Dear members of the "Eye-Movement" and "EYEMOV-L" list services:
I am preparing a manuscript in which I am comparing results from eye
tracking studies from various laboratories spanning several decades. To
facilitate the comparisons I'm attempting to define and use common terms for
measures that have been referred to by different authors using different
terms. Since the definition of "fixation" is so tightly linked to the size
of intervening saccades that can be detected, I only attempted to define the
term very generally. It seems like the best we, as a research community, can
do with this term is to report in detail our own individual definitions of
"fixation."
There is more controversy over what to call a series of consecutive
fixations within an area of interest. Authors have used "gaze," "dwell ",
"glance," or "fixation cycle" for this concept. The terms "dwell" and "gaze"
seem most popular. Some people seem to use "gaze" to refer to the
instantaneous point of regard with no temporal component. This is
particularly true when eye-in-head position is derived separately from head
position and the two signals are combined to determine the point of "gaze."
When "gaze" is used in this way, "gaze duration" seems an awkward term to
use to define a series of consecutive fixations within an area of interest.
There are also differences regarding what to call and how to compute the
cumulative duration and average spatial location of a such a series of
consecutive fixations within an area of interest. The cumulative duration
can be the sum of all consecutive fixations within an area of interest
(excluding the intervening saccades which connect them) or the duration
could be computed based on entry time and exit time from the area of
interest (thus including the time for the intervening saccades). While the
saccade time is relatively short compared to fixation time, including them
or not may still make a difference. Defining "dwell" or "gaze" duration
based on the sum of all consecutive fixations within an area of interest
(excluding the intervening saccades) is dependent upon the definition of a
fixation. Since we have less hope in using a common definition for
"fixation," I suggest that we simplify and unify the definition of "dwell"
or "gaze" duration by including the time taken up by the intervening
saccades (i.e., define it based on entry time and exit time from the area of
interest.
Below, I propose some "straw-man" definitions to start discussion. I look
forward to your thoughts.
Keith S. Karn, Ph.D.
Usability Engineer
Xerox Corporation, Industrial Design / Human Interface Department
1350 Jefferson Road, Mail Stop 0801-10C
Rochester, NY 14623 USA
office phone: +01-585-427-1561
e-mail: [log in to unmask]
Adjunct Assistant Professor, Center for Visual Science
Visiting Instructor, Department of Brain and Cognitive Sciences
University of Rochester
269 Meliora Hall
Rochester, NY 14627 USA
e-mail: [log in to unmask]
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"Fixation": A relatively stable eye-in-head position within some threshold
of dispersion (typically <2°) over some minimum duration (typically 100-200
mS), and with a velocity below some threshold (typically 15-100 degrees per
second). The definition of "fixation" is tightly linked to the size of
intervening saccades that can be detected and which the researcher chooses
to recognize. Minor changes in the parameters that define a fixation can
result in dramatically different results. Researchers in this field need to
be aware of the effects of the choices of these parameters and need to
report them fully in their publications.
"Area of interest": Area of a display or visual environment that is of
interest to the researcher or design team and thus defined by them (not by
the participant).
"Dwell": One fixation or, more typically, a series of consecutive fixations
within an area of interest. A dwell typically includes several fixations and
the relatively small amount of time for the saccades between these
fixations. A fixation occurring outside the area of interest marks the end
of the dwell.
"Dwell Duration": Total time from the time when gaze enters an area of
interest until a fixation begins outside the area of interest. This would
typically include the cumulative duration of a series of consecutive
fixations within an area of interest and the saccades which connect them.
"Gaze": Instantaneous point of regard in the visual environment.
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