Widespread vestibular activation of the rodent cortex

Abstract

Much of our understanding of the neuronal mechanisms of spatial navigation is derived from chronic recordings in rodents in which head-direction, place, and grid cells have all been described. However, despite the proposed importance of self-reference information to these internal representations of space, their congruence with vestibular signaling remains unclear. Here we have undertaken brain-wide functional mapping using both fMRI and electrophysiological methods to directly determine the spatial extent, strength, and time course of vestibular signaling across the rat forebrain. We find distributed activity throughout thalamic, limbic, and particularly primary sensory cortical areas in addition to known head-direction pathways. We also observe activation of frontal regions, including infralimbic and cingulate cortices, indicating integration of vestibular information throughout functionally diverse cortical regions. These whole-brain activity maps therefore suggest a widespread contribution of vestibular signaling to a self-centered framework for multimodal sensorimotor integration in support of movement planning, execution, spatial navigation, and autonomic responses to gravito-inertial changes.

Keywords: evoked potential; fMRI; rat; self movement; spatial navigation; vestibular cortex.

Figure 1.

VN stimulation during fMRI and LFP recordings. A, Still images from video recordings of the ipsilateral and contralateral eye before (i), during (ii), and after (iii) VN stimulation. The pupil, marked with a black ellipse, was tracked manually (iii) in NIH ImageJ Manual Tracker. B, Distance (in pixels) of the pupil from its start location. The time of the still images is marked. C, Time course of BOLD signal of the whole recorded volume and simultaneously recorded physiological parameters during VN stimulation. The nerve was stimulated for 6 s every 32 s with 0.1 ms pulses delivered at 333 Hz. D, LFPs recorded 1300 μm deep in retrosplenial cortex after VN stimulation (gray shaded area) of different intensities. Bottom, Electrooculograms (EOG) recorded from the contralateral eye. Averages of 48–52 trials. The different colors indicate different stimulation intensities expressed in multiples of eye-movement threshold determined at the beginning of the experiment (100 μA). E, LFP amplitude plotted against stimulation intensity (data from D). The dotted line represents the eye-movement threshold.

Figure 2.

VN stimulation triggered BOLD maps. A, Example of BOLD maps from a representative animal, thresholded (p < 0.001, uncorrected) and overlaid on coronal T2-weighted (RARE) anatomical scans, showing active brain areas during VN stimulation. Numbers on the images refer to approximate distances from bregma in millimeters. vc, Brainstem vestibular complex; lmn, lateral mammillary nucleus. B, Average BOLD map (n = 5; p < 0.05, corrected) during VN stimulation. Two horizontal slices are shown at the cortical surface level (top) and 4 mm below (bottom). Brain-wide average BOLD maps across all imaging slices and several statistical thresholds are presented elsewhere (see Notes). C, Calculated fMRI scores based on ROI analysis for brain areas in four functional groups. HD, Head-direction related. Abbreviations according to Table 1.

Figure 3.

Quantitative description of the fMRI results. Number of active voxels, percentage of active volume versus whole ROI volume, BOLD amplitude, and normalized fMRI score for all brain areas containing significant VN stimulation-evoked activity (p < 0.05, corrected). The areas have been grouped according to their function. The dotted line represents the active volume threshold applied. Areas below threshold are represented by gray bars in the fMRI score graph. Abbreviations according to Table 1.

Figure 4.

VN stimulation-evoked electrophysiological signals. A, Coronal brain slice 3.0 mm posterior to bregma, showing recording electrode track (DiI, red) over a DAPI (blue) background. B, Evoked LFPs (gray, individual trials; red, average) recorded from 1200 μm deep in primary somatosensory cortex (white circle in A) after VN stimulation (dark gray bar). Average of 55 trials. C, Peristimulus time histogram showing evoked multiunit activity after VN stimulation (dark gray bar). Bin size, 25 ms. Scale bar, 0.5 spikes per trial. Inset, Two hundred individual spikes (blue) and average spike waveform (red) of multiunit signals. Same recording as A and B. D, Normalized evoked LFP scores from several different brain regions. The color coding shows the amplitude of the first negative peak in the LFP. Abbreviations according to Table 1.

Figure 5.

Quantification of evoked LFP responses. The first column shows the probability of evoking LFP responses at three times the eye movement threshold. The second and third columns show the amplitude and time from stimulus onset of the negative peak of the evoked LFP signal, also at three times the eye-movement threshold. The fourth column shows the normalized stimulation threshold for evoking LFP responses expressed in eye-movement threshold units. Error bars denote SEM. Recorded brain areas have been grouped according to their function; the number of observations is in parentheses. No statistical comparisons between areas were made. Abbreviations according to Table 1.

Figure 6.

Examples of evoked LFP and multiunit spiking responses. Average (n = 45–60 trials) evoked LFP responses after VN stimulation (gray bar) at different stimulation intensities expressed in eye-movement threshold (numbers on the left). Bottom right, Peristimulus time histograms of multiunit spiking activity. Gray bar, Stimulation time. Bin size, 50 ms. Recordings are examples from six brain areas. Asterisks denote significant evoked activity. VPM, Ventral posteromedial nucleus.

Figure 7.

Functional connectivity fingerprint in response to VN stimulation. The average BOLD signal across all voxels in particular brain regions (ROIs) modulated by vestibular stimulation was first calculated and then cross-correlated between all ROIs. The strength of the functional coupling between ROIs, measured as the absolute correlation coefficient and the temporal aspects of the response, measured as phase shifts between the BOLD signals, were first calculated for each subject and the averaged across the population (n = 5). The combined strength (yellow-red scale) and phase (blue-red scale) connectivity matrix is presented. Abbreviations according to Table 1.