Emergent properties of the optic tectum revealed by population analysis of direction and orientation selectivity

Abstract

How local circuits within the brain process visual information has classically been addressed at the single neuron level. Such reductionist approaches, however, struggle to capture the full scope of functional properties associated with even "simple" brain nuclei. Using population functional calcium imaging, we aim to describe how local circuits within the zebrafish optic tectum process visual information. Specifically, how are previously identified direction-selective (DS) and orientation-selective (OS) retinal ganglion cell (RGC) inputs (Nikolaou et al., 2012) represented in tectal cells? First, we identify an emergent population of DS tectal cell with a direction preference not explicitly present in any one of the RGC inputs. Second, this is associated with a striking shift from a tiled and triangular representation of directional space (RGC inputs) into an overlapping cardinal representation by tectal cell populations. Third, and in contrast, we find that orientation space is represented similarly in both the RGC input and tectal cell populations illustrating feature-dependent differences in how tectal circuits process their inputs. Finally, we identify OS and two populations of DS cells at the superficial border of the tectal neuropil, one of which is an emergent population. This study, together with our previous one (Nikolaou et al., 2012), demonstrate that direction-selectivity is established in both the retina and tectum.

Figure 1.

Functional imaging of tectal cells. Ai, Schematic showing dorsal view of zebrafish tectum. Arrow indicates visual stimulation. Np, Tectal neuropil. Red box indicates imaged region. Aii,Direction of motion of bars relative to fish body axis. B, Montage showing responses of tectal cells to drifting bar stimulus. Direction of motion is shown bottom right in each panel. Voxel brightness is scaled according to response integral during each stimulus epoch. Red lines show approximate borders of the PVN region and neuropil. Red arrowhead indicates cells at the superficial neuropil. DS (Ci) or OS voxel (Cii) aggregations overlaid on an OGB fluorescence image of a single tuning experiment, color-coded according to median preferred angle (DS) or median complex angle (OS). White arrowheads, PVNs; red arrowheads, superficial neuropil cells. Color scales to right of images. Sagittal view of DS cells (Di) and OS (Dii) cells within standard tectal space color-coded as in C. RGC axons labeled with tagRFP (gray). Arrowheads as in C; green arrows, RGC axons; asterisks, skin surface. Approximate functional imaging plane of the experiment in B is shown as a dashed line in bottom left inset of Di. Scale bar, 20 μm.

Figure 2.

Re-encoding of direction space by periventricular neurons. Ai, Cumulative histogram of summed vector angles for DS-PVNs with four summed von Mises distributions fitted to the data (R² value relates to summed von Mises distributions). Colored arrows represent population peaks centers; 8°, 89°, 169°, and 261°. Each population is defined by ±2× bandwidth of each distribution; dashed lines. Aii, Summed distributions of previously identified DS-RGCs (red) and DS-PVNs (blue) populations; areas are normalized to populations tuned to caudorostral motion. Note population tuned to 89°, not present in the RGC population; asterisks, Welch's t test, p < 0.05 . B, Bar chart showing the relative populations of DS-RGC inputs and DS-PVNs color-coded according to population, normalized to the sum of the three populations that are present in both PVNs and RGCs (dashed line). n = 388 total number of identified DS-PVNs within all populations. C, Normalized responses of DS-RGCs (left) and DS-PVN (right) populations; mean (solid line) and dashed (±1 SD). Response plots within boxes are single, representative voxels (DS-RGCs) or cells (DS-PVNs) within each population. D, Cumulative histogram of summed vector angles for DS voxels of Tg(elavl3:GCaMP5) fish treated with 50 μm APV and 10 μm NBQX with three summed von Mises distributions fitted to the data. n = 7 fish.

Figure 3.

Direction-selective cells at the superficial neuropil. Ai, Cumulative histogram of summed vector angles for DS superficial cells- population peak centers; 21° and 126° (solid red curves). Aii, Summed distributions of previously identified DS-RGCs (red) and DS superficial cells (blue), areas are normalized to the most coherent population. Note population centered at 126° that is not present in the RGC population ; asterisk, Welch's t test, p < 0.05. B, Normalized responses of the two populations of DS-superficial cells; mean (solid line) and dashed (±1 SD). Response plots within box are single, representative DS superficial cells within each population.

Figure 4.

OS-PVN populations. Ai, Cumulative histogram of complex angles for OS-PVNs with four summed von Mises distributions fit to the data (red curves); colored arrows represent populations centered at 16°, 86°, 141°, and 174°. Each population is defined by ±1.5× bandwidth of each distribution; dashed lines. Aii, Summed distribution of previously identified populations of OS-RGCs (red) and OS-PVNs (blue), area normalized to area of all populations. B, Cumulative histogram of preferred complex angles for OS-superficial cells. Scale bar, 20 μm.