Acquiring novel speech categories is necessary in spoken language learning. The dual-learning systems (DLS) approach posits that two competitive systems underlie the category learning process: an explicit hypothesis-testing system, and an implicit procedural system. DLS assumes that the explicit system dominates early and control is passed to the implicit system when optimal. Evidence from our work, including the finding that minimally informative feedback enhances speech learning relative to fully informative feedback, supports the claim that the implicit system optimally mediates speech learning in adulthood. Experiment 1 replicates this finding. Experiment 2 tests the DLS prediction that explicit processing dominates early by comparing performance across two conditions. The optimal condition includes full feedback early and minimal feedback later. The suboptimal condition includes minimal feedback early and full feedback later. In both conditions, real-time computational modeling individualized when feedback transitions occurred. As predicted from DLS, learning was superior in the optimal condition.

Understanding brain structure-function relationships, and their development and evolution, is central to neuroscience research. Here, we show that morphological differences in posterior cingulate cortex (PCC), a hub of functional brain networks, predict individual differences in macroanatomical, microstructural, and functional features of PCC. Manually labeling 4511 sulci in 572 hemispheres, we found a shallow cortical indentation (termed the inframarginal sulcus; ifrms) within PCC that is absent from neuroanatomical atlases yet colocalized with a focal, functional region of the lateral frontoparietal network implicated in cognitive control. This structural-functional coupling generalized to meta-analyses consisting of hundreds of studies and thousands of participants. Additional morphological analyses showed that unique properties of the ifrms differ across the life span and between hominoid species. These findings support a classic theory that shallow, tertiary sulci serve as landmarks in association cortices. They also beg the question: How many other cortical indentations have we missed?