Probing top-down information in neocortical layer 1

Accurate perception of the environment is a constructive process that requires inte-gration of external bottom-up sensory signals with internally generated top-down information. Decades of work have elucidated how sensory neocortex processes physical stimulus features. By contrast, examining how top-down information is encoded and integrated with bottom-up signals has been challenging using tradi-tional neuroscience methods. Recent technological advances in functional imaging of brain-wide afferents in behaving mice have enabled the direct measurement of top-down information. Here, we review the emerging literature on encoding of these internally generated signals by different projection systems enriched in neo-cortical layer 1 during defined brain functions, including memory, attention, and pre-dictive coding. Moreover, we identify gaps in current knowledge and highlight future directions for this rapidly advancing field.