“…For small experimental animals, invasive measurements are acceptable, and electrical measurements by inserting electrodes into the brains of untethered animals are conventionally used. Besides simple metal electrodes, several types of sophisticated Si probes have been developed (such as the Utah probe [21], the Michigan probe [22], and the Toyohashi probe [23]) to measure brain activity at multiple points. It is, however, noted that electrical measurements are nonspecific, and thus, in many experiments, optical measurement using fluorescence is required, because such measurements are specific.…”