The detection of theta-oscillatory waves was performed as previously described (Csicsvari et al., 1999; O’Neill et al., 2006) by filtering the local field potential (5–28 Hz) and detecting Ibrutinib nmr the negative peaks of individual waves. Theta cycles that were detected globally using all electrodes located in CA1 and identified in each learning trial, were used as time windows to calculate

the instantaneous firing rate of the pyramidal neurons and establish a population vector. Each of these vectors during learning was correlated with the corresponding x-y vector representing the same location during the probe session before and after learning. A Fisher z-test was then used to test the null hypothesis that the correlation between the assembly patterns in learning and those expressed in the preprobe was the same as the correlation between the assembly Abiraterone manufacturer patterns during learning and those expressed during the postprobe (Fisher, 1921; Zar, 1999). The z values obtained from this procedure that compares pairs of population vector correlations in each theta cycle allow assessing the ongoing expression of hippocampal

maps: positive values indicate times at which the pyramidal activity patterns preferentially expressed the new cell assemblies developed during learning, while negative values suggest the expression of the old pyramidal assemblies. Standard errors were used when population means were compared. To measure the firing association of interneurons Metalloexopeptidase and pyramidal cells to the expression of pyramidal assemblies, the instantaneous firing rate (IFR, in Hz) of each neuron was calculated during learning for each theta cycles used as time window

for the analysis. Then the association of each cell was measured by calculating the correlation coefficient (Pearson-moment product) between the IFR and the z value of the assembly expression measured in the same window. However, we ensured that each pyramidal cell’s own activity did not influence the assessment of its assembly membership. To do so, we left out that cell from the population vector used for determining which cell assembly was expressed. Using the last 10 learning trials cells that exhibited significant correlations (p < 0.05) were divided by whether they exhibited positive or negative correlation coefficients. The firing associations to the new assemblies were confirmed using a logistic regression between the IFR and the time windows in which the newly-established cell assemblies were present (critical value: α > 1.960) (Zar, 1999). Isolation of monosynaptically-connected pyramidal cell-interneuron pairs were performed as described previously by identifying cross-correlograms between pyramidal cells and interneurons that exhibited a large, sharp peak in the 0.5–2.5 ms bins (after the discharge of the reference pyramidal cells) (Csicsvari et al., 1998).