The representation of abstract task rules in the human prefrontal cortex

Abstract

We have previously reported sustained activation in the ventral prefrontal cortex while participants prepared to perform 1 of 2 tasks as instructed. But there are studies that have reported activation reflecting task rules elsewhere in prefrontal cortex, and this is true in particular when it was left to the participants to decide which rule to obey. The aim of the present experiment was to use functional magnetic resonance imaging (fMRI) to find whether there was activation in common, irrespective of the way that the task rules were established. On each trial, we presented a word after a variable delay, and participants had to decide either whether the word was abstract or concrete or whether it had 2 syllables. The participants either decided before the delay which task they would perform or were instructed by written cues. Comparing the self-generated with the instructed trials, there was early task set activation during the delay in the middle frontal gyrus. On the other hand, a conjunction analysis revealed sustained activation in the ventral prefrontal and polar cortex for both conditions. We argue that the ventral prefrontal cortex is specialized for handling conditional rules regardless of how the task rules were established.

Figure 1.

The self-generated and instructed tasks. The instruction was followed by a varying delay (2–10 s). The task word appeared for 500 ms after which the participants responded (R1). They then made a second response (R2) reporting which task they had performed.

Figure 2.

(A) Time to make a decision about the target word in the self-generated and instructed conditions. (B) Time to decide on the task (self-generated) or to process the task instruction in session 6.

Figure 3.

All the plots in Figure 3 show the adjusted data for the BOLD signal (vertical axis), aligned to the time of presentation of the instructing word. Time is shown along the axis in the foreground. The numbers on the x-axis are adjusted to take into account a delay in the peak of the BOLD signal of 5 s. In other words, 0 (arrow) corresponds to 5 s after the instructing word. The data for the different delay lengths are shown along the left-hand receding axis, with the longest delay at the back and the shortest delay at the front.

Figure 4.

As in the plots in Figure 3, this figure shows the adjusted data for the BOLD signal (vertical axis), aligned to the time of presentation of the instructing word. Data are plotted for the longest delay only (10 s). Time is shown along the axis in the foreground. The numbers on the x-axis are adjusted to take into account a delay in the peak of the BOLD signal of 5 s. In other words, 0 corresponds to 5 s after the instructing word. The receding axis on the left indicates the 5 areas from which the data were taken. The data for the right-hand side of the BOLD signal for the preSMA are not shown in this figure. c = cortex, g = gyurs.

Figure 5.

Areas of sustained activation in common, irrespective of whether the rules were self-generated or verbally instructed.