FIG. 5.

Synaptic mappings from visual space to dendritic space. A: angular density of optical axes in visual space. The sphere representing the directions of optical axes is color-coded according to the angular optical axis density (in axes/°²) at that point in visual space. Adapted from Krapp and Gabbiani (2005). The solid rectangle is bounded at ± 30° in elevation, and azimuths of –10 and 20°. Arrows (top inset) indicate dorsal, frontal, and lateral directions in visual space. B: summary of the mapping process using Kohonen’s self-organizing map (SOM). The optical axes were projected onto a grid using an SOM (top). A 2nd SOM was used to map equal-length dendritic segments of the LGMD excitatory field onto an identical grid (bottom; same number of dendritic segments as optical axes: 7,322 segments of 0.95 μm each). To obtain the final, topographic map, these 2 grids were superimposed, yielding a direct mapping from optical axes to dendrite (lighter, thicker arrows). C: schematic representation of the three synaptic mappings employed. Top: topographic mapping, which is neighborhood-preserving and employs eye-like sampling of visual space. Middle: uniform mapping, which is neighborhood-preserving but samples visual space uniformly. Bottom: random mapping does not preserve neighborhood relationships but employs eye-like sampling of visual space. Color scale bar in A applies. D: topographic synaptic mapping of optical axes onto the LGMD excitatory dendritic field. Each optical axis was mapped to a dendritic point using the SOMs. The color of each dendritic segment reflects the optical axis density at the corresponding position in visual space from which it receives input. The solid outline delineates the region of the dendritic field receiving input from the visual region likewise denoted in A. Directional arrows indicate dorsal, posterior, and lateral directions relative to the animal (inset). E: synaptic mapping with uniform sampling of visual space. Each region of visual space is sampled by an equal number of dendritic segments. Same color code and region outline as in A.