Uncovering the relationship between neuronal activity (electric signal transmission) and structural connectivity (physical connection between neurons) is one of the major challenges in neuroscience. In this direction, we study activity changes caused by external perturbations in a network of living neurons. Our experimental set up is based on in vitro neuronal cultures, together with fluorescence imaging to monitor activity at single-neuron resolution. We carry out experiments with two types of cultures: those composed of individual neurons and those consisting of aggregates of neurons. These cultures show different dynamics and responses to perturbations that offer the possibility to increase our knowledge of several neurological diseases.