We show here by whole field monitoring of free intracellular Ca2+ ([Ca2+]i), locally recorded field potential (fp) and external [Ca2+], that low-[Mg2+] induces seizure like events (SLEs) accompanied by simultaneous fluctuations of [Ca2+]i and [Ca2+]e in cultured hippocampal slices. Within a SLE, complex [Ca2+]e fluctuations are seen throughout phases of Ca2+ depletion (tonic) and Ca2+ recovery (clonic) of the extracellular space. Information theory entropy-based analyses revealed strong asymmetric associations of [Ca2+]i and [Ca2+]e kinetics. By contrast, signal-associations between SLEs were found to be weak and of symmetric nature distinguishing seizure-like and interictal events by extensive coupling and decoupling of [Ca2+]i and [Ca2+]e fluctuations, respectively.