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INPUT CONDUCTANCE AT DIFFERENT LOCATIONS WITHIN V1 ORIENTATION MAP.

Dept. Brain & Cognitive Sci, Massachusetts Inst. Tech. (MIT), Cambridge, MA, USA

The mechanisms by which neurons in primary visual cortex (V1) generate orientation selectivity appear to be heterogeneous. Intracellular recordings have shown that subthreshold activity changes with cortical depth and with the position in the orientation map. Precise measurements of inhibitory and excitatory input conductances during visual stimulation have indicated a variety of combinations that may reflect structural inhomogeneities. Here we have calculated the changes in input conductances for neurons at known locations within the orientation map.
Experiments were conducted in adult cats. Optical imaging of intrinsic signals was performed to obtain the precise pattern of orientation domains and singularities (pinwheels). Next, we made whole-cell recordings at different locations in the map. The injection of negative and positive holding currents allowed us to measure changes in total conductance (Gt), excitatory conductance (Ge) and inhibitory conductance (Gi) during visual stimulation with drifting gratings. We selected for analysis 15 domain and 15 pinwheel neurons. Subthreshold activity, but not spike response, was significantly different between orientation domains and pinwheel centers. The averaged tuning curves of cells located at pinwheel centers, when compared with orientation domains, showed: a) a more flat profile, with increase in Gt for all stimulus orientations; b) a larger increase in both

Ge and

Gi for orientations orthogonal to the preferred. At the preferred orientation the mean value of the ratio Gi/Ge was > 1 for both types of neurons, but at orthogonal orientations, the ratio was > 1 for pinwheel cells and

1 for domain cells.
These results indicate that neurons located at specific locations in the orientation map receive very different combinations of excitatory and inhibitory inputs. In particular, inhibition sharpens orientation tuning much more at pinwheels than at orientation domains.
Support Contributed By: NIH Grant EY07023, MECD (Spain) and HHMI

Citation:
J. Marino, J. Schummers, M. Sur. INPUT CONDUCTANCE AT DIFFERENT LOCATIONS WITHIN V1 ORIENTATION MAP. Program No. 818.7. 2003 Abstract Viewer/Itinerary Planner. Washington, DC: Society for Neuroscience, 2003. Online.