A Head-Fixation System for Continuous Monitoring of Force Generated During Behavior

Abstract

Many studies in neuroscience use head-fixed behavioral preparations, which confer a number of advantages, including the ability to limit the behavioral repertoire and use techniques for large-scale monitoring of neural activity. But traditional studies using this approach use extremely limited behavioral measures, in part because it is difficult to detect the subtle movements and postural adjustments that animals naturally exhibit during head fixation. Here we report a new head-fixed setup with analog load cells capable of precisely monitoring the continuous forces exerted by mice. The load cells reveal the dynamic nature of movements generated not only around the time of taskrelevant events, such as presentation of stimuli and rewards, but also during periods in between these events, when there is no apparent overt behavior. It generates a new and rich set of behavioral measures that have been neglected in previous experiments. We detail the construction of the system, which can be 3D-printed and assembled at low cost, show behavioral results collected from head-fixed mice, and demonstrate that neural activity can be highly correlated with the subtle, whole-body movements continuously produced during head restraint.

Protocol for Recording from Ventral Tegmental Area Dopamine Neurons in Mice while Measuring Force during Head-Fixation

“… Turning the potentiometer changes the slope of the relationship between the applied force and the corresponding voltage. Counter clock-wise rotation (reducing resistance) will increase the slope and clock-wise rotation (increasing resistance) will reduce the slope ( Hughes et al., 2020a ). The resistance should be adjusted so the mice cannot max out the voltage signal in either direction.…”

“…Printing files are available by request. Additional step-by-step procedures, graphical descriptions, and videos of the assembly of the system can be found in a previous publication ( Hughes et al., 2020a ). Print out the components listed in Materials and Equipment.…”

“…The INA125 amplifiers are robust and should have a long life if the soldering and wire connections are done well. The circuit design described in ( Hughes et al., 2020a ) is recommended because it has been adapted to allow for bidirectional forces (e.g., forward/backward, left/right, and up/down). The most common source of circuit malfunctions is from faulty connections.…”

“…We further detail how this system can be used in conjunction with in vivo electrophysiology and optogenetics to study dopamine neurons in the ventral tegmental area. For complete details on the use and execution of this protocol, please refer to Hughes et al. (2020a , 2020b) …”

“…For complete details on the use and execution of this protocol, please refer to Hughes et al. (2020a , 2020b) …”