Eye Tracking Devices in Psychology of Programming Research:

Monday, 11 June 2012 0 comments


Probably the Eye tracking can be used in measuring point of gaze data that provides information concerning subject’s focus of attention. The focus of subject’s attention can be used as supportive evidence in studying cognitive processes. Despite the potential usefulness of eye tracking in psychology of Programming research, there exists only few instances where eye tracking has actually been used. This paper presents an experiment in which we used three eye tracking devices to record subjects’ points of gaze when they were studying short computer programs using a program animator. The results suggest that eye tracking can be used to collect relatively accurate data for the purposes of
psychology of programming research. The results also revealed significant differences between the
devices in the accuracy of the point of gaze data and in the times needed for setting up the monitoring
process
In the focus of psychology of programming research is the understanding of cognitive processes of
Programmers when they are writing, reading and learning computer programs. These cognitive
processes can't be observed directly. Instead, the researcher has to collect secondary data through
which the processes can be inferred. One way to gather this secondary data is to make observations of
the subject's actions. These observations can consist of for example errors the subject makes, time the
subject uses or location of the point of gaze (POG) of the subject.
In the eye tracking process, the collection of POG data can be performed without the need of the
subject performing any action. This can be seen as a benefit when studying cognitive processes that
can be easily disturbed. The collected POG data provides information of subject's attention, even
though the focus of attention is not necessarily at POG. Information about subject's attention can be
used as supportive evidence when studying cognitive processes.
Eye tracking has been used in several usability studies (Goldberg & Kotval 1999, Sibert & Jacob
2000, Byrne, Anderson, Douglass & Matessa 1999) and cognitive psychology studies related to
different search and reading strategies (Rayner 1992, 1998, Findlay 1992, Kennedy 1992). In
psychology of programming research, eye tracking has been used by Crosby (1989), who studied the
code viewing strategies of the subjects. Bednarik and Tukiainen (2004) have compared eye tracking
with blurred display.
Despite the potential usefulness of eye tracking in psychology of programming research, there exists
only few instances where eye tracking has actually been used. Therefore experience concerning the
benefits, disadvantages and problems of eye tracking in psychology of programming research is
needed. This paper reports an experiment in which we used three eye tracking devices to record
subjects’ POG when studying short computer programs using PlanAni animator(Sajaniemi &
Kuittinen 2003). We studied the easiness of use and accuracy of the three devices. We also observed
and estimated the amount of disturbance the devices caused to the subjects.
The rest of the paper is organized as follows. Next section gives an introduction to the eye tracking
process and to the devices used in this experiment. Then the experiment is described and results are
presented and discussed. The last section contains the conclusions.

Eye Tracking Methodology
Eye tracking process can be divided roughly into the following steps: subject set-up, adjustments,
subject calibration, and monitoring.
In the subject set-up phase, the subject is seated and her location in relation to the eye-tracking device
is adjusted. If head mounted optics is used, the eye tracking device is placed on subject’s head and its
position is adjusted.
The adjustments phase includes adjusting the settings of the eye tracking program; detecting and
ensuring the recognition of the subject's eye(s); and opening the file used for the recording of the eye
tracking data.
In the calibration phase, a calibration pattern consisting of a number of calibration points is shown to
the subject. The subject is asked to direct her gaze to each of the calibration points and the location of
the POG for each calibration point is recorded. The values from the calibration are used in calculating
the locations of points of gaze from the values received from the eye tracking device. The calibration
phase is repeated until satisfactory calibration values are recorded for each calibration point. One
significant problem in eye tracking is the drift effect, which indicates a deterioration of the calibration
over time (Tobii 2003). The drift effect can be reduced by ensuring the stability of the light conditions
of the environment and the equal light intensity between calibration stimuli and the experiment
stimuli.
The monitoring phase consists of viewing the status of the eye tracking and, if necessary, readjusting
the settings during the tracking of the actual experiment tasks.
In the experiment we used the following three devices: Tobii 1750 from Tobii Technology, ASL 504
Pan/Tilt Optics from Applied Science Laboratories and ASL 501 Head Mounted Optics from Applied
Science Laboratories. All three devices use video based combined pupil and corneal reflection eye
tracking.


