In order to better understand the user and visual interface, it is crucial to also understand human cognitive processes. Unfortunately, these processes are traditionally difficult to monitor without the use of cumbersome or expensive brain imaging equipment. In recent years, functional near-infrared spectroscopy (fNIRS) has emerged as a brain imaging technique that is both lightweight and easy to set up. In this paper, we demonstrate the potential of fNIRS to examine current visualization techniques and influence the design of visual interfaces. To validate fNIRS as a tool for visualization research, we present two studies based on previous work in brightness contrast in visual search and angle vs. position comparisons in form. Our results indicate there are significant and unintuitive cognitive differences in the prefrontal cortex during visual search tasks of positive and negative contrast polarity. Furthermore, we are able to differentiate between angle and position comparisons under specific experimental conditions. Finally, we outline the potential of fNIRS to give objective, continuous, and near real-time feedback of brain activity in future visualization research.