182602.fig.006
Figure 6: Extension of the context gating model to account for the learning of new combinations of contextual inputs and new attractor states. As well as the contextual and spatial inputs shown in Figure 4, there are also feedback connections from the commissural associational fibre system, which retrogradely depolarise the dendritic tree (shading). In those dendritic branches that also are receiving anterograde contextual depolarisation (solid black dendrite), the depolarisation reaches a level that allows synaptic plasticity to take place. In the example shown here, a weak context input (lemon) is paired with a strong context element (“white”) together with back-propagating dendritic depolarisation (shown by arrows). This level of depolarisation drives two kinds of plasticity—LTP at the weak, conjunctively active “lemon” synapse, and homeostatically scaling LTD at the other active context inputs (including “white”). Now, that dendritic branch will depolarise only following the conjunction of white and lemon. The collective processing of the context elements by the granule cell population will shape the feedback patterns from the commissural association system and allow refinement of the granule cell responsiveness to future contextual situations.