Saccular Projections in the Human Cerebral Cortex

Abstract

The cerebral cortical areas processing saccular information were investigated in human subjects using the fMRI method and loud clicks, which selectively activate the saccule. The results were compared with previous vestibular evoked potential (VEP) studies in anesthetized patients following vestibular nerve stimulation. Nine normal subjects participated in fMRI studies. By comparing the cortical areas activated by a click at 85 dB (auditory activation) with those activated by 102 dB (auditory plus saccular activation), the following cortical areas were selectively activated by saccular stimulation: intraparietal sulcus, frontal eye fields, prefrontal cortex, and postcentral gyrus, in addition to insula, supplementary motor area, and anterior and posterior cingulate cortex. Previous VEP studies also revealed similar activation areas by vestibular nerve stimulation with latencies at 6 ms, suggesting that the shortest pathways for activation of cerebral cortical neurons from the labyrinth are trisynaptic, with a relay in the thalamus. The activated areas are also consistent with results in previous studies using caloric stimulation, which primarily activates horizontal semicircular canals. These results suggest that canal and otolith information is processed largely by similar cortical areas in humans. Multiple cortical areas activated by these studies suggest that these areas are involved in different aspects of processing vestibular information. The saccular projections to the prefrontal and frontal cortex suggest that these areas are involved in planning motor synergies to counteract loss of equilibrium.