Supplemental Material
Results
MR signal change in the inferior frontal, anterior cingulate and superior parietal regions preceded the response. T-tests (p < .05) confirmed significant differences between the beta weights of the small and large rewards (inferior frontal gyrus: t(11)=8.6, p<.05; anterior cingulate: t(11)=8.1, p<.05; superior parietal gyrus: t(11)=7.5, p<.05) during these time points with greater signal for the larger reward value (see supplementary figure 1a-c and text). A post hoc comparison of beta weights during the early and late trials for these time points showed a significant decrease (t(11)=3.5, p<.05) in activity for these regions regardless of reward value (see supplementary figure 2 a-c).
Discussion
Maintenance and Withholding of Responses
Another circuit involving the inferior frontal, superior parietal and anterior cingulate regions, appeared to be sensitive to reward magnitude, but during the interval preceding the response rather than in anticipation to the reward. This pattern of activity is consistent with a role of these regions in selection and maintenance of a response until prompted to respond. As the subject had to refrain from responding prior to the response prompt, response inhibition was required as well. Activation of these regions as a result of maintenance and withholding a response until prompted is consistent with the imaging literature on brain circuitry implicated in response conflict and inhibition (Casey et al,
1997, 2000; Durston et al, 2002). This network has been implicated in goal-directed behaviors and response execution (MacDonald et al, 2000; Pochon et al, 2001; Durston et al 2003). Other studies have shown that stimulus-response conflict activity in the anterior cingulate predicts greater prefrontal cortex activity (Botvinick et al, 1999; Kerns et al,
2004). These results are consistent with the known reciprocal neuroanatomical projections between prefrontal and parietal regions and the anterior cingulate (Carmichael
& Price, 1996).
Work in macaques (Hadland et al, 2003) and humans (van Veen et al, 2001) supports the notion that the anterior cingulate cortex is more associated with response or action-reward associations than it is with stimulus-reward associations. Greater activity in these regions for the large relative to small reward magnitude may represent enhanced conflict while withholding a response that has been associated with a specific gain until prompted. This effect appeared to dissipate with learning as all three regions (i.e., anterior cingulate, parietal and inferior frontal cortices) significantly decreased in activity from early to late trials, regardless of reward value.
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Figure Legend
Supplementary Figure 1. Activation in the a) anterior cingulate (x=-6, y=19, z=25), b) superior parietal (x=-24, y=-57, z=43) and c) inferior frontal (x=-46, y=37, z=13) regions preceded the response. Data here is averaged across the experiment (all runs) and shows significantly greater activity to the large relative to the small rewards. Time is on x-axis and percent MR signal change is on y-axis.
Supplementary Figure 2. Activation in the a) anterior cingulate (x=-6, y=19, z=25), b) superior parietal (x=-24, y=-57, z=43) and c) inferior frontal (x=-46, y=37, z=13) regions significantly decreased from early to late trials (collapsed across rewards). Time is on x- axis and percents MR signal change is on y-axis.