Medial prefrontal cortex and self-referential mental activity: relation to a default mode of brain function

Abstract

Medial prefrontal cortex (MPFC) is among those brain regions having the highest baseline metabolic activity at rest and one that exhibits decreases from this baseline across a wide variety of goal-directed behaviors in functional imaging studies. This high metabolic rate and this behavior suggest the existence of an organized mode of default brain function, elements of which may be either attenuated or enhanced. Extant data suggest that these MPFC regions may contribute to the neural instantiation of aspects of the multifaceted "self." We explore this important concept by targeting and manipulating elements of MPFC default state activity. In this functional magnetic resonance imaging (fMRI) study, subjects made two judgments, one self-referential, the other not, in response to affectively normed pictures: pleasant vs. unpleasant (an internally cued condition, ICC) and indoors vs. outdoors (an externally cued condition, ECC). The ICC was preferentially associated with activity increases along the dorsal MPFC. These increases were accompanied by decreases in both active task conditions in ventral MPFC. These results support the view that dorsal and ventral MPFC are differentially influenced by attentiondemanding tasks and explicitly self-referential tasks. The presence of self-referential mental activity appears to be associated with increases from the baseline in dorsal MPFC. Reductions in ventral MPFC occurred consistent with the fact that attention-demanding tasks attenuate emotional processing. We posit that both self-referential mental activity and emotional processing represent elements of the default state as represented by activity in MPFC. We suggest that a useful way to explore the neurobiology of the self is to explore the nature of default state activity.

Figure 1

The distribution of significant regions of increased (▵) and decreased (▿) activity along the medial prefrontal cortex, the coordinates and Z scores of which are listed in Table 1. The triangle colors relate to image data shown in Figs. 2 (white), 3 (red), 4 (blue), and 5 (green).

Figure 2

T-statistic images of increases common to both active tasks (ECC, Left; ICC, Right) when compared with visual fixation. The white triangle (as in Fig. 1) is located in the vicinity of dorsal anterior cingulate and adjacent pre-SMA and SMA (Table 1, ICC = ECC).

Figure 3

T-statistic images of increases in ICC greater than in ECC in dorsal MPFC. The two conditions were directly compared [i.e., (ICC vs. fixation control) vs. (ECC vs. fixation control), where (ECC vs. fixation control) became the control condition for the comparison]. The approximate locations of significant regions within BA 8, 9, and 10 are listed in Table 1 (ICC > ECC).

Figure 4

T-statistic images of decreases common to both tasks (ECC, Left; ICC, Right) in ventral MPFC. The approximate locations of significant regions within BA 24, 25, and 32 are listed in Table 1.

Figure 5

Three T-statistic images comparing: ICC to ECC (see Fig. 3), and the individual task conditions, ICC and ECC, relative to their own controls (Left to Right). The green triangle (as in Fig. 1) denotes a region (Table 1) of so-called increased activity (Left) that arises entirely as the result of a decrease in ECC relative to its control (Right). Note that there is no change in the ICC relative to its control in this region (Center).