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2.1. Virtual Reality Exposure Therapy
Jacob proposed to use the eye as a replacement to the mouse [Jacob, 1990]. He proposed
an intelligent gaze-based informational display. In this system, a text window would unscroll
in order to give information about items selected by gaze. In this work, the author equally
identified one of the main problems in using the eyes as a pointing and selection tool: the
difficulty to know whether the eyes are scanning or selecting. This is also known as the
Midas touch problem. In order to sidestep this, he proposed to use dwell time for selection.
In their paper, Starker and Bolt presented an information display system [Starker and
Bolt, 1990]. In their system, a user equipped with an eye-tracker could control navigation
in a 3D environment by gaze. These 3D environments equally contained characters which
would change behavior when looked at. More specifically, they would start blushing or
speaking. They used synthesized speech to interactively describe the objects which were
being looked at on screen. Here, dwell times were used to zoom into the environment. Their
setup was in front of a monitor screen and the users had to use a chin-rest in order to avoid
head movements, which were not monitored.
Colombo et al. proposed a system coupling eye- and head-tracking [Colombo et al.,
1995]. They monitored the various types of possible movements to trigger different types of
events. Smooth gaze shifts were assimilated to image scanning, head movements were used
to drag objects on screen, and “eye pointing”, if long enough, was identified as a mouseclick.
They tested their method on a virtual museum application, where a user could explore
the museum environment and select desired information on the various paintings.
Cassel and Thórisson conducted an experiment in which they tested different types of
conversational agents [Cassell and Thorisson, 1999]. In a first phase, the agent gave content
feedback only. In the second, it gave content and envelope feedback (non-verbal behaviors
related to conversation such as gaze or tapping of the fingers), and in the third, content and
emotional feedback. Their aim was to confirm their hypothesis, that envelope feedback was
much more important than any other feedback. In their study, the subject was eye-tracked in
order for the conversational agents, which consisted in simple 2D cartoon characters, to be
able to respond with respect to where the subject was looking.
More recently, Tanriverdi and Jacob proposed an interactive system in VR [Tanriverdi
and Jacob, 2000]. They used eye-tracking to select objects in a 3D environment. They then
compared the efficacy of using eyes instead of hands for object selection. They concluded
that the use of eyes for selection was more efficient than hands for far away objects but not
for close ones. However, they also concluded that subjects had more difficulty remembering
interaction locations when using their eyes than when using their hands.
In most of the above mentioned work, there was a requirement for the user’s head to be
static because it was not tracked. Zhu and Ji developed an eye-tracking system which did not
require a static head [Zhu and Ji, 2004]. Moreover, their system did not require calibration.
One of the tests they did to evaluate their system, was to use eye-tracking for region selection
and magnification, which was achieved by blinking thrice.
Finally, Wang et al. developed a system in which they used eye-tracking in order to
change the behavior of a software agent in tutoring exercises [Wang et al., 2006]. When
eye movement fell under a certain threshold and/or when the pupil size was smaller than a
given threshold, indicating loss of interest, the software agent reacted by showing anger or by
alerting the subject. On the other hand, if both these values were above the given thresholds,
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