Whole-Brain Functional Ultrasound Imaging Reveals Brain Modules for Visuomotor Integration

Abstract

Large numbers of brain regions are active during behaviors. A high-resolution, brain-wide activity map could identify brain regions involved in specific behaviors. We have developed functional ultrasound imaging to record whole-brain activity in behaving mice at a resolution of ∼100 μm. We detected 87 active brain regions during visual stimulation that evoked the optokinetic reflex, a visuomotor behavior that stabilizes the gaze both horizontally and vertically. Using a genetic mouse model of congenital nystagmus incapable of generating the horizontal reflex, we identified a subset of regions whose activity was reflex dependent. By blocking eye motion in control animals, we further separated regions whose activity depended on the reflex's motor output. Remarkably, all reflex-dependent but eye motion-independent regions were located in the thalamus. Our work identifies functional modules of brain regions involved in sensorimotor integration and provides an experimental approach to monitor whole-brain activity of mice in normal and disease states.

Figure 1

Whole-Brain Functional Ultrasound Imaging in Behaving Mice (A) Schematic of the chronic cranial window (top view). The head bar is attached to the skull by a ring of dental cement. The craniotomy is stabilized by a layer of agar and closed by a plastic window, transparent to ultrasound. (B) Schematic of the chronic cranial window (coronal section) and of the ultrasound probe during imaging. Ultrasound gel is applied on top of the window, and the probe is positioned 3 mm above the window. (C) Schematic of the experimental setup, with an awake head-fixed mouse. Hemodynamic signals are measured through the chronic cranial window using the ultrasonund probe, stepped along the antero-posterior axis (black arrow). Three example image planes are labeled (numbered dashed lines). (D) Example coronal slices from a 3D anatomical map at three different planes (dashed lines in C). L, left; R, right. (E) Segmented brain regions (yellow outlines) after 3D registration of the anatomical map to the Allen Mouse Brain Reference Atlas overlaid on the same slices as in (D). L, left; R, right. (F) Schematic of the experiments for activity mapping in mice during the optokinetic reflex. Drifting gratings (0.05 cpd spatial frequency, 10°/s speed, an example direction is indicated by the black arrow) are presented binocularly on two visual displays. A camera records left eye position. The ultrasound probe is stepped across the brain. Three example imaging planes crossing the primary visual cortex (magenta) are labeled (numbered dashed lines). (G) Example coronal slices from a 3D activity map at three different planes (dashed lines in F) encompassing the primary visual cortex. Hemodynamic signals during visual motion stimulation relative to the signals during the presentation of a static gray background image—quantified as Z-scores—are color coded (bottom) for all voxels above a significance threshold and superimposed on the anatomical map (gray scale). Left and right primary visual cortices are outlined in white. L, left; R, right. (H) Top: single-voxel response curves in the primary visual cortex (3 examples). Middle: mean response curve of the primary visual cortex (across voxels from left and right hemispheres). Bottom: mean eye speed curve. Dashed lines indicate start and end of drifting grating stimulus. All signals are averaged across trials and the four motion directions (top, gray arrows). Same animal as in (G). (I) Schematic of the experiments for retinotopic mapping along the elevation axis. A horizontal bar (20° wide, corrected for planar distortion) is swept across a display monitor in front of the right eye (4°/s). Left eye is covered (red cross). Three example imaging planes crossing the primary visual cortex (magenta) and superior colliculus (cyan) are labeled (numbered dashed lines). See also Figure S3 for the azimuth retinotopic map. (J) Example coronal slices from a 3D elevation retinotopic map at three different planes crossing the visual cortex and superior colliculus (dashed lines and color scale in I) superimposed on the anatomical map (gray scale). Superior colliculi and primary visual cortex are outlined in white. L, left; R, right. (K) 3D elevation retinotopic map in the visual cortex. The position of the bar in the visual field eliciting the peak response is color coded (color scale in I) and superimposed on the anatomical structure. Gray surface shows the 3D outline of the cortex. V, ventral; L, left; P, posterior. (L) 3D elevation retinotopic map in the superior colliculus. The position of the bar in the visual field eliciting the peak response is color coded (color scale in I) and superimposed on the anatomical structure. Gray surface shows the 3D outline of superior colliculi. V, ventral; L, left; P, posterior.

