A device named Temptrode was devised, along with the two additional versions, to measure the temperature increases caused by optical stimulation. -type thermocouple wires with a radius of 0.12 mm (Copper/Constantan, Duplex type, Teflon coated, SANE CAL. Co. Ltd, Seoul, Korea) were used as the temperature sensor. In order to fit the wires along with the optical fiber into the limited space inside the cannula, the insulation in a thickness of 150 μm that wraps around the thermocouple wires was removed, and the exposed thermocouples were insulated by Parylene-C deposited in a thickness of about 3 μm. Next, the tips of the thermocouple wires were adhered together by soldering the wire tips to achieve spatial accuracy for temperature measurement near optical stimulation sites. After soldering, Parylene-C was deposited again on the newly soldered area to prevent possible noises coming out from the thermocouple wires through the DAQ board (NI USB-6353, National Instrument, Austin, Texas). Then, the wires were finally inserted into the stainless steel cannula and placed as close as possible to the tips of the optical fiber and electrode. In total, three types of Temptrode were devised for different measurement modes. In type 1, thermocouple wires were placed inside the cannula [Fig. 3(a)] while they were attached to the outer surface of the cannula in type 2 [Fig. 3(b)]. The distances between the tips of the optical fiber and the thermocouple wires ranged around 200–350 μm in Temptrode types 1 and 2. In type 3, three thermocouple wires were intertwined, with tips being placed at different distances from the tip of the optical fiber of 400, 700, and 1000 μm, respectively, in order to investigate the spatial distribution of temperature increases caused by NIS. In an additional experiment to investigate the spatial distribution of temperature increase, the temperature sensor, separated from the optical fiber, was attached to a microdriver and moved into the GP area by using the computer controlled manipulator (ARIS-22, Neurostar, Sindelfingen, Germany) while the optical fiber was already fixed at the target position. The separation between the optical fiber and temperature sensor enabled to measure the temperature increase at relatively farther distances from the stimulation site.