Abstract
Ultra-high field fMRI provides an opportunity to examine neural activity across cortical layers informing the directionality of neural signalings, such as feedforward and feedback. The gradient-echo (GE) BOLD signal has limited spatial specificity due to its sensitivity to draining veins at the cortical surface, whereas the spin-echo (SE) BOLD signal is sensitive to small vessels close to neural activity, leading to high spatial specificity. Here, we examined the layer-dependent neural activity of stimulus- and internally-driven representations in the human primary visual cortex (V1) using GE-BOLD and SE-BOLD signals. We acquired GE-BOLD and SE-BOLD signals simultaneously while participants viewed apparent motion (AM) stimuli. The AM stimuli comprised alternating presentations of two gratings whose orientations were orthogonal to each other. We localized regions of interest (ROIs) in V1 corresponding to the stimuli’s location and the mid-point between them, where actually presented and internally interpolated orientations are represented, respectively. The results showed that in the stimulus ROIs, GE-BOLD signal increased toward the superficial layers whereas this trend was less pronounced with SE-BOLD signal. On the other hand, in the internally driven ROIs, responses were absent in GE-BOLD signals whereas significant responses were found across layers in SE-BOLD signals. We also reconstructed orientation represented in each layer using an encoding model. With GE-BOLD signal, we found the highest orientation selectivity in the superficial layers for both ROIs. In contrast, with SE-BOLD signals, we observed distinct layer-dependent orientation responses for different ROIs: in the stimulus ROI, higher orientation selectivity was shown in the middle layers, where feedforward signals are dominant, whereas in the internally driven ROIs, orientation selectivity was higher in the superficial layers, where feedback signals are dominant. Our results suggest that SE-BOLD offers high spatial specificity across cortical layers, which enables us to distinguish internally driven feedback representation from stimulus-driven feedforward representation.