Time Courses of Cortical Vestibular Responses to Caloric Stimulation (fMRI)

The multisensory vestibular cortical circuit on the basis of monkey experiments as well as human functional brain imaging studies includes the posterior insula, central sulcus, superior temporal gyrus, and inferior parietal lobule with the intraparietal sulcus. The aim of this fMRI study was to determine whether one can differentiate the time course of activation of different vestibular and ocular motor areas. Ocular motor areas were considered since vestibular stimulation induces not only the perception of being rotated or tilted but also an ocular motor response, e.g. a vestibular nystagmus. The effects of caloric vestibular stimulation (right ear irrigation with 16°C water for 30s, 4 runs) on cortical activation were studied in 9 right-handed healthy volunteers. The protocol included 372 volumes, each consisting of 40 slices of a T₂*-weighted EPI sequence in alternating blocks of ten images at rest (eyes open in the dark), ten during caloric stimulation, and 60 images after caloric irrigation. Statistical analysis was done with SPM2 (p<0.001, uncorrected). Horizontal nystagmus was recorded by the MR-Eyetrack System while the subjects kept their eyes open in a completely darkened room. Caloric irrigation induced a direction-specific sensation of rotation and nystagmus away from the stimulated ear. Significant activations were found in a cortical network within both hemispheres but predominantly in the right. This network included the superior frontal gyrus/sulcus, the precentral gyrus, the superior temporal gyrus, the inferior and superior parietal lobule with the supramarginal gyrus, the precuneus, as well as the posterior and anterior insula. Analysis of voxel intensity over time showed that activation within the PIVC began immediately after the start of irrigation and reached its maximum about 16s later on the ipsilateral right hemisphere but 32s later on the contralateral left side (mean values of single subject analysis). Activation peaks of the areas in the superior temporal gyrus occurred about 33s after the irrigation onset on the right and after 45s on the left hemisphere. This is the first demonstration by means of fMRI that monaural vestibular stimulation causes bilateral vestibular cortex activation with different latencies. Activation peaks in the insular-temporal cortex seem to occur earlier within the right hemisphere during irrigation of the right ear in right-handers than in the left hemisphere.