The role of the presence of others in a social context has been debated widely. Although the importance of mutual cognitive functions between performer and observer is generally accepted, little is known about the neural correlates of paired performers and observers themselves. In this near-infrared spectroscopy (NIRS) study we measured the activation in the bilateral inferior parietal lobule (IPL) when driver-observer pairs of participants performed a driving video game task. The performer's task was to drive from start to goal using a default route map, while their partner observed the performance. According to the performer's subjective appraisal of the copresent observer obtained after the driving task, the pairs were divided into three groups: supportive, nonsupportive, and neutral. The driving time, error, and tension score did not show significant differences between the three groups. However, NIRS data of performers in the supportive group showed significantly higher activation in the left IPL than those in the nonsupportive group, but not in the right IPL. NIRS data of observers in the concerned two groups did not show significant differences bilaterally in IPL. These results suggest that the left IPL distinctively responds according to a performer's cognitive appraisal of a copresent observer.
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