What is the current state of knowledge on the roles and mechanisms of top-down information flow in the brain? I have heard that "90% of information flow in the brain (or maybe it was just the visual cortex) is top-down".  I'm not sure if this claim is substantiated, but I do get the impression that there is strong evidence that at least most of the information flow is top-down.  I'd like to know as specifically as possible what current theories in neuroscience say is going on here.  I'd appreciate details and reputable academic works.
No the brain doesn't in practice work in a single flow. It takes a combination of top-down and bottom-up at various frequencies to perform any task. Outside a laboratory the amount of time you spend going top-down or bottom-up will depend on what kind of life you lead.


  Hippocampal movement-related low-frequency oscillations are one of the most well-characterized and replicated findings across species. Previous literature has demonstrated that movement-related oscillations are distinct phenomenon from memory-related low-frequency oscillations. We suggest that movement-related oscillations arise, in part, due to entrainment from sensorimotor information inherited from upstream cortical areas and thus may represent largely bottom-up input. We also discuss memory-related low-frequency oscillations and their emergence when rodents retrieve information about which part of a maze to explore. These memory-related oscillations often appear coordinated between hippocampus and cortex and may reflect top-down, or rule based processing. The presence of both bottom-up and top-down low-frequency oscillations may be yet another example of spectral fingerprinting, in which different cognitive operations manifest in the same brain networks via different frequencies of synchronization. While many aspects of this idea remains to be tested, particularly in the context of navigation, we believe that these ideas represent one step in bringing some synchrony to an often desynchronized field on the study of neural oscillations.


-Multifaceted roles for low-frequency oscillations in bottom-up and top-down processing during navigation and memory