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Brazilian Journal of Motor Behavior
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Gaze behavior data in the vitrine of human movement science: considerations on eye-
tracking technique
TIAGO PENEDO
1
| SÉRGIO T. RODRIGUES
2
| GISELE C. GOTARDI
2
| LUCAS SIMIELI
1
| JOSÉ A. BARELA
3
| PAULA
F. POLASTRI
2
| FABIO A. BARBIERI
1
1
São Paulo State University (Unesp), Department of Physical Education, Human Movement Research Laboratory (MOVI-LAB), Bauru, São Paulo, Brazil
2
São Paulo State University (Unesp), Department of Physical Education, Laboratory of Information, Vision and Action (LIVIA), Bauru, São Paulo, Brazil
3
São Paulo State University (Unesp), Department of Physical Education, Institute of Bioscience, Rio Claro, São Paulo, Brazil
Correspondence to:!Fabio A. Barbieri. São Paulo State University (UNESP), Campus Bauru, School of Science, Department of Physical Education, Human
Movement Research Laboratory (MOVI-LAB). Address: Av. Eng. Luiz Edmundo Carrijo Coube, 14-01, Vargem Limpa. CEP: 17033-360 - Bauru, SP, Brazil.
Telephone: + 55 14 3103-9612
email: fabio.barbieri@unesp.br
https://doi.org/10.20338/bjmb.v17i4.352
ABBREVIATIONS
AOIs Areas of interest
DTU Display/transmission unit
PUBLICATION DATA
Received 15 02 2023
Accepted 19 06 2023
Published 20 06 2023
BACKGROUND: Eyes are the main gateway of visual information input. Moving the eyes is
essential to extract visual information from scenes while performing motor actions. This helps
to explain motor behavior, especially related to visual attention mechanisms, gaze training and
learning, and the relevance of visual information in controlling actions. Thus, collecting data on
gaze behavior is important for explaining motor behavior.
AIM: We present the main video-based eye-tracking techniques, briefly describe the anatomy
of the eyes, explain the operation of the eye-tracker (eye capture techniques, calibration, and
data analysis), and propose interpretations of the main variables that were extracted by the
technique. This way we develop considerations (limitations and advantages) on the eye-
tracking technique that placed gaze behavior data under the view of human movement
science.
INTERPRETATION: Eye-tracking has become an excellent tool to assist in the analysis of
human movement through gaze behavior. It is possible to make inferences, mainly from the
combination of sensory information, such as visual information, with performance during motor
tasks, about perception, cognition, and human behavior during the most diverse day-to-day
activities. Eye-tracker systems have been employed in different majors related to motor
behavior, such as medicine, commerce, and game development.
KEYWORDS: Eye movement | Vision | Motor behavior
INTRODUCTION
Vision is a striking source of sensory stimuli for humans, as well as other animals. Anatomically, the visual system includes the
eyes (which we will emphasize), connecting pathways to the visual cortex and other parts of the brain (e.g., primary visual cortex). Eyes
consist of eyelids, pupil, lens, retina, and many other components. All these components are connected to the nervous system and are
necessary for visual perception.
The richness of vision comes not only from the functional utilization of the anatomical parts of the visual system but also from
eyes movements to achieve the goal of “seeing”. While the specific anatomy of human eyes restricts the amount of precise information
that enters the nervous system, the ability to move the eyes coordinately allows the capture of relevant visual information from the
environment. Therefore, many characteristics of the acquisition of visual information during human movement still challenge researchers
who investigate the visual control of actions.
Eye movements are essential to collect accurate visual cues from relevant scene locations, allowing optimal control of human
movements in many contexts
1
, such as locomotion, balance activities, sports, and driving. Eye movements are particularly essential to
guide the movement during obstacle avoidance
2
– obstacle crossing or circumvention –, especially for people with gait impairments
3,4
.
Athletes’ gaze behavior also reflects perception-based decision-making and execution of motor responses involved in dynamic sports
settings. Literature reveals strong evidence that skilled athletes show more efficient patterns of visual search than their less-skilled
counterparts
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Eye movements are important to explain motor behavior, including questions related to visual attention mechanisms, the
relevance of visual information to action control, and learning and gaze behavior training
1
. Understanding the relationship between the
object or scene and the observer shows only part of the complexity of actions’ timing. More importantly, however, visual information is the
basis for humans to guide their actions in order to achieve behavioral goals and synchronize body movements with external events in a
perceptual-motor coupling. Despite being constantly used, such relationship is far from simple and trivial. To investigate this topic, it is
necessary to identify the association between the most relevant scene characteristics during visual information acquisition and the motor
control parameters compatible with this information
6
. Thus, for this research, it seems more important to investigate why humans look at
certain visual information when performing complex tasks instead of just knowing where they are looking at
1
.
