The Neural Encoding of Space in Parahippocampal Cortices

the complexity of various models. As proposed by Zilli (2012), an alternative perspective may be to classify models based on the mechanisms they use to encode, update, and read out position locations. A prominent feature of grid cells is their spatial scale, which is organized topographically, increasing progressively from dorsal to ventral medial entorhinal cortex (mEC; Hafting et al., 2005). Grid scale is characterized by two spatial measures; the distance between the grid nodes (spacing) and the size of the grid nodes (field size; Hafting et al., 2005). Soon after the discovery of the topographical organization in grid scale, a series of complementary in vitro studies described a myriad of biophysical properties which also showed a strong dorsal-ventral organization in mEC (Giocomo et al., 2007; Garden et al., 2008; Giocomo and Hasselmo, 2008; Boehlen et al., 2010 and for a review see Pastoll et al., 2012). The correlative changes in entorhinal cells at the systems and cellular level has resulted in several computational models utilizing single-cell mechanisms to determine the organization of grid scale (Hasselmo et al., 2007; Burgess, 2008; Navratilova et al., 2011). Some versions of oscillatory interference models have focused specifically on the in vitro finding that theta band oscillations measured in single mEC neurons decrease in frequency along the dorsal-ventral axis (Giocomo et al., 2007). These oscillatory interference models utilize elements of the intrinsic oscillatory activity, such as the oscillation frequency, to directly determine grid size and spacing. Barry et al. (2012) reviews how a cholinergic decrease in the frequency of theta band oscillations may mediate the expansion of grid scale observed in novel environments (Barry, et al., 2012), an idea supported by in vitro slice recordings showing a decrease in the frequency of the theta band resonance after the application of cholinergic agonists (Heys et al., 2010). However, as an alternative or even complementary mechanism, cholinergic activation may induce a change in grid scale by modulating the temporal spiking or integrative properties of layer II mEC neurons (see Pastoll et al., 2012; Yoshida et al., 2012). Spatial representations are thought to depend not only on grid cells but also on a multitude of other functionally specialized neurons in the parahippocampus. One of these specialized neuronal classes is head direction cells, which respond strongest when an animal faces a particular direction in the environment, irrespective of their behavior or location at the time (Taube et al., 1990; for a review see Clark and Taube, 2012). In mEC, head direction cells exist independently and co-localized with grid cells (Sargolini et al., 2006). This co-localization of head direction signals with grid cells raises the possibility that these The neural encoding of space in parahippocampal cortices