Geography Reference
In-Depth Information
sharp central vision (also called foveal vision), which is necessary during reading, watching
television or movies, driving, and any activity where visual detail is of primary importance.
The really high-resolution area covers only about 2° of the visual field. The fovea is
surrounded by the parafovea belt and the perifovea outer region [24]. The vision supported
by this part of the eye is so called peripheral vision, which in comparison with foveal vision
seem to be blurred [25].
Eye movement is not smooth. The eye moves in spurts and rests between each movement.
During a fixation, eyes are relatively steadily looking at one spot in the visual scene. In order
to achieve the most accurate visual impression of a visual scene, eyes move rapidly in
mostly ballistic jumps (i.e., saccades) from one spot to another. Among those rather large
saccadic eye movements that an attentive person can easily observe from his or her own
experience, there are three other, much shorter eye movements, i.e. tremor, drift, and
microsaccades. Their purpose is to avoid saturation effects of the visual receptors on the
retina which would lead to fading perception. However, people are unaware of those tiny
movements and they can be hardly detected by state-of-the-art unobtrusive eye-trackers.
[23]
The analysis of fixations and saccades requires some form of identification that results from
the processing of raw eye-movement data. Fixation and saccades identification is an
inherently statistical description of observed eye movement behaviours.
It is important to define the exact detection algorithm for eye movement analysis, because
different parameterizations of an algorithm might lead to different results [26]. Plenty of
algorithms exist, but mostly used are I-VT and I-DT. In the case of the I-VT (Velocity-
threshold fixation identification) algorithm, the eye-velocity value is compared to the
threshold. If the sampled velocity is smaller than the threshold, the corresponding eye-
position is marked as a part of a saccade, otherwise the eye-position sample is assigned to be
a part of a fixation. The I-DT (Dispersion-Threshold Identification) algorithm takes into
account the close spatial proximity of the eye position points in the eye movement trace. [27]
Based on statistical analysis of fixations, saccades, their mutual relationship and other
characteristics, it is possible to identify certain attributes of respondent behaviour.
For example, long average fixation durations can be interpreted in two different ways as
either: (1) the user has difficulties extracting information; or (2) the user is more engaged
with interpreting a representation [28]. Hence, distinguishing between the two is case
specific.
Saccade/Fixation ratio describes the ratio between search activity (represented by the
number of the saccades) and processing activity (represented by the number of the
fixations). A small saccade/fixation ratio indicates that the user is spending more “cognitive
resources” on the task and less cognitive resources on gathering important background
information [16].
A large number of saccades indicate a low degree of search efficiency or poor interface
layout. User roams from place to place finding no satisfactory answer. Saccadic amplitude
