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David Lyon, Yongjun Liu, Moritz Negwer, Hanjuan Shao, Markus Ehrengruber; Investigating Inhibitory Circuits of Visual Cortex. Journal of Vision 2013;13(9):1262. doi: 10.1167/13.9.1262.
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© ARVO (1962-2015); The Authors (2016-present)
In primary visual cortex (V1), inhibition is essential for several basic functional characteristics of individual neurons, including preferences for stimulus contrast, size, and orientation, that provide the building blocks for object perception. While inhibitory neurons serve to control excitation it is not known whether inhibitory and excitatory cell-types are mediated through the same set of cortical circuits, nor whether the functional selectivity of these circuits differ. This is largely due to technical limitations, since inhibitory and excitatory neurons are intermingled and the study of one population over the other is difficult to achieve. To overcome this, we developed a technique for labeling pre-synaptic inputs to either inhibitory or excitatory neurons that uses viral vectors with cell-type specific promoters and a modified rabies virus. The method labels thousands of connected cells with fluorescent proteins that distinguish them from the inhibitory or excitatory starter cells. Initial experiments reveal that the dominant source of input to both inhibitory and excitatory V1 neurons is local in origin. In addition, we find that long-range intrinsic inputs to inhibitory neurons are as sharply tuned to the preferred orientation as has been shown using non-specific tracers. Third, feedback from higher visual areas is also present, but the percentage of feedback to excitatory cells is much greater (~20%) than found for inhibitory cells (<10%). Together these results suggest that, compared to intrinsic circuits feedback has less direct control over inhibition than excitation. This difference could have important implications for such processes as contextual modulation of V1 cells which is thought to be mediated in different ways through intrinsic and feedback pathways. Because the rabies virus used in this technique can also be engineered to simultaneously deliver genes for optogenetics, future applications can involve the direct manipulation of each circuit to determine their functional impact on target neurons.
Meeting abstract presented at VSS 2013
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