Cognitive load is determined by a variety of methods. According to Yuksel, the most frequently
used measures are self-reporting questionnaires, followed by dual-task techniques and physiological
parameters. This latter measurement primarily includes heart-rate variabilities or electrodermal
activity; however, in the past years, HCI research increasingly applied task-evoked pupil diameter
to index cognitive processing demands. Applying pupil size changes as a measure
of cognitive load has many advantages but pupil diameter is also sensitive to ambient lighting
conditions and cannot be applied in any type of ubiquitous computing systems. Therefore, latest
research applied another oculomotor parameter to determine mental workload: smooth pursuit
eye-movements (SPEM). Smooth pursuit eye-movements are defined as a voluntary behavior for the
purpose of keeping the image of a moving object within the foveal area of the eye. And recently,
two groups observe that mental workload influences the quality of the pursuit performance.
The present work aims to contrast the sensitivity of both pupil diameter
and smooth pursuit eye-movements for indexing cognitive processing demands.
We applied a dual task scenario. After a 9-point calibration procedure, the level of difficulty
was varied on basis of an n-back task. We decided for an auditory n-back task to avoid interferences
with the pupillary light reflex. The impact of task load variation was determined through
changes in pupil diameter and on the basis of the performance in a simultaneous SPEM task.
N-back tasks are continuous-recognition measures. During easy trials of the current auditory n-back
task, participants were instructed to press a button as soon as a number was repeated.
Or, in the moderate and hard conditions,
whenever a number recurred with one or two others in between.
In the parallel smooth pursuit eye-movement task, participants were instructed to follow
a fixation cross that moved horizontally from left to right across the screen.
Our results indicate once again that task load influences pupil diameter.
The signals depict differences from baseline mean. During control trials, participants carried out
the smooth pursuit eye-movement task only. As can be seen, pupil size increases with the level of
difficulty of the n-back task. Target onset is not always but in most cases, followed by a transient
pupil dilation. However, overall pupil diameter reaches a plateau phase already during the 2-back
task and cannot be differentiated from pupil sizes during high workload in the 3-back task.
During smooth pursuit eye-movements, deviations in gaze behavior increase with increasing task load
in the parallel n-back task. However, in contrast to pupil diameter, smooth pursuit performance
differentiates only roughly between the different levels of task load. The control trial
differs from all experimental conditions but we observe only a strong tendency
towards differences in gaze behavior between very easy and very difficult tasks.
Due to the button presses during the auditory n-back task, we can calculate various behavioral
responses. Descriptive analyses of the reaction times show an increase with increasing task load,
however, this parameter also does not discriminate between the 2-back and 3-back condition.
In contrast, the number of omissions is higher during 2-back compared to 3-back trials
but there are no differences among the lower conditions.
And the false positive responses revealed a counterintuitive pattern with more false
hits during 2-back compared to 3-back trials. So, we can conclude that cognitive load exerts
a considerable influence on pupil diameter. However, it seems hardly possible to differentiate
task load in the upper ranges. This is a problem since high levels of cognitive
load are exactly those that are associated with impairments in behavioral performance. For smooth
pursuit eye-movements we observe an increase in gaze deviation with increasing task load.
However, SPEM performance distinguishes only between conditions of very high and very low
task difficulty. According to a recent study, this might be different for smooth pursuit
during circular and sinusoidal movements. Further research apart from simple horizontal trajectories
needs to be applied on this issue. And last but not least, a combination
of behavioral measures adds valuable information on cognitive processing
and may also help to distinguish even highest states of mental workload.