Chunk #40 — Open questions in layer-fMRI connectivity analyses — Inherent connectivity across layers — The neuro-vascular coupling favors the first input.
Previous time-resolved layer-fMRI studies in animals have shown that the onset of the fMRI signal to secondary connected layers has a surprisingly small amplitude and is surprisingly late. It has been shown with line scanning in the rat barrel cortex that the feed-forward induced fMRI signal starts rising in input layer IV ≈ 500ms after the stimulus onset. fMRI activity in secondary connected layers II/III is delayed by another ≈ 300 ms – 400 ms and has a much smaller magnitude (Yu et al. 2014). The transition time that it takes until the secondary connected layers show fMRI signal change is longer than what is expected from the electrophysiology (Godlove et al. 2014; Ninomiya et al. 2015). Similarly, recent layer-dependent work across multiple areas in the sensory system has revealed that the fMRI onset of secondarily-connected areas can be delayed by up to several seconds (Jung et al. 2019). Based on this empirical evidence, it has been hypothesized that the fMRI response might be more susceptible to the initial input of a brain area. Secondary activity in inter-connected layers might need more time to accumulate sufficient neural activity until the fMRI signal reaches a significant increase.
