Where, how and why do top-down and bottom-up signals interact in the primate brain?

Hierarchical predictive coding theory (hPC) proposes that perception arises from the integration of bottom-up (BU) pathways carrying sensory evidence with top-down (TD) pathways carring priors and expectations. However, there is surprising little evidence concerning the location and functional mechanisms underlying such BU-TD integration. This can be overcome by future parallel experiments in humans and non-human primates (NHPs). Molecular characterization in NHP of cortico-cortical BU and TD projection cell types will allow tracing these circuits and exerting causal control to explore functional mechanisms in visual paradigms in behaving NHPs. Identical paradigms in human subjects undergoing ultra-high field laminar fMRI scanning will indicate homologous pathways in the human brain with access to perceptual reports. In NHP and human, long-distance priors and expectations suggest that the claustrum might be involved in hPC. Recent connectomic and transcriptomic characterisation of cortical-claustral-cortical loops will enable a similar approach to explore these pathways participation in the integration of TD and BU signals as used in the cortico-cortical pathways.