Brain structural correlates of individual differences in heartbeat counting and discrimination: A voxel-based morphometry study.

Individual differences in interoception have drawn attention because of their relevance to emotion-processing abilities and mental health. To assess individual differences, researchers have employed methods using cardioception, including the heartbeat counting task (HCT), where participants count their heartbeat for a certain duration, and the heartbeat discrimination task (HDT), where they judge whether presented tones with various delays to the R-peak are synchronized with their heartbeats. However, the processes and neural substrates underlying individual differences in cardiac interoception remain unknown. Based on our hypothesis that these tasks recruit fronto-insular brain regions, we investigated the neuroanatomical substrates underlying individual differences in HCT and HDT indices by conducting voxel-based morphometry analyses using brain structural images of 138 participants who underwent both tasks. HCT-derived interoceptive accuracy (IAcc) was positively associated with gray matter volume (GMV) in the left frontal pole, whereas an HDT index, reflecting response consistency across different delay conditions, was positively associated with the GMV of the left orbitofrontal cortex and right posterior insula. Conversely, the correlation between accuracy and confidence in the HCT, defined as interoceptive awareness, was associated with GMV, including the left posterior insula. These variations in the brain regions may reflect differences in the demands of the HCT and HDT.