In Tobii 1750 (Tobii 2003), the eye tracking device is embedded into the panels of the monitor that
the subject is viewing (Figure 1a). The device uses a wide-angle camera to capture images of the
subject and near infrared light emitting diodes for eye illumination. The device uses both eyes of the
subject for tracking. Tobii 1750 records data at the rate of 30 Hz (30 gaze data points/second). When
the device does not detect the subject’s eye(s), the recording rate is slowed down until proper
detection is regained. The theoretical accuracy of POG coordinates provided by the device is 1 degree
visual angle (approximately 1 cm error when the subject is seated at 50 cm distance from the display).
In ASL 504 pan/tilt optics (ASL 2003b), the eye tracking device is placed below the monitor the
subject is viewing (Figure 1b). The device has an adjustable wide angle camera that repositions itself
according to the movements of the subject. The device uses the wide angle camera to capture an image
of the subject's eye and near infra-red light emitting diodes for eye illumination. The device uses one
eye for tracking. ASL 504 pan/tilt optics records data at the rate of 50 or 60 Hz. The theoretical
accuracy of POG coordinates provided by the device is 0.5 degree visual angle (approximately 0.5 cm
error when the subject is seated at 50 cm distance from the display).
In ASL 501 head mounted optics (ASL 2003a), the optics device is placed on subject's head (Figure
1c). The device uses one wide angle camera to capture image of the subject’s eye and another wide
angle camera to capture the subject's field of view (the scene camera). The device uses near infra-red
light emitting diodes for eye illumination. The device uses one eye of the subject for tracking. ASL
501 head mounted optics records data at the rate of 50 Hz. The theoretical accuracy of POG
coordinates provided by the device is 0.5 degree visual angle.

Experiment
In our experiment, we studied the easiness of use of eye tracking devices by measuring the total
amount of time needed for the preparations of the subject. The preparations consist of subject set-up,
adjustments and calibration. We also observed and estimated the effort these activities required from
the subject. The accuracy of the devices was measured by calculating mean distances between
recorded points of gaze (in the data files) and requested points of gaze (measured with the eye tracking
software). The experimenters were using eye-tracking devices for the first time.

