where \(\left| \cdot \right|\) denotes the number of elements in the set. This "overlapping value" lies between 0 and 1, with large values providing strong evidence that the new spikes in cluster \(\tilde C_k^{t_i}\)(i.e. the ones resulting following the removal of those from the overlapping sets) should be incorporated to the class \(C_j^{t_i}\). From here on, when we say that spikes from a given cluster should be incorporated to a given class, we always assume that all spikes from the overlapping sets have been removed in advance. To remove artifacts, only the clusters with a firing rate higher than 0.03 Hz (computed after removing the spikes from the overlapping set) will be considered for tracking on each block; otherwise, these spikes are discarded.