Operator state monitoring via pupilometry: measuring mental workload under varying luminance conditions

Abstract

Normal human pupil diameter (PD) ranges between about 2 to 8 millimeters (mm) depending on a number of factors. While the inverse relation between PD and luminance is well known, less well known and dramatic is the dependence of PD on mental workload (MWL). The PD response to MWL states are typically of small amplitude and high frequency, reflecting differential sympathetic and parasympathetic enervation. The literature shows that with contemporary objective eye tracking technology, PD behavior may prove to be a practical indication of psychophysiological status, including attention and MWL. Despite the pupil’s limited response repertoire; i.e., diameter, it is capable of potentially encoding a spectrum of information about operator state. The challenge is disambiguating PD response characteristics. A commercial, off-the-shelf eye tracker recorded PD of volunteers performing the N-back task,a standard MWL task involving auditory attention and short term memory with three levels of difficulty (easy, moderate, and difficult). The measurements were made at 3 levels of average luminance selected to assess PD when it is small, medium, and large. Each of these luminance levels fluctuated around its mean level in a sinusoidal fashion (with a ±25% modulation) at one of 4 frequencies: 1.0 hertz (Hz), 0.2 Hz, 0.1 Hz, and 0.0 Hz. The last frequency being the control condition of a 0% light modulation. Multivariate analysis revealed the effects of MWL on PD were detectable across all combinations of luminance and modulation frequency. The next step is to determine whether pupil diameter can predict MWL.