September 21, 2016
The Eyes Have It: The Foundation
I learned very little about vision in PT school. When clients came to me, parents would often say their child’s vision had been checked and it was fine. Except that as I watched them move, their vision didn’t seem fine. And so I began yet another educational journey – this time to learn about functional vision.
The term functional vision is in itself a generality, referring to the approximately 17 skills required for the eyes to work together to help us navigate the world. Here’s a wonderful discussion of functional vision by Cam McCroden, a developmental optometrist in Victoria BC.
In explaining the importance of functional vision, professionals often focus on reading skills (did you see what I did there? 🙂 ). But PTs should be thinking about the impact of functional vision on posture1, balance2 and the ability to move through space3, which contribute to visual-perceptual skills and yes, eventually to reading.
Vision is the primary sensory system used for postural control, until we approach 6 years old, when our brain is able to pay attention to somatosensory information at the same time. 2 In order for visual information to be used for postural control, the eyes need to progress from working separately (as they do at birth), to both eyes working together (binocular vision) during various skilled eye movements. This prerequisite of binocular vision is required for depth perception. Without depth perception, there cannot be proficient balance.4
When a child is born, the eyes work independently of each other – it’s a little disconcerting to watch a newborn and see the eyes move separately. At birth, we know the baby sees 8 – 15 inches but vision is a little fuzzy. However, the vestibular system is already hard at work partnering with the visual system to orient the eyes relative to gravity as visual acuity improves. Neck flexion when holding/feeding the infant places the horizontal semicircular canal in an improved position and thus provides input to the medial and lateral rectus muscles of the eye; this is the beginning of the eyes working together (binocular vision) and developing convergence (there is more to this connection but not enough space to delve into it here).
Development in all systems continues and by 8-10 weeks of age, the neck musculature displays anti-gravity activation during postural activity. This is termed the visual-vestibular-cervical triad5. This activation of the neck musculature is a critical component of the base of support for the eyes.
There are interconnections between these three systems. The vestibulocollic reflex (VCR) connects the vestibular system to the muscles of the neck, so the neck muscles can stabilize the head in space. It is foundational to the visual-vestibular-cervical triad. The vestibulo-ocular reflex (VOR) connects the vestibular system and the visual system to keep the image in the middle of the fovea when we move our head and is foundational to our visual processing abilities.
When development is typical, dynamic systems create central stability which provides a secure postural base for the eyes, such that the eyes can move efficiently and independently of the head/body, in all positions and during all movements. However, if any of the components are disrupted – visual, vestibular, musculature – then the ability of the eyes to work together for skilled, functional movements can be compromised. This in turn compromises function 3.
Next up in this functional vision series, we’ll talk about visual efficiency vs. visual-perceptual skills and how they impact gross motor skills.
1. Lidbeck C, Bartonek A, Yadav P, et al. The role of visual stimuli on standing posture in children with bilateral cerebral palsy. BMC Neural. 2016; 16(1): 161.
2. Foudriat BA, Di Fabio RP, Anderson JH. Sensory organization of balance responses in children 3-6 years of age: a normative study with diagnostic implications. Int J Pediatr Otorhinolaryngol. 1993;27(3):255-71.
3. Land M. Eye movements and the control of actions in everyday life. Prog Retin Eye Res. 2006; 25(3): 296-324.
4. Barry, S. Fixing my gaze. Philadelphia, Basic Books, 2009.
5. Padula W, Munitz R, Magrun WM. Neuro-visual processing rehabilitation: An interdisciplinary approach. Optometric Extension Program Foundation, Santa Ana, 2012.