Obtaining spatial-temporal data on eye movements assists in the investigation of most relevant aspects of visual information,
perceptual mechanisms, and visual control during motor action. Thus, accurate and reliable eye movement data collection is important to
explain and uncover many aspects of human motor behavior. This manuscript is a tutorial for eye-tracking technique use and data
analysis. So, in the following sections, we present the main video-based eye-tracker techniques used in human motor behavior and sport
science studies. To understand the functioning of eye-trackers, we present a brief description of the human eye anatomy and functional
gaze behavior. Two main types of corneal reflection techniques, as well as calibration methods, data analysis, and the potential
interpretation of main dependent gaze variables are discussed. Finally, we focus on pointing out limitations and advantages that, in our
view, should be taken into account when buying an eye-tracker for empirical work.
HUMAN EYES
The main structures of the human eye include the cornea, sclera, lens, pupil, iris, and retina. Externally, the cornea, sclera, and
lens play important roles, while internally, the pupil, iris, and retina are key components. The sclera is the opaque white part of the eyes
that provides structural support and prevents the passage of light that could degrade the image on the retina. The lens is a gelatinous
structure that can adjust its shape to focus incoming light precisely.
The cornea is a thin tissue that covers the front of the eye and reflects a portion of the light as it travels toward the pupil. The
pupil, which is controlled by the iris (eyes’ colored part), is the opening that allows light to enter the eye. The size of the pupil is
intrinsically linked to various factors such as the amount of light stimulus
7
, the autonomous
8
and limbic system
9
, and cortical areas
10
.
Located at the back of the eye, the retina is a small tissue composed of neurons that play a vital role in visual processing. Within the
retina there are two types of photoreceptor cells responsible for converting light into electrical signals: rods and cones. These signals are
then transmitted via the optic nerve to the primary visual cortex, where images are interpreted and understood. However, it is important to
note that not all information the retina sends to the cortex is processed.
Humans primarily perceive detailed vision through the fovea, which is a small area located in the middle of the macula on the
retina. To direct luminous stimuli to the fovea, the eyes need to be moved. The fovea encompasses approximately 1 to 2° of the central
retinal area and provides the highest level of acuity or clarity
11
. The fovea consists of specialized photoreceptor cells called cones, which
transmit electrical impulses to the brain for interpretation in the presence of ambient light
12
. Cones enable finer and more precise visual
discrimination due to their one-to-one relationship with bipolar and ganglion cells within the retina. However, as the stimulus moves
toward the periphery, specifically the parafovea, the resolution of the image decreases because of the reduction of the number of cones.
The parafovea covers roughly 10° of the visual arc and is composed of photoreceptors called rods. Rods are more sensitive to
light and motion but are unable to provide detailed resolution or color perception due to their many-to-one mapping with the underlying
retinal ganglion cells
13
. Due to eyes’ structural and functional characteristics, individuals need to continually adjust their eye position by
moving their eyes toward the target of interest in order to maintain visual resolution
14
. This dynamic eye movement is crucial for
exhibiting well-defined visual behaviors.
GAZE BEHAVIOR
Visual inputs that enable the identification of world characteristics for the brain to process are obtained through the eyes’
structure but are heavily influenced by eye movements. Eyes’ movements play a crucial role in the interaction between an individual and
their environment, contributing to perception and attention
15
mechanisms associated with cognitive and visual processing
16
. In recent
years there has been significant growth in the study of gaze behavior, which focuses on examining eye movements
7,17,18
. Gaze behavior
encompasses the specific eye movements an individual performs, which are personally relevant to them
19
. These movements’ purpose is
to seek relevant information and facilitate interaction with the environment. Studying eye movements during everyday tasks such as
driving, walking, and crossing obstacles, but also during sports-related skills like decision-making and anticipation provides valuable
insights into both visuomotor and visual-cognitive behavior and impairments. Analyzing eye movements in these contexts is an important
tool for understanding the intricate relationship between vision, motor actions, and cognitive processes
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There are five types of gaze behavior:
1) Gaze fixation: it plays a crucial role in human vision by allowing us to stabilize our gaze on specific informative areas in the
visual field, primarily using the fovea region for detailed information processing. During gaze fixations, the eyes remain stationary at a
point of interest in the environment for a certain time. The duration of gaze fixations can vary, typically ranging from 80 to 1500 ms,
depending on factors such as the scene or environment’s complexity and the task being performed
22
. In contexts that require high
performance, such as sports, medical situations, and other motor tasks, the concept of the “quiet eye” becomes relevant. The quiet eye
phenomenon refers to the last fixation on a specific object or location within a visual angle of 3°. This fixation must endure for at least 100
ms and is associated with maintaining visual focus and concentration
5
.
