Despite all that we have learned about the nervous system, from molecules to networks, we still cannot draw the schematics of, for example, curiosity. Nevertheless, bridging the gap between psychology and biology is a key goal for the neuroscience community (Human Brain Project, n.d.). In order to actually link a behavioral function to its underlying neural mechanisms, animal models must be free from anesthesia and constraints, thus allowing neural circuits to be probed during the behaviors that engage them. Hence, bridging the psychology-biology gap depends upon freely moving animals (Figure 14.1). Concepts of optical stimulation systems. The delivery of light to the brains of behaving rodents can be divided into strategies based on the provided degrees of freedom. (A) Benchtop opto-stimulators (e.g. a two-photon microscope) can be used to record and/or manipulate neuronal activity in head-fixed mammals. (Redrawn with permission from Figure 14.1A in <xref ref-type="bibr" rid="CH014_CIT00015">Dombeck et al., 2007</xref>.) (B) Animals large enough to receive implants can be tethered to optical fibers (<xref ref-type="bibr" rid="CH014_CIT00021">Gradinaru et al., 2007</xref>). (C) By placing the light source/s on or in the head of the animal, rigid optical fibers can be replaced with flexible electrical wires, upscaling the number of independently controlled targets and providing many more degrees of freedom (<xref ref-type="bibr" rid="CH014_CIT00063">Stark et al., 2012</xref>). (D) Head-mounted light sources can be powered without a tether, by a battery or wireless power transfer (<xref ref-type="bibr" rid="CH014_CIT00070">Wentz et al., 2011</xref>). (E) Closing the loop between neural activity and optical stimulation on the head may enable pacemaker-like user-independent systems. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429194702/3dec76b0-5bb2-4f88-814e-59a5922496cc/content/fig14_1_B.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>