Depth-Dependence of Visual Signals in the Human Superior Colliculus at 9.4T

Purpose/Introduction: The superior colliculus (SC) consists of 7 layers. The most superficial layers receive input mainly from the retina. The intermediate layers contain neurons mostly involved in oculomotor control, and the deep layers contain multisensory and visuomotor neurons [1, 2]. The benefit of UHF is the increase in the blood oxygen level dependent size and the possibility to go for higher resolutions enabling to study layer specific function of the SC. In this study UHF 9.4T MRI is used and the increase in sensitivity allowed us to get functional consistency across subjects with a 10 min paradigm. Subjects and Methods: Functional measurements were conducted in 8 human healthy volunteers at the 9.4T scanner with a 31 channel head coil system [3] Left or right–sided half circles of a checkerboard were shown in a block design (15 s duration). For each subject two sessions of 10 min each were acquired (1 9 1 9 1 mm3, TR: 1 s, TE: 21 ms FA: 53). The images were preprocessed using a modified hemodynamic response [4]. Regions-of-Interest (ROI) of the left and right SC were drawn manually for each subject. We divided the SC in three zones that we called: superficial layer (SL), intermediate layer (IL) and deep layer (DL). Results: We were able to detect signal increases in the left and right SC in most of the subjects (Fig. 1). We also found a lateralization of BOLD responses: right SC is activated when the left hemifield is stimulated (red) and vice versa (blue). This lateralization was consistent with cortical signal. Averages of the beta estimates of the GLM model were calculated within each of the three zones (SL, IL and DL) for each subject (Fig. 2 top plots) and as a group (Fig. 2 bottom plots). The highest signals were localized in the SL of the SC for both right and left colliculi and for both right and left stimulation. The activations encountered in the SC are not present outside the structure, in the CSF, evidencing the localizing capacity of our technique. Discussion/Conclusion: We conducted the first group fMRI study at 9.4T aiming at a functional description of SC in the human midbrain. We demonstrated a response lateralization and depth-dependence of visual signals in the SC in good agreement with its functional architecture known from non-human primates