Insula and striatum mediate the default bias

Abstract

Humans are creatures of routine and habit. When faced with situations in which a default option is available, people show a consistent tendency to stick with the default. Why this occurs is unclear. To elucidate its neural basis, we used a novel gambling task in conjunction with functional magnetic resonance imaging. Behavioral results revealed that participants were more likely to choose the default card and felt enhanced emotional responses to outcomes after making the decision to switch. We show that increased tendency to switch away from the default during the decision phase was associated with decreased activity in the anterior insula; activation in this same area in reaction to "switching away from the default and losing" was positively related with experienced frustration. In contrast, decisions to choose the default engaged the ventral striatum, the same reward area as seen in winning. Our findings highlight aversive processes in the insula as underlying the default bias and suggest that choosing the default may be rewarding in itself.

Figure 1.

A, The experimental paradigm. B, Frequency of switching for 18 participants. Red horizontal line indicates the mean frequency of switching. C, D, Participants felt more frustrated when they lost after switching away from the default than when they lost after staying with the default (C), and more satisfied when they won after switching away from the default than when they won after staying with the default (D).

Figure 2.

A, B, Right anterior insula (yellow) activated by switch versus stay choices (A), and left ventral striatum (red) activated by the reverse comparison (stay versus switch) (B), overlapped with striatum (yellow) activity responding to winning outcomes. A, C, For switch versus stay comparison, insula (green) activity was negatively correlated with the tendency to switch, while caudate (blue) activity was positively associated with switch frequency. D, Significant activity associated with the residual frustration scores (i.e., frustration for L_switch minus frustration for L_stay) in right anterior insula (red) for the comparison of L_switch versus L_stay outcomes and significant activity in the right anterior insula (yellow) associated with switch versus stay decisions. E, Significant activity associated with the residual satisfaction scores in ventral striatum (red) for the comparison of W_switch versus W_stay and significant activity in bilateral ventral striatum (yellow) associated with winning versus losing. All activations are significant at p < 0.05, small volume corrected.