Method
Design: A within-subject design was used with one independent variable (the eye tracking device
used for collecting the data) and two dependent variables (the time needed for the preparation of the
subject, and the accuracy of the device).
All subjects were measured using all three eye tracking devices (Tobii 1750, ASL 504 Pan/Tilt Optics,
and ASL 501 Head Mounted Optics) and the order of the devices was counterbalanced. Each device
occurred in each of the chronological position (1st, 2nd or 3rd measuring device) equal number of
times. In the experiment we used two different versions of PlanAni. The order of the versions was
varied so that with each tracking device and each of the viewed programs two of the four subjects
used the animator with code view first and the other two used the animation view first.
Subjects: Twelve subjects, eight male and four female, participated in the experiment. The subjects
were required to have at least basic programming skills and some experience in programming. They
were recruited from third year courses in computer science and were given a coffee ticket for their
participation.
Materials: For the purpose of the experiment, PlanAni was modified so that it showed either only the
code-view that is located on the top left corner of the animator (Figure 2) or only variable animationview
that is located on the top center of the animator (Figure 3). All variables were depicted by the
same neutral image. Both versions showed notifications for the subject and the input/output area. For
the task of focusing at specific targets on the screen, screenshots of PlanAni were used. The PlanAni
version was v0.53.
Procedure: The subjects used PlanAni to comprehend six short computer programs—two programs
with each eye tracking device . They were allowed to view each program one time step by step. The
POG of subjects during these tasks was measured. With each device, the subject was first seated and
the eye tracking device's location in relation to the subject was adjusted (subject set-up phase). The
movement of the subject was minimized by using a chair without wheels, by setting the chair close
enough to the desk to minimize the horizontal rotation and advising the subject to avoid quick and
large movements of her head. The subject was not explicitly demanded to stay perfectly immobile
during the task. After set-up, the settings of the interface program were adjusted, detection of the
subject's eyes was performed and the file used for storing the POG data was opened (adjustments
phase). After this the calibration of the subject was performed (calibration phase). Time needed for
these preparations was measured by the experimenter using a special program that required a single
key-press to start and stop time measuring. With each device, the subject performed two program
comprehension tasks so that she used both versions of PlanAni. After each viewing task, the subjects
were asked to give a short program summary. The program summaries were collected for the purpose
of motivating the subjects to study the program but they were not analyzed further in this experiment.
After studying the programs, subjects were asked to look at eight specific targets on the screen before
proceeding to the next eye tracking device.
Eye Tracking in Psychology of Programming Research
In psychology of programming research eye tracking can be used as an implication of the focus of
subjects’ attention. The POG is not, however, the same as the focus of attention. Attention is not
necessarily always associated with the visual scene, even though POG is. The subject can also
voluntarily target his attention slightly off the POG (Posner 1980).
The general unobtrusiveness of an eye tracking device can be seen as a factor when using this
technology in psychology of programming research. Subjects’ cognitive processes can be easily
disturbed with objects in the field of view, sounds in the room, and extra activities required by the
experimental settings. Some of the subjects commented that the scene camera of the ASL 501,
positioned according to the manual, was disturbingly in their field of view. The visor of the ASL 501
remained in the lower part of the subject’s field of view during the measuring. This did not, however,
invoke any comments from the subjects. With ASL 504, the adjustable camera produced a buzzing
sound when it repositioned itself, causing the subject to be aware of the device’s existence. Tobii 1750
looks like a normal display device and makes no visible or audible interference.
When considering the required effort and caused disturbance, Tobii 1750 seemed to be the most
unobtrusive for the subjects. With ASL 504, the subject was required to keep her head perfectly still
during the detection of the eye, since the auto-follow property of the camera could be turned on only
after the pupil and corneal reflection were found. The positioning of the optics device of ASL 501 on
the subject’s head was time consuming and caused physical discomfort to the subject.
Tobii 1750 enabled a subject to easily observe the tracking status before the calibration phase, and to
take part in the detection of the eye. The calibration was not dictated by the operator but the tracking
program performed the calibration by showing the subject calibration points in random locations at a
slow pace.
In eye tracking, the quality and amount of recorded data is influenced by the amount of subjects’
motions. The more immobile the subject is, the better data eye tracking devices usually record (Tobii
2003, ASL 2003a). When eye tracking is used in psychology of programming research, however, the
immobilising of the subject can disturb the cognitive processes that are being studied. It seems that
there is a trade off between the accuracy and the ecological validity of data. With the subject seating
used in our experiment, we reached an accuracy that was quite near to the theoretical accuracy of
Tobii 1750. With ASL devices, however, the measured accuracy was considerably behind the
theoretical values.
Tobii 1750 and ASL 504 require the subject to be seated and tolerate limited movements of the head,
only. ASL 501 allows the subject to move aroundan activity needed in some experimental settings
in psychology of programming.
Conclusions
We have conducted an experiment comparing the use of three eye tracking devices in a psychology of
programming experiment in which subjects studied short computer programs using a program
animator.
The results show that there are significant differences in the accuracy and easiness of use between the
devices. The ASL 501 Head Mounted Optics required approximately twice as much time for the
preparation than the other two devices. The ASL 501 was also the least accurate of the devices when it
was used for the task in which the subject viewed a computer screen. This can be partly explained by
inaccuracies in the manual correction of the shifting effect. This effect can be removed by using
magnetic head tracker with ASL 501.
When considering the required effort and caused disturbance, Tobii 1750 seemed to be the most
unobtrusive for the subjects. The device allowed the subject to take part in the detection of the eyes
and the calibration process was performed without step-by-step dictation of the operator.

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