Methodological advancements of functional Magnetic Resonance Imaging (fMRI) contrasts and readout strategies in recent years have allowed researchers to approach the mesoscopic spatial regime of cortical layers (De Martino et al. 2015; Huber et al. 2017; Kok et al. 2016; Koopmans et al. 2010; Olman et al. 2012; Polimeni et al. 2010b). This has allowed investigations of the connectivity and function in neural microcircuits across cortical layers within cortical areas (Callaway 1998; Lund 1988). Non-human primate studies have shown that in hierarchically organized brain systems, inter-area neural input arrives in different layers for bottom-up or top-down connections. Specifically, feed-forward, bottom-up activity (e.g. V1 → V5/hMT) terminates predominantly in the middle granular layer, while feedback, top-down activity (e.g. V1 ← V5/hMT) terminates predominantly in superficial and/or deeper layers (Angelucci et al. 2002; Felleman and Van Essen 1991). Thus, the ability to map brain activity across cortical layers revolutionizes the ability to tackle cortical information processing within brain systems and might become an important tool for cognitive network neuroscience (Lawrence et al. 2019; Stephan et al. 2019).
