November 18, 2010
Every motor event is a sensory event first
Often we tend to think of voluntary motor tasks as events involving the musculoskeletal system and the motor control system. But long before the motor task comes into being, there is a plan for the motor task. And long before the plan for the task is the sense of where our body is in space and the sense of midline is for our body. This is where the sensory systems come into play for postural control. It is our postural control that allows us to keep our balance during that voluntary motor task.
There are 3 senses that are hugely important to postural control/balance – proprioception, vestibular and vision.
Vision: our visual sense not only registers the characteristics of the object but also where it is in space in relation to our body.
Proprioception: the sense of where our body is in space, as registered by our joint and muscle receptors.
Vestibular: the sense of where our body is in space, specifically in relation to gravity as registered by the vestibular mechanism in the inner ear.
Vestibulo-ocular reflex: the connection between the visual and vestibular systems that keeps the image in the centre of our visual field as we move our heads.
We know that as children we use vision as our primary sense for balance until we are 6 years old. After that we begin to be able to combine the visual input with our proprioceptive and vestibular input between the years of 6 to 12 years. Finally, after 12 years of age we begin to be able to successfully solve balance tasks that involve sensory conflict. Consider the classic example of sensory conflict: you are sitting in a train, looking out the window and the train right next to you begins to move. It takes you a moment to figure out that the other train is moving but you are not; this is sensory conflict. This progression in the sophisticated use of sensory information is secondary to the maturation of the vestibulospinal tract and the consequent maturation of the vestibulo—ocular reflex.
It is not surprising that many children who experience challenges with movement retain their reliance on their vision for postural control. Children who experience difficulties with their registration and processing of proprioceptive and vestibular information, have difficulty knowing where they are in space. They consequently cannot move beyond vision to more complex sensory processing for balance. In addition, the vestibular system is critical in setting the amount of anti-gravity muscle tone and it modulates the amount of activity at the respiratory diaphragm (via the vestibulospinal tract). The diaphragm happens to be one of the 4 key inner Core muscles responsible for anticipatory postural control. When the vestibular system is not functioning properly, our postural control is compromised in this way as well. Unfortunately all of this makes the coordination for higher level balance skills and higher level gross motor skills a challenge.
Also, some of our children are sensitive to visual input. Over 50% of the brain is devoted to processing visual information on some level, and a lot of visual information can be overwhelming for these children. They may avoid paying attention to information regarding their position in space in order to avoid visual overload.
Finally, many of our children do not have a solid connection between their visual and vestibular systems. The vestibulo-ocular reflex enables the eyes to stay centred and steady as our head moves. This reflex functions even when we are “at rest”, as we have a natural postural sway in quiet sitting and standing. When your eyes don’t keep up with the movement of your head, you process information about where you are in space (and the objects in that space) much slower, and incoordination can result.
All in all, the sensory systems make an enormous contribution to postural control and the development of motor skills. It’s best for us to remember that every motor event begins as a sensory event and pay equal amounts of attention to the sensory systems, the musculoskeletal system and the motor control system.
Kim Barthel, OT, will be teaching Seeing, Moving May 4 – 6, 2011 in St. Catharines. This course will increase our understanding of the neurobiology of the visual and vestibular systems and discuss the brain mechanisms that support the integration of these systems through functional treatment strategies. For information, email firstname.lastname@example.org