We sought to test the prediction directly that frontally distributed potentials should precede the P3b and be systematically related to context updating. To do so we used the simplest task that requires context updating, a variant of the continuous performance task (CPT, Rosvold, Mirsky, Sarason, Bransome, & Beck, 1956
) that is closely related to, but simpler than most task switching paradigms. In the standard version of the CPT, participants view a continuous stream of letters and respond to specified targets with a button press. In AX-CPT variants of the task, however, the response to the target is contingent upon the previous stimulus. For example, in one version participants are instructed to respond with a button press whenever a target is detected (e.g., the letter “X”), but only when it follows a particular cue (e.g., the letter “A”). Thus, the response to a target (e.g., button press to X) is contingent on the context provided by a preceding cue (e.g., A or non-A), and each appearance of a cue should elicit context updating. Typically, such cue-target sequences are embedded within a stream of stimuli that carry no contextual information and do not require a response. Like cues, these “control” stimuli must be encoded (in order to know whether or not they are a cue), but unlike cues they do not provide any information needed for processing subsequent stimuli. Accordingly, these control stimuli should not elicit context updating (since there is no context to represent). Thus, a comparison between cues and control stimuli should provide a useful contrast for identifying signals related to context updating.
Such a contrast may be more reliable than ones that have been used in previous task switching studies. As noted above, contrasting switch and repeat trials has produced inconsistent results with regard to potentials preceding the posterior positivity (Brass et al., 2005
; Rushworth, Passingham, et al., 2002
; Rushworth, et al., 2005
; Wylie, et al., 2003
). This inconsistency may be due to the assumption that, in task switching paradigms, context is updated only on switch trials, and is simply maintained (from the previous trial) on repeats. However, this may not always be so. Depending on task conditions (e.g., trial sequence, intertrial interval, emphasis on accuracy), participants might choose to (re)activate context on some or all repetition trials as a strategy to avoid maintenance costs or maximize accuracy; or they may be forced to do so because of a distraction following the previous trial (Rushworth, Hadland, Paus, & Sipila, 2002
; Todd et al., 2009
; Wylie & Allport, 2000
). Consequently, context updating could occur on repeat as well as switch trials, minimizing the switch-repeat differential, and compromising its reliability across studies. In contrast, the comparison between cues and control stimuli in the AX-CPT task should be more reliable. This is because cues always require context updating while control stimuli neither require a response nor provide information that bears on any subsequent response. Accordingly, in the present study we used the AX-CPT to explicitly control the conditions under which context updating was required.
We used a version of AX-CPT in which cue-target pairs were imbedded in sequences of control stimuli, and varied the dependency of target response on the preceding cue in order to manipulate the likelihood that participants engaged in context updating. To make the task more consistent with task switching paradigms, we used a two-alternative forced choice design for responses2
. On context-dependent trials the cue played the same role as the instruction in an exogenously cued task switching paradigm, specifying the response rule for the upcoming target. For instance, following an “A”, participants were required to press the right button for an “X” and the left button for a “Y, and following a B these rules were reversed. Thus, on such trials cues should trigger context updating to ensure the correct rules are applied in responding to the subsequent target. In these respects, context-dependent trials were comparable to trials in an instructed (exogenously cued) task-switching paradigm; cue switches (e.g., A-X followed by B-X or B-Y) were analogous to switch trials and cue repetitions (e.g., A-X followed by A-X or AY) analogous to repeat trials.
These context-dependent trials were compared with two other types of trials. Context-independent trials paralleled context-dependent trials in all respects, except that response to the target was not
contingent on the preceding cue. For instance, following a “C” or “D” cue participants would see a “W” or “Z” target, but would always be required to press left for a “W” and right for a “Z”. Thus, in these trials, the target-response contingency was the same, irrespective of the preceding cue. In other words, participants could make the correct response based only on target identity without relying on the preceding context. It should be noted that participants could still use the context provided by the cue to prepare for the upcoming target, by reminding themselves of the target-response rule. However, with practice, we would expect participants to learn these simple, and consistently-mapped target-response associations and come to respond automatically to the target, without relying on the context provided by the cue (Shiffrin & Schneider, 1977; Cohen et al., 1990
). Therefore we expected that these trials would be associated with substantially less context updating than context-dependent trials.
Finally, we included a set of control trials, in which stimuli were random single letters interleaved among the cue-target pairs of the context-dependent and context-independent trials. These stimuli did not require any response. Participants had to visually and semantically encode these stimuli in order to determine their identity in order to know not to respond, however they were not associated with any future response rule. It is possible that during early performance, participants may have activated a context representation indicating that they were not to respond to these stimuli. However, as with the context-independent targets, again we would expect that with practice participants would come to automatically recognize that they did not need to respond to these stimuli. Thus, we did not expect control stimuli to trigger context updating.
In summary, to identify the onset of context updating, we compared the effects of cue processing during context-dependent trials with cue processing during context-independent trials and with processing of control stimuli. We predicted that these comparisons would reflect a progressive decrease in the likelihood of context updating during cue processing, and that a parallel effect in voltage potentials would index the engagement of this process. More specifically, we predicted that the onset of such changes would indicate the onset of context updating and its scalp distribution should be maximal over frontal electrodes. We show that the first effect of context updating occurred at approximately 200 ms following cue onset and preceding cue offset. It occurred over frontal electrodes and its scalp distribution was consistent with a source in the PFC. Additional effects were observed 400 -700 ms after cue onset and distributed over posterior electrodes (P3b), and from 700 ms until response onset and distributed over frontal electrodes. We interpret these as reflecting the influence that context-updating in PFC has on reconfiguring posterior mechanisms responsible for task execution.