**1. Introduction**

Visual and motor development involves a physical and physiological process that allows perceiving precise details of an image (eyesight) and a perceptual process (vision) that requires multisensory integration: vision, hearing, touch, and proprioception for the interpretation of visual information. Vision delivers a key sensory input required for the proper functioning of neural circuits and is therefore a brain process of sensory integration [1] that offers important information in most daily activities [2]. Binocular vision offers improvements over monocular vision. It allows precise depth perception (stereopsis). It also enables the exploration of the visual scene during movement planning of the hands and feet directed at a target in space, such as the hand to grasp items [3] and the feet to walk [4], climb obstacles [5], or even for sitting postural control [6]. Three- and six-month-old babies can better assess if objects are within reach and move toward them using both eyes instead of one [7], as demonstrated during crawling [8].

Toddlers follow a pattern of skill development that allows them to know when they are progressing adequately. The first years of life are critical to a child's overall development. The central nervous system grows at a very fast rate, similar to the skills that the child develops. During this period, the development of postural reflexes and reactions, basic motor skills, and gross voluntary motor skills lay the foundation for mature motor behavior [9]. Many developmental tests are based on motor activity, such as the Peabody Developmental Motor Scale-II (PDMS) [10]. Gross motor development assesses muscle control, coordination, and locomotion. Fine motor skills include the development of control and coordination of body segments to perform more precise and complex tasks, integrating muscle coordination and perceptual skills [11]. Motor development can be influenced by other areas of development above and beyond sensory systems during childhood [12] For example, babies learns motor skills through observation [13].

Many studies have established a relationship between visual function and motor development [14–19]. They claim that binocular vision disorders such as amblyopia and strabismus can adversely affect skills that depend on eye movements, including reading. Many of these studies assessed only fine motor skills or eye-hand coordination in school-age children and their relationship to amblyopia. They studied groups of participants with different age ranges, between 5–9 years [14], 8–12 years [15–17], and 10–30 years [18]. Other research determined that children with poor stereoacuity have significant visuomotor deficits compared to typical developing children [20], while other reports affirmed that stereoacuity may be limited to specific tasks. In a group of children aged 5 to 13 years without visual impairment, Alramis et al. [21] showed that binocular vision and stereoacuity were associated with higher performance of certain fine motor tasks, and task performance decreased in younger children. They concluded that the role of vision in the performance of fine motor skills depends on both the task and age.

In addition to fine motor skills, few studies measured postural stability and control or gross motor skills in typical developing children. Some reported a delay in motor development and skill acquisition in children with visual disabilities and described a delay in gross motor skills such as control of the head and ability to sit, crawl, and walk during the first year of life [12,22]. Souza et al. [23] observed that a group of 15- to 22-month-old toddlers with visual impairment presented an overall delay in neuropsychomotor development, mainly in coordination. Celano et al. [24] suggested that there may have been a delay in fine motor function and balance in a group of 4.5-year-old children with unilateral visual impairment secondary to congenital cataracts. Chakraborty et al. [25] demonstrated a close relationship between stereopsis and fine and gross motor skills in a group of 4.5-year-old toddlers. Thompson et al. [26] studied a large group of 2-year-olds to fully evaluate the state of vision (visual acuity, stereopsis, alignment of visual axes, eye motility, and self-refraction) and its possible relationship with motor development. Their results demonstrated that global perception of movement and binocular vision are associated with motor function at an early stage of development as measured by the Bayley Scale of Infant and Toddler Development 3rd edition (BSID-III). All of the scientific literature reviewed included babies and toddlers [12,22–24] with some form of visual impairment or born with risk factors for neurological development [25,26]. To the best of our knowledge, this is the first report to study visual and motor development in a sample of between two and three-year-olds both visually and from the perspective of motor development.

The purpose of this study was to describe the vision development differences between slow and fast motor development in typical developing toddlers. Vision study included ocular health screening, visual acuity, refractive errors, ocular alignment, motor fusion and suppression, ocular motility and stereopsis. Among motor development parameters, we studied fine motor development through grasping and hand-eye coordination and gross motor development, analyzing static, locomotion, and object manipulation.
