Pupil Responses during Discrete Goal-directed Movements Xianta Jiang 1,2 M. Stella Atkins 2 Geoffrey Tien 2 Roman Bednarik 3 Bin Zheng 4 1 Zhejiang University, Hangzhou, China, jxt@zju.edu.cn 2 Simon Fraser University, Burnaby, BC, {xiantaj, stella, gtien}@sfu.ca 3 University of Eastern Finland, Joensuu, Finland, roman.bednarik@uef.fi 4 University of Alberta, Edmonton, AB, bzheng1@ualberta.ca ABSTRACT Pupil size is known to correlate with the changes of cognitive task workloads, but how the pupil responds to requirements of basic goal-directed motor tasks involved in human-machine interactions is not yet clear. This work conducted a user study to investigate the pupil dilations during aiming in a tele-operation setting, with the purpose of better understanding how the changes in task requirements are reflected by the changes of pupil size. The task requirements, managed by Fitts’ index of difficulty (ID), i.e. the size and distance apart of the targets, were varied between tasks, and pupil responses to different task IDs were recorded. The results showed that pupil diameter can be employed as an indicator of task requirements in goal-directed movements—higher task difficulty evoked higher valley to peak pupil dilation, and the peak pupil dilation occurred after a longer delay. These findings contribute to the foundation for developing methods to objectively evaluate interactive task requirements using pupil parameters during goal-directed movements in HCI. Author Keywords Pupil diameter; goal-directed movement; movement-evoked pupillary response; Fitts’ Law. ACM Classification Keywords H.5.2 [Information interfaces and presentation]: User Interfaces- Evaluation/methodology INTRODUCTION The extent of achievement of a complex task critically relies on the task requirements while performing the task, since the human mental capacity is a finite resource [5]. This especially holds true in goal-directed movements with high task requirements. For example, surgeons undergo higher mental workload in a minimally invasive laparoscopic surgery than in open procedures [3], since the task requirements in laparoscopic surgery are higher than in open surgery, due to the long tool shaft and the lack of depth information when projecting the scene inside the body to a 2-D screen. Therefore, it is highly desirable to objectively and continuously measure the task requirements as a surrogate for mental workload of the users during goal- directed movements. Such a method would allow indirect evaluation of the performance during the procedure. Pupil size has been well proven to correlate with the information processing load in a variety of cognitive tasks [2], but has not been thoroughly explored in the field of HCI [16]. In order to apply pupillary indicators to task workload in the field of HCI research, pupil size changes during goal-directed movements should be explored first. For example, how do the pupil size changes correlate with the level of the task difficulty in an eye-hand coordination movement? Only a few works have been published in this research area, and to a very limited extent. One of the pioneer fundamental works was done by Richer and Beatty [24], who found that the pupil dilated as a function of the complexity of finger movements. However, this is not a real situation of goal-directed movement; the participants in this 1985 study simply flexed their fingers while looking at a blank screen for the purpose of pupil size recording. This work aims to explore the pupillary responses during a discrete goal-directed movement with the task difficulty level clearly defined by Fitts’ law [8] (the size and distance between targets were varied corresponding to difficulty level) to quantify the pupil dilation during movements. The findings from this study constitute the foundation for constructing methods towards objectively measuring the task requirements during complex goal-directed movements. Such measurements can be used for adjusting proactive responses of user interfaces, for instance in medical educational simulations. RELATED WORK Mental Workload and Measurement Methods Past research introduced three main categories of mental workload measurement techniques: subjective rating scales (self-assessment), performance measures (including primary and secondary task measures), and psychophysiological measures [10]. 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