In one aspect, a system for in vivo and transcranial stimulation of brain tissue of a subject may include at least one light source, a controller to control operation of the light source, a signal detecting unit and a processor configured to receive signals from the signal detecting unit, analyze the signals and generate a feedback signal to the controller to control the light source until optimal results are obtained. In one embodiment, the light source is a laser instrument and the wavelength can range from 800 to 1100 nm. In another embodiment, the irradiance of the laser instrument can range from 50 to 1000 mW/cm2.