Optogenetics has been used to study regions of the brain involved in Parkinson's disease, inspiring new methods of treatment. The optogenetic inhibition of DYSK-SPNs, a small subset of D1-SPNs, with halorhodopsin in mice has been shown to reduce involuntary movements - a side effect of L-DOPA medication. Optogenetic stimulation of cortical neurons projecting to the subthalamic nucleus reduces motor deficits, helping to explain the mechanism by which deep brain stimulation works. Additionally, optogenetics has been used to identify a subset of external globus pallidus (GPe) neurons that result in long-lasting restoration of movement when stimulated.