Functional organization of the primary visual cortex (areas 17 and 18) of the tree shrew revealed by optical brain imaging

Abstract

The tree shrew is the closest relative to primates and is considered as an excellent model for investigating the structure-function of the visual system. In recent years, optical brain imaging studies have revealed the presence of a clear modular organization for orientation in area 17, as well as a structured visuotopic map. To date, there is no evidence of the functional organization of areas beyond area 17. The aim of this study was to further document the organization of area 17 and uncover that of the neighboring area 18 by recording intrinsic optical signals in anesthetized animals. The visual cortex was exposed and illuminated at a wavelength of 700nm. Functional maps were recorded either through episodic or periodic paradigms. Drifting sine-wave gratings of varying orientations were presented to acquire orientation and direction maps. Bright bars (100% C) drifting periodically along vertical or horizontal axes were used to uncover the visuotopic organization. Robust orientation selectivity maps were obtained in area 17 with both paradigms. The spatial period of orientation modules was ∼600 microns, a value inferior to that reported in cats and ferrets. Direction modules were sometimes observed but with a low selectivity. High definition visuotopic maps along azimuth and elevation were obtained with the periodic paradigm. The area activated was greater than that observed for orientation as a second cortical zone (2–3mm in width), lateral to area 17, was visible. This zone exhibited a mirror representation of that in area 17 along the azimuth and corresponded to the anatomically-defined area 18 (revealed by Nissl staining). No orientation and directions maps were observed. These results indicate that optical imaging can be used to visualize area 18 and that this area does not exhibit a modular organization such as the one described in area 17. Supp. by NEI grant R01EY016155.