Figure 2

Brain Regions Activated During Visual Motion Eliciting the Optokinetic Reflex (A) Top: coronal slices from the mean 3D activity map in wild-type mice (n = 12 mice). Stimulus: drifting gratings. Z-scores (per voxel) are color coded (color scale, bottom) and superposed on the anatomical map of one animal. All voxels significantly active across mice are displayed (p < 0.05, t test). Bottom: 3D views of the same data, presented within a “ghost” surface reference brain in gray (note that the “ghost” is larger, in both rostral and caudal extremities, than the field [6 mm] imaged with functional ultrasound). See also Video S1. (B) Left: standardized responses of the 181 imaged brain regions, ordered by major brain structures (color scale, bottom). Black thick line, presentation of drifting gratings. Responses are averaged across trials, the four directions of motion (top, gray arrows), and mice (n = 12). Black arrows point to the six regions shown in (C). Right: p values displayed on a log scale. Red circles indicate the 87 active regions (p < 0.001, generalized linear model followed by t test). See also Table S1 for the names of the regions in the same order. (C) Response curves of the six regions indicated in (B) (black arrows). Curves are averaged across trials and the four directions of motion (top, gray arrows). Thick line, mean over the 12 mice; widths of gray bands, 2 × SD.

Figure 3

Functional Ultrasound Signals Reflect Modulation of Local Spiking (A) Schematic of spike recordings with a multielectrode probe positioned in the primary visual cortex (left) or in the amygdala (right) during visual motion eliciting the optokinetic reflex. See also Figure S5. (B) Standardized spike rate for all neurons recorded in each region (n = 3 mice). Black thick line, presentation of drifting gratings. Gray arrows, axis of visual motion. (C) Spike rate averaged across all neurons with significant responses in the primary visual cortex (red) and amygdala (blue). Thick line, mean; widths of colored bands, 2 × SD.

Figure 4

Clusters of Brain Regions Respond Differentially to Horizontal and Vertical Visual Motion (A) Normalized responses to horizontal visual motion (left panels, average of left and right motion) and vertical visual motion (right panels, average of up and down motion) of the 87 active regions (color scale, bottom) clustered into three sets. Axis of motion, gray arrows on the top. Black thick line, presentation of drifting gratings. The mean response curve of the cluster is shown below each cluster. (B) Scatterplot of response amplitudes (n = 12 mice) to horizontal and vertical motion for each active region. Each circle represents a brain region, colored according to its cluster (color codes, in legend). Dotted lines, axis of origin lines and unity line. (C) Distribution of the three clusters within major brain structures.

