Mnesic evocation occurs under the action of a stimulus. A successful evocation is observed as the overrun of a certain threshold of the neuronal activity followed by a medical imaging instrument like a PET-scanner. Within the neural system, this successful evocation corresponds to an effective activity that induces other activations in other parts of the brain and conscious actions. Populations of coupled neuronal oscillators can dynamically store information in the form of a periodic attractor of large dimension. In this context, the overrun of such an activity threshold is due to a maximization of the global activity of the population of oscillators. It is allowed by a synchronized activity of the - neuronal - oscillators, which can be provided by the action of an external stimulation. One can hold this process as an elementary process of mnesic evocation. We use an isochron-based analysis to understand the relevant aspects of this synchronization phenomenon. The temporal gap resulting from the perturbation of a large population of uncoupled oscillators (initially distributed in an equal manner on the latent phase) gives us a direct characterization of the phase space. We obtain a method to classify the phase space into fast an slow synchronization regions, thus allowing a qualitative understanding of the behaviour adopted by oscillators in response to perturbations.