The necessity of conscious awareness in human learning has been a long-standing topic in psychology and neuroscience. Previous research on non-conscious associative learning is limited by the low signal-to-noise ratio of the subliminal stimulus, and the evidence remains controversial, including failures to replicate. Here, functional MRI decoded neurofeedback (DecNef) allowed participants to generate brain activity patterns consistent with a perceptual state without awareness of the represented content. We aimed to instill perceptual meaning into a meaningless hiragana by conditioning it through reinforcing neural patterns associated with perceptual content (i.e. living), thereby bypassing the conscious mapping between symbol and the perceptual referent. Despite being unaware of the goal of DecNef training, participants learned to induce the target representation in fusiform cortex. Then, the behavioural significance of DecNef training was evaluated in a visual search task. DecNef and control participants searched for living or non-living targets that were pre-cued by the targeted hiragana during DecNef or by a novel hiragana. The targeted hiragana did not prime search for the associated target but, strikingly, participants were impaired at searching for the perceptual content targeted by DecNef. Hence, conscious awareness may function to support higher-order associative learning, while lower-level forms of re-learning, modification, or plasticity in established neural representations can occur unconsciously, with behavioural consequences outside the original training context. The work also offers an account of DecNef effects in terms of neural representational drift.

Aging is often associated with a decrease in cognitive capacities. However, semantic memory appears relatively well preserved in healthy aging. Both behavioral and neuroimaging studies support the view that changes in brain networks contribute to this preservation of semantic cognition. However, little is known about the role of healthy aging in the brain representation of semantic categories. Here we used pattern classification analyses and computational models to examine the neural representations of living and non-living word concepts. The results demonstrate that brain representations of animacy in healthy aging exhibit increased similarity across categories, even across different task contexts. This pattern of results aligns with the neural dedifferentiation hypothesis that proposes that aging is associated with decreased specificity in brain activity patterns and less efficient neural resource allocation. However, the loss in neural specificity for different categories was accompanied by increased dissimilarity of item-based conceptual representations within each category. Taken together, the age-related patterns of increased generalization and specialization in the brain representations of semantic knowledge may reflect a compensatory mechanism that enables a more efficient coding scheme characterized by both compression and sparsity, thereby helping to optimize the limited neural resources and maintain semantic processing in the healthy aging brain.

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