Scenario ii: simulated ictal source with evolving frequency at SNR = 0.9. (a) CPD decomposition. The frequency signature (F1) of the first component, corresponding to the ictal source, shows a single peak at 6 Hz, i.e. at the average of the start and end frequency. (b) BTD decomposition. The spatial mode of the BTD components were set to be rank-1, while the frequency and temporal modes were set to rank-2. Therefore, this block component comprises the spatial signature S1, the frequency signatures F1a and F1b and the temporal signatures T1a and T1b. The frequency signature F1a and F1b, corresponding to the ictal source, represent a spectrum peaking at 4 and 7 Hz, respectively. From the corresponding temporal signatures, one can deduce that the ictal pattern is slowing down, as T1a gains amplitude towards the end. (c) The time×frequency matrix obtained with CPD. No frequency shift can be seen. (d) The time×frequency matrix obtained with BTD. The frequency shift from 8 to 4 Hz can be easily assessed.