Discussion of nunuphar's comment: The authors use two transgenic mouse lines D1-Cre and A2A-Cre with Cre-dependent Channelrhodopsin to express Channelrhodopsin in direct pathway MSNs or indirect pathways MSNs in dorsomedial striatum. They then record in striatum or Substantia Nigra pars reticularis (SNr) which is thought to subserve much of the functions of the primate GPi due to the small size of GPi in rodents. They find that activation of the direct pathways results in an increase in the velocity of the mice in an open field, and that activation of the indirect pathway results in a decrease in their velocity. In both mice they record from a small number of striatal cells that are activated by the blue light and are putative direct or indirect projection neurons. When recording the response of the SNr cells to the light in striatum, they find both excited and inhibited units in about equal proportions. In the direct pathway expressing mice, they find that the latency of the response of the inhibited units is faster than that of the excited units and that the activity of these units is a better predictor of when locomotor starts will occur pointing to the inhibition as the primary effect and the excitation as the secondary effect perhaps through disinhibition from the nearby inhibited units, interneurons, or network effects. In the indirect pathway expressing mice the latency of the response of the excited units is faster than of the inhibited units, and the activity of the excited units is a better predictor of when immobility starts will happen. These results generally support the classical model of the direct and indirect pathway as inhibiting and disinhibiting SNr, respectively, and thus activating or inhibiting motor programs, but show that a large proportion of SNr neurons behave in an opposing manner.