Saccular activations in the brainstem and the cerebellum (fMRI)

The saccular projections in the brainstem and the cerebellum to the uvula and the nodulus have recently been demonstrated in the macaque. The aim of this fMRI study was to determine whether vestibular evoked myogenic potentials (VEMP) which are a routine diagnostic instrument for sacculus function for the past years can activate areas other than the vestibular nuclei in the brainstem and the cerebellum in humans. If so could one also demonstrate a dominant saccular input from either ear in right-handers? Therefore, the differential effects of unilateral VEMP stimulation (500Hz tone burst signal in the right ear while the left ear was plugged for 26s, 13 runs) on subcortical activation were studied in 18 right-handed healthy volunteers (9M, 9 F; mean age 24 years) lying supine in a clinical 1.5 T scanner wearing MRI-suitable piezo-electric headphones (Jaencke, Zuerich). Each volunteer underwent three randomised sessions: 1. One with a 95 dB 500Hz tone burst signal for which VEMPs had been recorded from the sternocleido-mastoideus muscle outside the scanner before. 2. One control session with a similar sub threshold 65 dB 500Hz tone burst signal which could not trigger VEMPs. 3. The third control trial consisted of a 95 dB white noise signal to differentiate the expected bold responses from the pure loudness of the auditory stimulation. Random effects statistical analysis was done with SPM2 (p<0.005, uncorrected) after motion correction, coregistration to a new specific template for the brainstem and the cerebellum, masking, normalization and smoothing. Unilateral saccular stimulation from the right ear gave activations only in the right dentate nucleus and the nodulus. These could not be seen after stimulation of the left ear. But the VEMP stimulation from either side resulted in a significant deactivation of the declive and culmen bilaterally. In a paired t-test right side vs. left side, which excluded the auditory effects (95 dB click tone bursts vs. 65 dB click tone bursts) significant activations were found in the right dentate nucleus, the nodulus, the right inferior semilunar lobule and the inferior vermis. This is the first demonstration of vestibular otolith activations in humans in the brainstem and cerebellum by means of fMRI. We found a dominance for the saccular input from the right ear. Monaural otolith stimulation from the right side in right-handers caused significant activation in the nodulus and right dentate nucleus and simultaneous deactivations of the culmen and declive.