  Objective: Optogenetic manipulations of excitable cells enable activating or
silencing specific types of neurons. By expressing two types of exogenous
proteins, a single neuron can be depolarized using light of one wavelength and
hyperpolarized with another. However, routing two distinct wavelengths into the
same brain locality typically requires bulky optics that cannot be implanted on
the head of a freely-moving animal.
  Methods: We developed a lens-free approach for constructing dual-color
head-mounted, fiber-based optical units: any two wavelengths can be combined.
  Results: Here, each unit was comprised of one 450 nm and one 638 nm laser
diode, yielding light power of 0.4 mW and 8 mW at the end of a 50 micrometer
multimode fiber. To create a multi-color/multi-site optoelectronic device, a
four-shank silicon probe mounted on a microdrive was equipped with two
dual-color and two single-color units, for a total weight under 3 g. Devices
were implanted in mice expressing the blue-light sensitive cation channel ChR2
and the red-light sensitive chloride pump Jaws in parvalbumin-immunoreactive
(PV) inhibitory neurons. The combination of dual-color units with recording
electrodes was free from electromagnetic interference, and device heating was
under 7{\deg}C even after prolonged operation.
  Conclusion: Using these devices, the same cortical PV cell could be activated
and silenced. This was achieved for multiple cells both in neocortex and
hippocampus of freely-moving mice.
  Significance: This technology can be used for controlling spatially
intermingled neurons that have distinct genetic profiles, and for controlling
spike timing of cortical neurons during cognitive tasks.
