V1 connections reveal a series of elongated higher visual areas in the California ground squirrel, Otospermophilus beecheyi

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The mouse is fast becoming the model of choice for studies of visual cortex organization due to the animal's genetic tractability (Baker, 2013; Glickfeld, Reid, & Andermann, 2014; Huberman & Niell, 2011) and because the relatively small region devoted to visual processing can make it easier to relate cortical structure to function (Glickfeld, Andermann, Bonin, & Reid, 2013; Marshel, Garrett, Nauhaus, & Callaway, 2011; Wang & Burkhalter, 2007; Wang, Gao, & Burkhalter, 2011). Compared to monkeys, both have a proportionately large primary visual cortex (V1), however the amount of relative space devoted to higher visual cortical areas in mouse is small (Figure 1). This is at least partly due to mice relying largely on somatosensation through vibrissa on the snout (and their sense of smell) resulting in a cortex dominated by the barrel fields representing the vibrissa, (Campi and Krubitzer, 2010; Krubitzer, Campi, & Cooke, 2011; Lyon, 2007; Rosa & Krubitzer, 1999). One major difference between mouse and monkey is that higher visual cortex in mouse is comprised of a series of relatively small visual areas found mainly along the V1 border (Figure 1a; Marshel et al., 2011; Olavarria & Montero, 1989; Wang & Burkhalter, 2007; c.f., Kalatsky & Stryker, 2003; Wagor, Mangini, & Pearlman, 1980), compared to a sequential series of relatively large, elongated cortical areas extending well beyond V1 in monkey, with an additional number of smaller higher visual areas found in posterior parietal and inferior temporal cortex (Figure 1b; Gattass, Lima, Soares, & Ungerleider, 2015; Kaas & Lyon, 2001; Kaas, Roe, Baldwin, & Lyon, 2015; Lyon & Connolly, 2012; Orban, Van Essen, & Vanduffel, 2004; Rosa & Tweedale, 2000).
While comparisons between mouse and monkey can be made, they could be aided by a detailed understanding of visual cortex organization in more highly visual rodents such as squirrels. Squirrels likely share common features of cortical organization with other rodents, including nocturnal mice. One known example is the absence of robust orientation columns in V1 of mouse (Schuett, Bonhoeffer, & Hübener, 2002), rat (Ohki, Chung, Ch'ng, Kara, & Reid, 2005), and squirrel (Van Hooser, Heimel, Chung, Nelson, & Toth, 2005), although a degraded version has recently been shown in mouse (Ringach et al., 2016). Yet, similar to primates, and the close primate relative, tree shrews (Murphy et al., 2001), squirrels are diurnal and have retinas comprised primarily of chromatic‐sensitive cones (80–95%; Jacobs, Tootell, Fisher, & Anderson, 1980; Long & Fisher, 1983), an extremely high density of retinal ganglion cell input to the lateral geniculate nucleus (LGN) and superior colliculus (Johnson, Geller, & Reese, 1998; Major, Rodman, Libedinsky, & Karten, 2003) allowing for higher visual acuity (Van Hooser, Heimel, & Nelson, 2003), and a large expanse of cortex between V1 and S1 (Figure 2; Kaas, Krubitzer, & Johanson, 1989; Krubitzer et al., 2011; Van Hooser & Nelson, 2006), estimated at eight times more than that of a rat (Paolini & Sereno, 1998).
Compared to the number of studies on mouse and monkey (see above), relatively few have examined the organization of higher visual cortex in squirrels (Hall, Kaas, & Killackey, 1971; Kaas et al., 1989; Paolini & Sereno, 1998; Wong & Kaas, 2008). Kaas et al. (1989), for example, described an elongated V2 along the lateral V1 border (Figure 2a) based on myelo‐architecture, connectivity patterns with V1, and limited retinotopic mapping. However, the organization beyond V2 was not clearly defined.
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