Measuring the actual timing of liquid-reward delivery using a thermistor

Background: Determining the exact timing of reward delivery is critical in neurophysiological experiments. Despite the importance of this parameter, techniques for precisely measuring the exact delivery time of a liquid reward have been underdeveloped. Although there is always some latency between the computer command and reward delivery, the former has been used routinely to mark reward onset time. New method: To address this issue, we developed two methods of estimating the time of liquid reward delivery. First, electrical conduction between two contacts at the outlet of a pipe was detected. This technique was impractical during normal experiments but was a direct and reliable method of measuring the precise timing of water outflow. Second, a self-heating thermistor inside the fluid delivery pipe was used. The thermistor detected a decrease in temperature upon commencement of water flow. Results: The electrical contact began 50-80. ms after the computer command. The thermistor-based delivery timing estimation was ~40. ms later than that measured by the electrical contact. The time lag was constant with minimal variance between trials within the same experimental setup. Comparison with existing method: As far as the time difference between the two methods is measured, the timing of water outflow can be estimated using the thermistor-based device. This is the first method of estimating the onset time of water flow during experiments. Conclusions: Our new system could be used to quantify setup-dependent changes in the timing of reward delivery, improving the sophistication of reward assessments in neurophysiological experiments.