<p>A<sub>1</sub>, Summary plot of fEPSPs showing associative LTP induced by simultaneous (paired) theta burst stimulation (TBS) of two SC pathways, while prior independent (unpaired) TBS does not cause potentiation (n = 4). Inset, recording configuration. A<sub>2</sub>, fEPSPs and cell ensembles evoked by each pathway (red, green) in one experiment at the times indicated on the summary plot. Scale bar, 0.2 mV and 20 ms. B, Associative LTP significantly increases the overlap ratio (OLR) of the two SC ensembles. B<sub>1</sub>, OLR was measured as the cells common between the two ensembles (SC<sub>1+2</sub>) divided by the total cells in the two ensembles (SC<sub>1</sub> + SC<sub>2</sub> - SC<sub>1+2</sub>, we subtract SC<sub>1+2</sub> in order not to count cells common to both ensembles twice). Summary data plot the increase in total cells (SC<sub>1</sub> +SC<sub>2</sub>) and OLR of the two ensembles normalized to control conditions (n = 4 slices; **, p<0.01). B<sub>2</sub>, Overlay of the two SC-evoked neuronal ensembles (red, green) shown in (A<sub>2</sub>). Yellow cells indicate neurons common to the two ensembles. (C,D) Increasing afferent input by increasing stimulus strength expands the size of cell ensembles but associative LTP causes a greater increase in overlap between two SC ensembles. C, Associative LTP was induced by pairing a weak stimulus (one TBS, black arrow) in one pathway (black traces) with a strong stimulus (four TBS, gray arrow) to the other pathway (not shown). Cell ensembles were measured under control conditions (i), following an increase in stimulus strength (ii), when stimulus strength was returned back to control (iii) and following associative LTP (iv). D, Summary data showing change in total number of cells and OLR relative to control conditions for changes in stimulus strength and associative LTP (n = 3; *, p<0.05).</p
