, III, Carter, Flory, Ferrell, & Manuck, 2007; A. W. MacDonald, III, et al., 2005). Both of these tasks measure an aspect of goal representation known as context processing. In both the expectancy AX and DPX tasks, there are four types of trials: AX, AY, BX and BY. AX trials are "target trials"; in these types of trials a valid cue is followed by a valid probe. The 3 other trial types are "Non-target trials" in which either a valid cue is followed by an invalid probe ("AY" type trials) or an invalid cue is followed by either a valid or invalid probe ("BX" or "BY" probes, respectively). Participants are required to make one response for target trials, and another response for non-target trials. The nature of the cue (valid or invalid) provides the "context" for responding on a given trial. The majority of trials are "target trials" (AX trials). This feature is intended to encourage participants to "expect" a valid probe to follow a valid cue. A consequence of this manipulation is that participants develop a prepotency to respond with "target" responses on trials for which valid cues are presented, regardless of whether the trials were of the target (AX) or non-target (AY) type. Non-target cues provide the context that a non-target response will be required, regardless of the nature of the probe (valid or invalid). The expectancy manipulation is designed to increase AY error rates. On BX trials participants with normal context processing must maintain the cue in order to inhibit the prepotency to respond to valid probes with a target response. Compared with controls, persons with impaired context processing may be expected to make more BX errors, whereas the AY condition is more difficult for people with intact context processing.
The original AX-CPT has a large body of evidence supporting its use as a measure of goal maintenance and context processing, including both behavioral and imaging evidence (Barch, et al., 2001; Barch, Carter, & Cohen, 2003; Barch, et al., 2004; Cohen, et al., 1999; Delawalla, Csernansky, & Barch, 2008; Edwards, Barch, & Braver, 2010; A. MacDonald, et al., 2005; A. W. MacDonald, 3rd & Carter, 2003). However, sometimes error rates can be low in healthy individuals, leading to some challenges with range and ceiling effects. Thus, one potential advantage of the DPX over the AX is that dot patterns used are more amenable to parametrical manipulations than are letters. By manipulating the similarity between valid probes (Xs) and invalid ones (Ys), the proportion of errors in the AY difficulty control condition can be manipulated. However, instructing participants in the DPX version can sometimes be more challenging, though the version available here includes instructions designed to simplify this process. Performance on AY and BX trials has been formally modeled, such that these predicted changes in performance can be achieved through manipulation of the units responsible for representation and maintenance of current goals. The AX-CPT also shows convergent validity with other tasks that require top-down control. For example, performance on the BX condition was associated with color-naming performance on incongruent trials of the Stroop task (r=.44, p<.01), and the ability to use semantic context to constrain word completion (r=.32, p<.01) (Cohen, et al., 1999). Confirmatory factor analysis supported the convergent validity of the letter version of the expectancy AX and the formally equivalent DPX (A. W. MacDonald, 3rd, et al., 2005).
The versions of the AX-CPT and DPX available here are implemented in Eprime and present 144 trials in 4 blocks of 36 trials. There are 104 AX (72%) 16 AY (11%), 16 BX (11%) and 8 BY (6%) trials. The cue is presented for 1000 ms (with a target or non-target response expected), the ISI is 2000 ms, and the target is presented for 500 msec with a response window of 1500 msec. The ITI is 1200 ms. Practice trials are embedded in the task, along with instructions. Participants should practice until criteria are obtained (80% accuracy and no less than one 1 BX trial correct).
These versions are currently being used in our test-retest reliability, and could change based on the results of that study.