A feed-forward model of spatial and directional selectivity of hippocampal place cells

In recent years a wealth of studies have focused on the role of the Hippocampus in spatial learning and navigation, triggered by the finding of place sensitive cells in this area. These cells have been interpreted as being responsible for coding a representation of space [9, 10]. Even though the term place cell suggests that location is the unique determinant of firing of hippocampal cells, there exist several other factors which influence hippocampal activity. Experimental data have shown that the firing of place cells doesn’t depend on the heading direction when the animal is foraging freely in an open environment. In contrast, on radial mazes or linear tracks the firing of these cells becomes directionally selective [8, 4]. Furthermore, place directionality also appears in open environments when the animal is constrained to follow the same path between points of special significance [7] We have developed a model of hippocampal place cells, which reproduces both spatial and directional selectivity of these cells. In our model, place cells are intrinsically directional (responding to specific local views), and their directionality is reduced during exploration when the rat is not constrained in the direction of its movements (like in open fields or the centre of mazes). We have modelled changes in the place cell directionality as the result of experience-dependent changes (through unsupervised Hebbian learning) in feed-forward connections from a population coding for specific local views. When the animal is able to explore the same location with different head directions, it allows a single post-synaptic cell to be connected to pre-synaptic cells coding for several local views. Therefore, the directional selectivity of that cell is reduced.