Abstract
At ultra-high field it is now possible to acquire functional images with unprecedented spatial precision, spanning the submillimeter range. These measurements allow investigations of some fundamental units of neural computations, such as cortical layers and columns, that had previously only been accessible in animals using invasive electrophysiology. With submillimeter fMRI it is therefore possible, in principle, to study fine scale organization of high-level cognitive processes that are unique to humans.
Here we evaluated top-down effects of high-level, socially relevant task demands, such as face perception, across cortical depths in lower and higher-level visual areas using functional images recorded with 0.7mm isotropic voxels.
To this end, we instructed participants to perform either a face detection or a stimulus-irrelevant fixation task with identical phase scrambled (ranging from 0 to 40% phase coherence) faces. Using an independent functional localizer, we identified 3 regions of interest (i.e. Fusiform and Occipital face areas and V1) and segmented each region into 3 cortical depths. To evaluate task-related top-down modulations, we calculated the ratio of the activation during the face relative to the fixation task at each cortical depth.
Task-related top-down modulations were more pronounced in the inner than the outer layers of V1; and in the outer compared to the inner layers in the FFA (p<0.05).
These findings are consistent with feedback exchange between deeper and superficial layers, and with apical dendritic amplification being a key mechanism of conscious perception. This work represents a promising step towards characterizing laminar functional profiles for complex human-specific cognitive processes.