Invariance detection in the brain: revealed in a stepwise category induction task.

A critical sub-process of category learning is detecting the invariance between categorical members. To examine brain activation associated with invariance detection at different steps of category learning, a stepwise category induction task was used in the present study. Within each trial, three stimuli were displayed sequentially, and participants were asked to learn the target category corresponding to the invariance among stimuli. Results revealed that invariance detection activated the fronto-parietal network. However, the frontal and parietal cortices functioned differently throughout the different steps of invariance detection. The left middle frontal gyrus (BA 9) was highly activated in both steps of invariance detection, but the posterior parietal regions, especially the right superior parietal lobule (BA 7), were more active in the final step of invariance detection, reflecting increased attention to the completion of category learning and the preparation for a subsequent response. Furthermore, a psychophysiological interaction analysis (PPI) revealed increased connectivity between the left middle frontal gyrus and the bilateral parietal cortex during the final step of invariance detection. Overall, the present findings imply the necessary role of the fronto-parietal network in variance detection.