2) Smooth pursuit: it is an eye movement characterized by slow and continuous tracking of a moving object, wherein the
velocity of the eyes closely matches that of the object being pursued. Human smooth pursuit velocities typically range from 10 to 90 °/s,
with eye speed reaching saturation at around 87 °/s, depending on the visual target’s
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characteristics. However, when there are rapid
and sudden changes in an object’s position within a visual scene, the execution of smooth pursuit movements is impractical. In such
situations, it is challenging to extract relevant visual information due to the limitations of smooth pursuit in responding to quick object
displacements.
3) Saccades: rapid and sudden eye movements are referred to as saccadic eye movements. These movements are
characterized by preprogrammed and ballistic rapid shifts of the eyes, reaching speeds of up to 700 °/s
7
. The primary function of
saccades is to rapidly bring a new part of the visual field onto the fovea, the area in the retina responsible for detailed vision. During
saccades, the retinal image undergoes a high-velocity displacement that results in a momentary reduction of visual processing. This
phenomenon is known as saccadic suppression. As the eyes rapidly move, the visual system experiences a temporary “blinding” effect,
leading to poor visual perception during saccades
7
.
4) Vergence movements: it occurs when the fovea of each eye aligns with targets located at different distances from the
observer. Eye movements are typically conjugate, which means that both eyes move in the same direction. However, when viewing
objects at varying distances, convergence movements occur in a deconjugate (or disjunctive) manner. This means that the vision’s lines
from each eye converge or diverge in relation to the object being observed. The convergence movements are part of the “near reflex
triad”, which involves three components: 1) convergence; 2) accommodation of the lens - which brings the object into focus, and 3)
pupillary constriction - increasing the depth of field and sharpening the image on the retina
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.
5) Vestibulo-ocular movements: they compensate for head movements and stabilize the eyes in relation to the observed
environment. These reflex responses play a crucial role in preventing visual images from slipping across the surface of the retina as the
head’s position changes. Vestibulo-ocular movements can be observed when an object is fixed and the head moves from side to side. In
this scenario, the eyes move in the opposite direction to the head movement, ensuring that the object’s image remains in the same place
on the retina. This compensatory eye movement occurs because the vestibular system detects transient changes in head position and
generates rapid corrective eye movements. The sensory information received from the semicircular canals determines the direction of
eye movement. It is important to note that the vestibular system is “insensitive” to slow or rotational movements of the head but can
effectively counteract fast movements. During continuous rotations without visual cues, such as in the dark or with eyes closed,
compensatory eye movements cease after approximately 30 s of rotation. However, when the same test is conducted with visual cues,
compensatory eye movements continue due to the activation of the smooth pursuit system, which detects movement in the visual field
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.
Considering the five types of gaze behavior (fixation, pursuit, saccades, vergence, and vestibulo-ocular movements), there is a
growing interest in understanding and obtaining information about the first three types—fixation, pursuit, and saccades—, as they relate
to the human visual control of actions. These measurement processes have been used for some time, but technology advancements
have significantly enhanced several aspects of these processes. As a result, the quality of eye-tracking data and analysis has improved
considerably. In this context, we discuss several relevant issues regarding eye-tracking.
DESCRIPTION OF EYE-TRACKING EQUIPMENT
Modern eye-tracking equipment has been specifically designed to track eyes’ movements and gather visual information during
more complex everyday tasks, mobility activities, and sports activities. Depending on the type of device that is used, the components
required for tracking and acquiring gaze data may vary.
For static eye-tracking devices such as video-based systems, the necessary components include a mounting bracket that is
positioned directly on a surface (such as monitors or laptop screens) or supported on a tripod. The device is connected to a computer via
a USB cable, which is equipped with software for recording, analyzing, and visualizing gaze data. These systems utilize a camera that
captures images of both eyes, allowing some tolerance to head movements. On the other hand, mobile eye-tracking devices consist of
glasses or a head-mounted unit that includes cameras for tracking the eyes (eye-camera) and capturing the scene (scene camera).