Neural correlates of categorization uncertainty: evidence from a category induction task

Introduction Categorization uncertainty occurs when novel stimuli are classified as category members or non-members, with frontal–parietal and insular regions involved in its monitoring, but graded uncertainty in similarity-based category induction—especially implicit processing without explicit confidence judgments—has not been well characterized. Methods The present study investigated these mechanisms using a similarity-based category induction task combined with functional magnetic resonance imaging (fMRI). Participants simultaneously viewed three stimuli: two reference stimuli (S1 and S2) that defined a target category, and a probe stimulus (S3) for which they judged category membership. Levels of categorization uncertainty were manipulated by varying the degree of feature similarity between the probe and reference stimuli. Results The fMRI data revealed that activity in the left fronto-parietal network—including the medial frontal gyrus (BA11), middle frontal gyrus (BA46), cingulate gyrus (BA32), inferior parietal lobule (BA40), and left insula (BA13)—increased systematically with higher categorization uncertainty. Notably, positive activations were observed in the left cingulate gyrus, middle frontal gyrus, and inferior parietal lobule, whereas negative activations were detected in the left medial frontal gyrus and insula. Psychophysiological interaction (PPI) analyses further demonstrated enhanced functional coupling between the left cingulate cortex and inferior parietal lobule under high uncertainty conditions. Discussion These findings suggest that distributed fronto-parietal and insular systems support the processing of graded uncertainty during similarity-based category induction, even in the absence of explicit confidence judgments.