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Characterizing depth-dependent and task-related BOLD modulation in the ventral temporal lobe with ultra-high field fMRI.

From 12/5/2022 to 12/5/2022

Luca VIZIOLI, Medical School University of Minnesota, has given a talk on Zoom on December 5th.


FMRI has become a key tool for human neuroscience. At ultra-high field (=> 7T), recent technological advances permit the acquisition of functional images with unprecedented submillimeter resolutions, allowing the study of the human brain at the mesoscale level. At this scale, we can target some of the most fundamental units of neural computations, such as cortical layers and columns, which historically could only be studied invasively in animals. Previous animal work suggests that cortical layers subserve specific functions, with the middle layers primarily receiving feedforward/bottom-up inputs, and the outer and inner laminae processing feedback/top-down signals.
While several research groups have begun exploiting submillimeter resolutions in fMRI to investigate the response profiles of layers and columns in humans, most of the work focuses on early sensory cortices, such as V1 – where SNR is highest – or M1 – where the cortex is thickest.  Moreover, to balance current imaging acquisitions capabilities against signal-to-noise constraints, many of these studies are carried out using 512 µL voxels (corresponding to 0.8mm isotropic), which can be insufficient to fully characterize mesoscopic functional profiles. In order to exploit the full potential of ultra-high field fMRI, we need to tackle higher level areas as well as early sensory cortices and study processes that are unique to humans, while continuing to push resolution.
To this end, in our current work, we record BOLD responses elicited by face stimuli across 3 cortical depths at 7T in different areas of the visual hierarchy (i.e. V1, Occipital face and fusiform face area - FFA). We push the spatial resolution of our images (from the typically employed 512 µL to the 343 µL used here) and, to assess top-down modulations across cortical depths and areas, we varied the task demands to identical visual stimuli and measured its impact on BOLD responses. To deal with the high levels of thermal noise, we use NORDIC denoising, which suppresses thermal noise without compromising the precision of BOLD responses (Vizioli et al., 2021). Our results indicate that task-related top-down modulations on BOLD responses were larger in the inner compared to the outer layers of V1; and in the outer compared to the inner layers in the FFA.
Our depth-dependent findings are consistent with animal reports of feedback exchange between deeper and superficial layers and with the notion of apical dendritic amplification as a key mechanism of conscious perception. Importantly, the dissociation in task-related top-down modulations across depths between the FFA and V1 indicates that these results cannot be accounted for by neurovascular architecture.
Our approach showcases the potential of “laminar-fMRI” to explore large scale network activity along distant regions of a processing hierarchy. These results represent a promising step towards characterizing laminar functional profiles in humans for complex, cognitively meaningful, and socially relevant stimuli such as faces. 

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