A cortical-depth-dependent analysis of bottom-up and top-down driven fingertip maps in human somatosensory cortex measured at 7T

Functional MRI (fMRI) at ultra-high field (7T) is more frequently being used to resolve fine detail on and across the cortex. Whereas most layer/laminar fMRI studies probe the degree of activation at varying depths by averaging all responses at a certain depth, far less work has been conducted to assess the consistency of the spatial distribution of those responses across depth. To address this, we performed a depth-dependent analysis of bottom-up- and top-down-driven somatotopic digit maps in human S1. Maps were generated via phase encoded vibrotactile stimulation (sensory condition; bottom-up maps), or by sweeping attention across fingertips using the Attentional Drift Design (attention condition; top-down maps). High resolution anatomical (MP2RAGE; 0.5mm) and functional BOLD (3D-EPI; 0.8mm) imaging data were acquired using Siemens’ Magnetom 7T scanner. We segmented the anatomical data to generate a family of surfaces at specific cortical depths, interpolated functional data onto them, and applied within-layer smoothing – before finally running delay analyses for both experimental conditions. Our findings in S1 are in line with previous work in visual cortex examining how gradient echo EPI responses vary across depth with the strongest and most spatially spread responses being found most superficially. Notably, smoothing data tangential to the surface was effective in reducing variance in the maps, particularly for the deepest cortical depths, while maintaining each digit representation.