Prefrontal cortex involvement in processing incorrect arithmetic equations: evidence from event-related fMRI

Abstract

The main aim of this study was to investigate the differential processing of correct and incorrect equations to gain further insight into the neural processes involved in arithmetic reasoning. Electrophysiological studies in humans have demonstrated that processing incorrect arithmetic equations (e.g., 2 + 2 = 5) elicits a prominent event-related potential (ERP) compared to processing correct equations (e.g., 2 + 2 = 4). In the present study, we investigated the neural substrates of this process using event-related functional magnetic resonance imaging (fMRI). Subjects were presented with arithmetic equations and asked to indicate whether the solution displayed was correct or incorrect. We found greater activation to incorrect, compared to correct equations, in the left dorsolateral prefrontal cortex (DLPFC, BA 46) and the left ventrolateral prefrontal cortex (VLPFC, BA 47). Our results provide the first brain imaging evidence for differential processing of incorrect vs. correct equations. The prefrontal cortex activation observed in processing incorrect equations overlaps with brain areas known to be involved in working memory and interference processing. The DLPFC region differentially activated by incorrect equations was also involved in overall arithmetic processing, whereas the VLPFC was activated only during the differential processing of incorrect equations. Differential response to correct and incorrect arithmetic equations was not observed in parietal cortex regions such as the angular gyrus and intra-parietal sulcus, which are known to play a specific role in performing arithmetic computations. The pattern of brain response observed is consistent with the hypothesis that processing incorrect equations involves detection of an incorrect answer and resolution of the interference between the internally computed and externally presented incorrect answer. More specifically, greater activation during processing of incorrect equations appears to reflect additional operations involved in maintaining the results in working memory, while subjects attempt to resolve the conflict and select a response. These findings allow us to further delineate and dissociate the contributions of prefrontal and parietal cortices to arithmetic reasoning.

Figure 1

Surface rendering of brain areas that showed significant activation during the processing of incorrect, compared to correct, arithmetic equations. Activation was limited to two clusters in the left dorsolateral and ventrolateral prefrontal cortex (Brodmann areas 9/46 and 47). Results are from a random effects analysis of event‐related activation in 16 subjects; each activated cluster was significant after corrections for multiple spatial comparisons (P < 0.01).

Figure 2

Coronal sections showing activation during the processing of incorrect, compared to correct, arithmetic equations. Other details as in Figure 1.

Figure 3

Overlap between brain areas activated during: 1) the processing of incorrect, compared to correct, arithmetic equations (shown in the yellow‐red‐black color scale), and 2) arithmetic processing (correct and incorrect equations compared to the control condition: shown in cyan). The left dorsolateral prefrontal cortex (DLPFC) in the middle frontal gyrus showed an overlap in activation in the two analyses (circled in yellow). The left ventro‐lateral prefrontal cortex (VLPFC) activation in the inferior frontal gyrus, on the other hand, was activated only during the differential processing of incorrect equations (circled in green).

Figure 4

Surface rendering of brain areas that showed significant activation during the processing of 3‐operand, compared to 2‐operand, arithmetic equations, irrespective of trial type. Activation was limited to the right angular gyrus/intra‐parietal sulcus.