Diffusion-weighted imaging (DWI) provides information that allows the estimation of white-matter (WM) fibre orientation and distribution, but it does not provide information about myelin density, fibre concentration or fibre size within each voxel. On the other hand, quantitative relaxation contrasts (like the apparent transverse relaxation, R2∗) offer iron and myelin-related contrast, but their dependence on the orientation of microstructure with respect to the applied magnetic field, B , is often neglected. The aim of this work was to combine the fibre orientation information retrieved from the DWI acquisition and the sensitivity to microstructural information from quantitative relaxation parameters. The in vivo measured quantitative transverse relaxation maps (R and R2∗) were decomposed into their orientation-dependent and independent components, using the DWI fibre orientation information as prior knowledge. The analysis focused on major WM fibre bundles such as the forceps major (FMj), forceps minor (FMn), cingulum (CG) and corticospinal tracts (CST). The orientation-dependent R parameters, despite their small size (0-1.5 Hz), showed higher variability across different fibre populations, while those derived from R2∗, although larger (3.1-4.5 Hz), were mostly bundle-independent. With this article, we have, for the first time, attempted the in vivo characterization of the orientation-(in)dependent components of the transverse relaxation rates and demonstrated that the orientation of WM fibres influences both R and R2∗ contrasts.

No abstract

No abstract

The continuity illusion occurs when visual stimuli are presented at a sufficiently high frequency, thereby triggering a shift from the static to the dynamic vision mode. This facilitates perception of continuous and moving objects, which is key for interactions with the surrounding environment. However, how the continuity illusion is encoded along the entire visual pathway remains poorly understood, with disparate Flicker Fusion Frequency (FFF) measured at the retinal, cortical, and behavioural levels. Here, we combine a behavioural paradigm, functional–MRI (fMRI), and electrophysiological validation for studying the mechanisms underlying the encoding of the continuity illusion effect in the rat. Our behavioural measurements reported a Flicker Fusion Frequency (FFF) of 18±2 Hz. Remarkably, whole–pathway fMRI revealed marked zero–crossings from positive to negative fMRI signal regimes at the FF in the superior colliculus (SC) – an important visual saliency detector – but not in higher cortical or thalamic visual areas. Our electrophysiological recordings in SC explained the sources of these observations as arising from strong neuronal suppression when the continuity illusion is achieved. Combined, our data suggests that activation and suppression balances in SC play a critical role in encoding the continuity illusion effect.

Monocular visual stimulation with short inter-stimulus intervals (ISIs) evokes (i) negative BOLD responses (NBRs) in the contralateral superior colliculus (cSC), and (ii) positive BOLD responses (PBRs) in the ipsilateral superior colliculus (iSC). This pattern suggests a potential "push-pull" mechanism between SCs possibly evoked by (mostly inhibitory) tectotectal projections. Here, we mapped activity in the entire visual pathway using fMRI, and modulated cSC inputs through lesions in visual cortex and SC to dissect collicular communication mechanisms. While, cortical context potentiated a putative “push-pull” mechanism, further silencing of iSC resulted in PBRs in cSC, suggesting that iSC exerts suppression on cSC.