NeuroNexus optoelectrode polytrodes (A1x16-Poly2-5mm-50s-177-OA16LP) were used for recordings in the mouse medial septum (MS). Nearby groups of electrode sites along the polytrode were sorted as tetrodes. The optoelectrode configuration with fiber termination 200µm above the electrode sites enabled opto-tagging of four different neuron types in transgenic mice. The Nature Communications results from the laboratories of Dr. Li Zhang and Dr. Huizhong Tao at the University of Southern California demonstrate that the somatostatin-positive (SOM) subtype of GABAergic neurons in the medial septum mediate reward association in learning.