However, there are some disadvantages of the method worthy of discussion. From a purely technical point of view, besides being restricted to surface cortical regions, the method does not permit the use of freely moving subjects. Thus, head-fixed preparations must be applied, at least at the time of this writing. From a physiological viewpoint, although some recently reported interaction with GABAA receptors121 might be of negligible size,122,123 it remains largely unexplored how far pharmacological side effects might possibly affect the signal.120 In addition, the often found relatively low signal-to-noise ratio (significantly influenced by “biological noise”) frequently requires averaging across repeated trials. However, it should be noted that advanced postprocessing algorithms31,90,124,125 and excellent handling of the method allows single-trial read-outs in anaesthetized, awake, and also in the behaving animal (for example, see Refs. 84, 90, 115, and 116). The fact that the portion of activity originating from distinct (e.g., inhibitory or excitatory) populations of neurons cannot be distinguished is a disadvantage for addressing questions related to interactions between different cell-specific circuitries (note that, on the upside, it avoids biased sampling of neurons and captures net population activity irrespective of preferred feature selectivity, thus providing an accurate picture of the global state of the cortex). Moreover, the wide spread of activity observed with VSDI (up to several millimeters) might include contributions from fibers of passage that shadow spatiotemporal interactions between more closely coupled groups of neurons. Finally, the use of voltage-sensitive dyes requires craniotomy in animal models and in most cases also removal of the dura to obtain proper staining. The staining itself may occupy potential measurement times. Most important, however, is that the invasive nature of the procedure hinders chronic monitoring as it bears the risk of tissue irritation or invoking inflammatory processes. Recently developed genetically encoded voltage indicators (GEVIs) may improve upon such technical shortcomings related to VSDI.126 In particular, GEVIs allow targeting of specific cell populations and, thus, provide the unique property of selective staining of specific cell types. We may await even more exciting times once these methodologies are available in species other than the current dominant mouse model.