Pupil Response in Visual Tracking Tasks: The Impacts of Task Load, Familiarity, and Gaze Position

Abstract

Pupil size is a significant biosignal for human behavior monitoring and can reveal much underlying information. This study explored the effects of task load, task familiarity, and gaze position on pupil response during learning a visual tracking task. We hypothesized that pupil size would increase with task load, up to a certain level before decreasing, decrease with task familiarity, and increase more when focusing on areas preceding the target than other areas. Fifteen participants were recruited for an arrow tracking learning task with incremental task load. Pupil size data were collected using a Tobii Pro Nano eye tracker. A 2 × 3 × 5 three-way factorial repeated measures ANOVA was conducted using R (version 4.2.1) to evaluate the main and interactive effects of key variables on adjusted pupil size. The association between individuals' cognitive load, assessed by NASA-TLX, and pupil size was further analyzed using a linear mixed-effect model. We found that task repetition resulted in a reduction in pupil size; however, this effect was found to diminish as the task load increased. The main effect of task load approached statistical significance, but different trends were observed in trial 1 and trial 2. No significant difference in pupil size was detected among the three gaze positions. The relationship between pupil size and cognitive load overall followed an inverted U curve. Our study showed how pupil size changes as a function of task load, task familiarity, and gaze scanning. This finding provides sensory evidence that could improve educational outcomes.

Keywords: biosignal; gaze position; pupil responses; task familiarity; task load; visual tracking.

Figure 1

Experimental setting. Note: (a) the Tobii Pro Nano eye tracker is denoted by yellow text; the head support is denoted by blue text. (b) Participants were seated in front of the computer, positioning their chins on the designated support.

Figure 2

Illustration of Areas of Interest. Note: red regions represent the Eye on Arrow (EOA), blue regions represent the Eye Before Arrow (EBA), and the remaining areas indicate the Eye on Other Areas (EOO).

Figure 3

Illustration of baseline pupil size and measured pupil size. Note: for every task condition, the pupil size obtained from the initial ten sampling frames was averaged to determine the baseline pupil size. All remaining frames within that specific task condition were averaged to determine the measured pupil size. The sampling rate of pupil size was 60 Hz.

Figure 4

Interaction plot displaying the influence of block and trial on the participants’ performance scores. Note: error bars represent standard errors of the performance score for each group.

Figure 5

Interaction effects in a three-way repeated measures ANOVA. Note: this plot illustrates the interaction effects of task load (x-axis, represented by varying blocks), repetition (unique colored lines), and gaze position (three faceted subgraphs representing different AOIs) on the dependent variable, adjusted pupil size. Faceted subgraph EOO, Eye on Other Areas, EOA, Eye on Arrow, EBA, Eye Before Arrow. Significant interactions among variables are indicated by non-parallel lines. Error bars represent standard errors of the adjusted pupil size for each group.

Figure 6

Linear mixed-effects regression plot. Note: this plot displays the relationship between adjusted pupil size and NASA-TLX scores using a linear mixed-effects regression model, accounting for participants as a random effect. Different colored lines represent individual trials or the overall trend. The shaded area represents the 95% confidence interval for the regression line.