**6. Key role of hub neurons**

Since some neurons can be identified by their particular connectivity as hub neurons, the next step is to know what class of neurons they are. There are evidences that some hub neurons are interneurons (unpublished). The functional connectome in the striatum was observed in an area of about 1 mm<sup>2</sup> with hub neurons connecting many neurons at distances larger than 500 μm, while synapses between projection neurons can only be found at a distances less than 100 μm [16–18]. Indeed, only interneurons can extend their axons to connect neurons at distances up to 1 mm. This inference was confirmed by whole cell patch clamp recordings of some hub neurons identifying fast-spiking (PV), low-threshold spiking (LTS) and cholinergic interneurons (ACh) [9, 36]. Transgenic mice in which optogenetic stimulation activates a particular neural population [59] shows that hub neurons connect with different groups of neurons perhaps inducing the coactivity that underlies network states, and therefore, are responsible for their alternation. However, further experiments using transgenic animals and optogenetics are needed to identify the classes of neurons that form striatal modular circuits and under what conditions.
