Keywords

1 Introduction

The visual system of human obtains external information through the pupil. The role of pupil is to adjust its size to change the amount of entering the eyes. The pupil will dilate when light is strong and it will constrict while the light is weak. Pupil size is determined by a balance between the competing forces of two smooth muscle systems, controlled by the parasympathetic and sympathetic nervous system, respectively [1]. The pupil is constricted by parasympathetic nervous system (PNS) activity which is responsible for the light reflex and near response. Pupil dilation is effected by the sympathetic nervous system (SNS). Certain mental process (such as perception, memory, thinking, language, and processing, motivation, emotion and other senior psychological activities), will also affect the size of the pupil. Because of pupils’ important role to people, it has attracted the interest of many researchers.

One of the earliest researchers to examine pupil size variations was Bumke, who, in 1911 (as cited in the document of Hess), stated that intellectual processes, psychical effort, active mental images, and all affect the pupil size [2]. Today the measurement of pupil size was use to evaluate mental workload (MW), they think that the pupil size was related to task difficulties and it can effetely reflect the fluctuation of the MW which was closely related to the change of task difficulty. The pioneer work measuring pupil size changes relating to mental workload and task difficulty should be Hess and Polt who studied the pupil in a mental arithmetic experiment [3]. They found that the pupil size of the subjects gradually dilated along with the time elapse of presentation of a multiplication problem and reached a peak immediately before the production was orally reported; then constricted rapidly back to the original size. Following their work, extensive studies showed that the changes of pupil size reflect mental workload and the level of difficulty in different tasks and pupils dilate as task difficulty increases [4–7]. The relationship between increments in pupil diameter and mental workload is a very well known and it can be found from the investigations of Ahlstrom and Friedman-Berg [8] or Jainta and Baccino [9].

Numerous studies have shown the high sensitivity and validity of this index in providing an adequate measure of the cognitive demands imposed by a task (e.g. LeDuc et al. [10]). However, some authors casted doubts on the validity of pupil size as an appropriate index of mental workload. Schultheis and Jameson have evaluated the validity of pupil size in adaptive hypertext systems and the results showed no significant differences in pupil diameter in relation to text difficulty [11]. Conati and Merten also found that pupil size was not a reliable predictor of self-evaluation when the user interact with an environment for exploration-based learning [12].

In this study, target search task was made to investigate whether pupil size reflect mental workload differences in a situation with different task search difficulties and whether the motivation or interesting can take effect on the change of pupil size. We expected that the pupil size would be more cognitively demanding, reflecting the task demands.

2 Method

2.1 Participants

The experiment was completed by 12 male undergraduate students who were recruited through the internet from the Beihang University and their ages ranged form 18 to 23 (the average age was 20). Each was tested by a standard near-visual chart to ensure that they had a near fovea acuity that was normal. Participants provided informed consent and received RMB 100 yuan in compensation.

2.2 Apparatus

Pupil size were recorded using a eye tracking system of Eyelink II measuring system manufactured by Canada’s SR Research CORP.

2.3 Stimulus Material

Thirty pictures were prepared. They were modified by drawing software with three ways. In the first way, there was one difference to the corresponding raw picture after each picture was modified; in the second condition, there were two differences between the modified picture and the corresponding raw picture; there were three differences to the corresponding picture of the modified picture. So there were four groups of pictures: the raw pictures group(RG), one difference group(OG), two differences group(TG) and three differences group(TDG) (Fig. 1). There were 30 pictures in each group. The subjects’ task was to find difference between the modified pictures and raw pictures.

Fig. 1.
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Experiment pictures

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The searching of picture differences was done in two experimental conditions. The first condition(FC) was that the subjects were asked to find these differences as soon as possible without time limited. The second condition(SC) was that the subjects were given additional rewards except the basic remuneration according their searching performance that the more quickly they found the differences, the more rewards they would get and the slowest 3 people would only get the basic payment of RMB 100 yuan. The pupil data of subjects were recorded with the eye movement measuring system in the experiment.

2.4 Procedure

At first, the subjects searched difference according FC and sequence was as follows.

  1. (1)

    To find the difference between RG and OG and have a rest for 5 min.

  2. (2)

    To find the difference between RG and TG and have a rest for 5 min.

  3. (3)

    To find the difference between RG and TDG and have a rest for 5 min.

After finishing the experiment of FC, the subjects made the searching task of SC as the same sequence of above.

3 Result and Discussion

The same characteristic of pupil size change for all subjects was that the pupil size of the OG, TG and TDG was larger in FC than that of the corresponding in SC. There were also different characteristics of pupil change among subjects and. The first difference: the pupil size dilated from one difference to two differences while constricted at three differences (e.g. subject 1, Fig. 2) and two subjects were in this condition. The second difference: the pupil size constricted from one difference to two differences while dilated at three differences (e.g. subject 2, Fig. 3) and four subjects were in this condition. The third difference: the pupil size continuously decreased from one difference to three differences (e.g. subject 5, Fig. 4) and six subjects were in this condition.

Fig. 2.
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Changes of pupil size of subject 1 (Color figure online)

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Fig. 3.
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Changes of pupil size of subject 2 (Color figure online)

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Fig. 4.
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Changes of pupil size of subject 5 (Color figure online)

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With the SC, the additional rewards might be an encouragement to drive subjects try their best to search the differences between the pictures, so all the subjects’ pupil size was larger than the corresponding FC and it could be shown that motivation or interesting could take an effect on pupil size. After the experiment, we have asked the feeling of visual fatigue of all subjects, they all reported that they feel tied with their vision in different experiment stage. Some subjects reported their fatigue when they search the TDG (e.g. subject 1) while the most subjects reported their fatigue when they search the TG (e.g. subject 2 and subject 5). So it could been seen that if the subject don’t feel tied of their eye, the pupil size will dilate when the task become more difficult (Fig. 2). When the subjects felt tired of their eye, the pupil size will constrict (Fig. 4), but the results didn’t always show such trend, some subjects the pupil size will dilate in the final of the experiment session (Fig. 3). It might be an interesting phenomenon and perhaps it could be named as “the terminal effect”. When the subject saw the terminal of the task, they be excited and they will spend more effort to perform the task, so the pupil size in Fig. 3 dilated at the final session.

In this experiment, difficulty level was according to the difference number, it was more difficult with the more difference number. It was easier for subject to find the one difference between the pictures. The subject need to spend more time and pay more attention to the more difference to discriminate the detail of the pictures, so it was easier to cause the feeling of visual fatigue. Fatigue will lead to the decrease of pupil size while the increasing of the task difficulty will make the pupil size dilate and they have been proved by researchers. In this study, the results agreed to the present research.

4 Conclusion

It can be seen from the results that the increasing of task difficulty will make pupil dilate, the positive motivation or interesting also dilate the pupil size, while the visual fatigue constrict the pupil size. Because of the effect of these factors, it is hard and complicate to use pupil size to the evaluation of mental workload. Thought it is difficult, controlling or eliminating the impact of these variables, the pupil size is a reliable index of a person’s mental workload change. The terminal effect that may have influence on pupil size may be an interesting phenomena and it worth to make a further study.

As a physiological index not controlled by consciousness, pupil size can reflect well the people’s mental workload. Because there are more factors affecting the pupil size and mental workload, to precisely evaluate the mental workload with pupil size, it need to eliminate the interference of all the factors. Also the experiment task need to be refined, so only single dimension of mental workload can be evaluated.