The primary visual cortex (V1) in highly visual mammals is organized into two-dimensional maps of stimulus features including orientation, spatial frequency and visual space. Intrinsic signal optical imaging, which defines maps at a scale of ca 100 um resolution, has revealed an inverse-gradient relationship between certain sets of maps: at regions where the orientation map changes rapidly, spatial frequency and ocular dominance maps change relatively slowly. We have now probed a crucial hypothesis underlying map relationships, that individual cells that are each tuned to multiple features show subtle variations in response tuning on a cell-by-cell basis, consistent with the inverse-gradient principle of multiple maps. Furthermore, we have examined whether additional relationships, such as between the maps of orientation and visual space, exist at a scale below the resolution of intrinsic signal measurements. We used bulk loading of fluorescent calcium indicators and two-photon imaging in ferret V1 in vivo to measure multiple tuning properties of individual, spatially identified, adjacent neurons in the same patch of cortex. By combining calcium imaging with intrinsic signal optical imaging, we targeted injections of calcium indicator specifically to orientation pinwheel centers, as well as to specific locations within spatial frequency maps. We find that neurons with the same preferred spatial frequency cluster in a columnar way, and different spatial frequency columns separated by less than 100 um can be clearly distinguished. Furthermore, the orientation and spatial frequency maps measured at single cell resolution by calcium imaging match closely the maps obtained by intrinsic signal imaging. At orientation pinwheels, where the preferred orientation of neurons changes rapidly, preferred spatial frequencies of the same group of neurons are very similar and show very little scatter. At spatial frequency column borders, where preferred spatial frequencies change rapidly, preferred orientations change very little. Thus, the inverse-gradient relationship between orientation and spatial frequency representations indeed exists at the level of single cell responses. Retinotopic mapping and receptive field progression is evident across cells spread over 100 um of cortical distance. Receptive field centers have small but significant jumps near pinwheel centers, yet at locations distinct from them. Such sites are seen consistently at multiple depths of superficial visual cortex. Thus, the retinotopic map also appears to have a fine-scale relationship with the orientation map.

Supported by EY07023 and EY017098.