P ARTÍCULAS COMERCIALES DE CIRCONA ESTABILIZADA CON ITRIA

Beyond the neurophysiological aspects of primary cells in the hippocampus, the idea of how animals represent or learn about their environment and the objects within it goes back to the prominent work of Tolman and his colleagues in 1930 (Best, White, 1999). Contrary to the general belief of Stimuli-Response (S-R) or Stimuli-Stimuli (S-S) learning, Tolman proposed that animals may have an allocentric map based representation of their world called a cognitive map. In his words (Tolman, 1948):

‘We believe that in the course o f learning, something like a field map o f the environment gets established in the rat's brain. '

pp 192 The idea of the cognitive map had been the subject of controversy until a major paradigm shift by the work of O'Keefe & Dostrovsky (1971), which was the discovery of place cells.

Their prominent work provided a solid foundation for a new approach for studying the brain’s behaviour and also put forward the idea that place cells are the fundamental element of a non-centred (allocentric) and distributed representation of the environment. Place cells, head-direction cells, grid cells and border cells are components of a system that seems to integrate the trajectory of the animal pathway and stores landmarks to be used for correcting path integration drifts. This storage of the allocentric map of the environment can support the cognitive map theory (Best, White, 1999; Fuhs, Touretzky, 2000; McNaughton et al., 1991;

Muller et al., 1996; Redish, 1999; Redish, Touretzky, 1997; O'Keefe, Dostrovsky, 1971;

O'Keefe, Nadel, 1978). However, it is missing some components, such as a mechanism for relating different positions (McNaughton et al., 1991). Although in some models of path integration (Will be studied in the next chapter) there is a relational dependency between cells representing positions in the environment, the mechanism by which animals relate to their position is unclear.

The discovery of grid cells in addition to the functional and anatomical properties of the grid cells have resulted in a shifting the focus of navigation studies from the hippocampus to other brain areas, which is contrary to the original idea that the hippocampus is the locus of the cognitive map theory. Moreover, it seems that spatial representation is only one of the functions of the hippocampus (Eichenbaum et al., 1999; Buzsaki, 2005; Leutgeb et al., 2005).

Nonetheless, the theory still seems to find more solid support compared to the previous ideas of navigation (Best, White, 1999). Moreover, it may have gained even more support in terms of metric positioning of tlie environment with the fact that grid cells in the entorhinal cortex have a type of relational dependency (Saigolini et al., 2006b). Therefore, the cognitive map theory could be still valid regardless of the anatomical location of such a map in the brain.

Contrary to eaiiier hypothesis, it seems that a group of brain structures contribute to construct a map like representation of the environment and in this thesis we have considered the cognitive map theory as a hypothetical form of representation of space in .the brain.

Therefore, when we discuss the biological spatial navigation in the brain we seek for form of representation that allocentric, integrates path to recognise positions and uses environment cues to correct the accumulated error in path integration.

2.11 Summary

Anatomical properties of the hippocampus and entorhinal cortex (especially the medial entorhinal cortex) which are constituents of a system most likely to represent spatial information in the brain were discussed in this chapter. The principal cells and the information pathways between several elements of this system were explained. As Figure 2-2 shows, the information flow between the entorhinal cortex and the hippocampus suggests that the source of spatial information in the hippocampus is the entorhinal cortex. In this chapter, we also explored the firing pattern properties of place cells, head direction cells, grid cells and recently discovered border cells. Head direction cells signal the orientation of an animal and it is bound to the visual cues in the environment. They reside in the subiculum areas which have direct projections to the hippocampus and medial entorhinal cortex. The medial entorhinal cortex hosts head direction cells too, which most possibly derive their activities from head direction cells in other brain areas. Also, the medial entorhinal cortex includes grid cells which tessellate an environment with periodic hexagonal firing pattersn which are also visual cue bound. The computational models which will be discussed in the following chapters suggest that the orientation of grid cells could be directly obtained from head direction cells. A combination of grid cells, head direction cells and border cells could give input to the monotonie firing of place cells at the hippocampus. These components describe how an allocentric spatial representation of the environment could be used to model navigation.

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The missing components are those which represent the visual or other surrounding cues that could conect the accumulated errors in head direction cells, grid cells and place cells. A possible candidate for the existence of such information is the lateral entorhinal cortex which has projections to the subiculum, the medial entorhinal cortex and the hippocampus, while receiving strong input from the neocortex, which is believed to have highly processed stimuli information (O'Keefe, Burgess, 2005). Also, the lateral entorhinal cortex exhibits strong functional differences to the medial entorhinal cortex (Hargreaves et al., 2005). These missing components and their effect on creating an spatial representation of the environment will be discussed in the next chapter.