Effect of System Response Time on Brain Activity and Sense of Agency

The operation of display-equipped devices, such as smartphones and car navigation systems, is based on sensory input and visual information provided by the display. Therefore, it is necessary to clarify the effect of the system response time between the input by the user and the response displayed by the device during operation. The purpose of this study was to clarify the effects of the length of and variance in the system response time on brain activity and on the users’ experience when operating a device. The experiment entailed a task in which participants followed a target by rotating a human-machine interface (HMI) controller. The experimental conditions consisted of four levels of constant delay from 0 ms to 120 ms and a randomized variable delay. Near-infrared spectroscopy, which measures brain activity based on changes in oxygenated hemoglobin concentration, was used to assess brain activity during the experiment. Participants’ brain activity during the trials suggested that the SoA was impaired when the delay was not constant. NASA-TLX was also used to investigate participants’ mental workload, the delay discrimination rate, and participants’ work efficiency in each condition. The results confirmed that an increase in system response time increased mental workload and decreased work efficiency. In addition, the delay discrimination rate varied among individuals; thus, differences in activation were confirmed in the prefrontal cortex and supplementary motor cortex between the group of subjects with a high discrimination rate and the group of subjects with a low discrimination rate.