Figure 5

Brain Regions Associated with the Optokinetic Reflex (A) Schematic illustrating retinal direction selectivity and eye motion in wild-type and FRMD7^tm mice during horizontal visual motion. Black arrows, directions of visual and eye motions; red crosses, directions that lack retinal direction selectivity or tracking eye motion. (B) Scatter plot of response amplitudes to horizontal visual motion in FRMD7^tm mice (n = 4 mice) and wild-type mice (n = 12 mice) for each active region. Responses are averaged across trials, two directions of motion (rightward and leftward), and mice. Each circle represents a brain region, colored according to its cluster (color code, in legend). Dotted lines, axis of origin lines and unity line; gray arrows (top), axis of visual motion. (C) Normalized responses of the 87 active brain regions to horizontal visual motion for wild-type mice (left panels) and FRMD7^tm mice (middle panels; color scale, bottom) grouped by their cluster (as in Figure 4A). Responses in wild-type and FRMD7^tm mice were normalized by the same values. Black thick line, presentation of drifting gratings. The mean response curve of each cluster is shown in the right panels (black, wild-type; gray, FRMD7^tm mice). Dotted lines, start and end of drifting gratings; gray arrows (top), axis of visual motion. (D) Schematic illustrating retinal direction selectivity and eye motion in wild-type and FRMD7^tm mice during vertical visual motion. Black arrows, directions of visual and eye motions. (E) Scatter plot of response amplitudes to vertical visual motion in FRMD7^tm mice (n = 4 mice) and wild-type mice (n = 12 mice) for each active region. Responses are averaged across trials, two directions of motion (upward and downward), and mice. Each circle represents a brain region, colored according to its cluster (color code, in legend). Dotted lines, axis of origin lines and unity line; gray arrows (top), axis of visual motion. (F) Normalized responses of the 87 active brain regions to vertical visual motion for wild-type mice (left panels) and FRMD7^tm mice (middle panels; color scale, bottom) grouped by their cluster (as in Figure 4A). Responses in wild-type and FRMD7^tm mice were normalized by the same values. Black thick line, presentation of drifting gratings. The mean response curve of each cluster is shown in the right panels (black, wild-type; gray, FRMD7^tm mice). Dotted lines, start and end of drifting gratings; gray arrows (top), axis of visual motion. (G) Example of eye movements during the optokinetic reflex evoked by visual motion in a horizontal direction (top, gray arrow). Horizontal eye position (y axis) in a single trial shown in a wild-type (top, black) and a FRMD7^tm (bottom, gray) mouse. Gray rectangles, presentation of drifting gratings. Inset (top left): example image of the left eye from the eye-tracking camera. White region in middle, pupil. (H) Mean tracking eye speed over time in wild-type (top, n = 12 mice) and FRMD7^tm (bottom, n = 4 mice) mice during the optokinetic reflex. Thick line, mean; gray band, 2 × SD; dotted lines, start and end of drifting gratings. Direction of visual motion, gray arrows on top. Small responses indicated by green arrows are pupil dilation artefacts. (I) Bar plot (mean ± SEM) of correlation coefficients between paired responses of the three clusters and eye speed of wild-type mice (n = 12 mice).

Figure 6

Direction Selective Responses in the Nucleus of the Optic Tract (A) Schematic illustrating the logic behind hemisphere-asymmetric responses of the nucleus of the optic tract (NOT). Leftward visual motion (blue arrow) stimulates temporal-to-nasal-tuned retinal direction-selective ganglion cells in the right eye, which then stimulate the NOT in the left hemisphere. The same logic holds for rightward visual motion (orange arrow). (B) Map of correlation coefficients of single-voxel responses with leftward and rightward stimulus for an example wild-type mouse (color scale at the bottom; gray arrow, direction of visual motion) superposed on the anatomical map. White dotted lines, outline of NOT. L, left; R, right. (C) Asymmetry index of the 87 active regions in wild-type mice (black, wild-type; gray, FRMD7^tm mice). Dotted line, mean + 3 × SD of values in wild-type mice. Red arrows, NOT.

Figure 7

Functional Modules Activated during Visual Motion Eliciting the Optokinetic Reflex (A) Schematic illustrating retinal direction selectivity and eye motion in FRMD7^tm and eye-block mice during horizontal visual motion. Black arrows, directions of visual and eye motions. Red crosses, directions that lack retinal direction selectivity or eye motion. (B) Scatterplot of the responses of brain regions (quantified by Z-scores) during stimulation with horizontal visual motion in FRMD7^tm (n = 4) and eye-block (n = 4) mice. Each circle represents a brain region. Black lines, threshold (p = 0.001, Bonferroni corrected) for defining a brain region as active or inactive. Colored panels indicate the three modules defined by the two threshold lines: RI, RDMD, and RDMI. One region from the RI module is out of range (x = 27.8, y = 10.8). (C) Standardized responses during horizontal visual motion in both FRMD7^tm and eye-block perturbations of the optokinetic reflex (color scale, bottom). Left pair of panels, regions are organized according to clusters (as in Figure 4A). Right pair of panels, regions are organized according to functional modules, defined in (B). Lines between the left and right pairs of panels link the position of each brain region across the two classifications. Black arrows indicate the regions from the RDMI module. Black thick lines, presentation of drifting gratings. Gray arrows (top), axis of visual motion. (D–F) Top: distribution of brain regions of the RI (D), RDMD (E), and RDMI (F) modules in major brain structures. Bottom: coronal slices showing the spatial distribution of each module region in the reference brain (from bregma + 0.1 mm to bregma + 5.6 mm, 500 μm steps).