Depth-dependence of visual signals in the human superior colliculus: Comparison between 3T and 9.4T

The SC is composed of 7 layers where the most superficial layers process visual information. The intermediate layers contain neurons involved in oculomotor control, and the deep layers contain multisensory and visuomotor neurons. Until now there were several studies which demonstrated reliable activation in the SC after visual stimulation at 3T. In this study we investigated the benefits of using 9.4T MRI for humans to study small brain structures in the brainstem like the SC in a comparison with 3T data. Left or right-sided half circles of a flickering checkerboard (8Hz) were shown in a block design interleaved with fixation baseline periods with a duration of 15s. For the 3T measurements each participant underwent three experimental runs, in which each condition was repeated 10 times. For the 9.4T measurements each participant underwent two experimental runs, in which each condition was repeated 20 times. Regions-Of-Interest (ROI) of the SC were drawn manually for each subject considering specific anatomical landmarks. The SC was then divided in three zones: superficial zone (SZ), intermediate zone (IZ) and deep zone (DZ). With the data acquired in the 9.4T we were able to detect signal increases in the left and right SC and lateralization of BOLD responses consistent with the cortical signals in each subject. With the 3T data the same results were only apparent in a group analysis. Averages of the beta estimates of the GLM models were calculated within each of the three zones (SZ, IZ and DZ) for the 3T and the 9.4T data. For the 9.4T data the highest signals were localized in the SZ of the right and left colliculi as expected. Activations encountered in the SC were not present outside the structure (CSF) evidencing the localizing capacity of our technique. For the 3T data there is also a higher value for the SZ compared to the other zones. However, this difference was less clear compared to the results obtained for the 9.4T data. In summary, despite of several technical issues, e.g. larger B1 inhomogeneities and distortions, we demonstrated the feasibility of a functional mapping of the human brainstem with UHF neuroimaging and found a substantial benefit of the 9.4T in comparison with the 3T for the functional mapping of upper brainstem structures