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Kuo-Sheng Lee, David Fitzpatrick; Complex organization of V1 projections to extrastriate cortex in tree shrew. Invest. Ophthalmol. Vis. Sci. 2018;59(9):5850.
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© ARVO (1962-2015); The Authors (2016-present)
The tree shrew has long been recognized as a useful model for early visual system because of its highly developed visual capabilities and close evolutionary relationship to primates. However, fundamental questions about the functional organization of visual areas beyond the primary visual cortex (V1) remain unresolved. The goal of the present study was to use the patterns of anatomical connections to define the number of V1 recipient extrastriate areas in the tree shrew and characterize the modular organization of the projections within these areas.
Tree shrews (Tupaia belangeri, n=14, 2-4 months of age, male and female) were injected with adeno-associated viral vectors (expressing Ruby2sm-Flag, GFPsm-myc, or tdTomato) in V1, either by pressure or iontophoretically at 200-400 µm below the pial surface, followed by a postsurgical survival period of 2-3 weeks. Post hoc immunohistochemistry was performed on the flattened brain sections to enhance fluorescence. Elevation and azimuth dimensions of visual space were mapped by simultaneously tracing axonal projections from 3 different points of V1.
Consistent with previous results from the Kaas lab, we found three distinct areas that receive direct V1 inputs: secondary visual cortex (V2), temporal dorsal area (TD), and temporal posterior area (TP). Unexpectedly, single large injections of tracers in V1 (~ 2 mm) produced a stripe-like organization in V2, with multiple patches separated by unlabeled regions. Smaller triplet V1 injections with different fluorescent markers resulted in 6 separate patches in V2, each patch exhibiting an orderly visuotopic representation. We also observed 5 visuotopically-organized modules in TD.
These observations suggest that individual tree shrew extrastriate cortical areas contain repetitive representations of visual space, similar to the interleaved mapping organization found in in primate V2 (functional stripes), and contrasting with the single visuotopic representation found in the mouse homologue of area V2 (lateral medial area). To further characterize the physiological properties of the extrastriate cortex in tree shrew, we are currently applying two-photon calcium imaging to examine the functional organization of visual features with single cell resolution.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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