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In a syncytial tissue, the intracellular and extracellular spaces may have different degrees of anisotropy. This property allows generation of a re-entrant wave front in a two-dimensional sheet of tissue after two successive stimuli are applied through the same electrode, if the second stimulus is delivered during the vulnerable phase of the first action potential. The re-entrant wave front contains four phase singularities placed symmetrically about the position of the electrode.
Roth et al. (Sun,) studied this question.