Excerpts from a coming book by Valerie Dejean
B ILATERAL COORDINATION is another function particularly influenced by the vestibular system. It provides the opportunity for the two sides of the body to communicate with each other at the level of the brain stem via the vestibular nuclei. In this manner it supports the ability of the body to use both sides in a coordinated manner. We see this initially when the baby starts to develop equilibrium reactions where one side of the body responds differently yet in a coordinated manner with the other side of the body. From this activity the baby develops a sense of where his center is, and then how to move around it (rotation), and across it (crossing midline). This awareness provides the foundation for the development of laterality (sometimes incorrectly referred to as dominance), and for the specialization of skills on each side of the body. Many of our advanced human skills, such as language, are dependent on a good foundation of lateralization and specialization.
The vestibular system is anatomically joined with the cochlear system, and it has many close neuronal associations with the pathways for auditory processing and language. Decreased vestibular processing can impact on the area of speech and language development.
P roprioceptors: Our proprioceptors give us information about the position of our body parts. These receptors are located in our muscles, tendon, ligaments and joints. Our vestibular system is actually a specialized proprioceptor. We don't have to look at our arms and legs to know where they are and exactly what position they are in.
Kinesthetic Perception Body Perception or Body Scheme: Body Percept is like an internal map or blue print of our body. It is at a higher level of sensory integration because the brain needs to integrate tactile, vestibular, visual, auditory, and propriocetive messages to form an internal perception that tells us exactly the position of each body part in relation to the other parts, as well as each body part's potential for movement, the kinesthetic sense. It is essential for the kinesthetic sense to have a feeling of Gravitational Security. This is our feeling of security when we are moving our body in space and in changing our relationship to gravity. It is essential to have good security in our body's relationship to gravity to release ourselves from being earth bound.
Body Map or Body Schema Once we have put this all together we have the resultant internalized picture, referred to as our body map or body schema. Sensory information is registered from the body and organized into neuronal models, which are replications of the environment and our mechanical selves. The vestibular, tactile, proprioceptive (information about our body position from our muscles and joints)and kinesthetic systems enable an individual to develop an understanding of self. We need a 'self' and a 'non-self' in order to interact with and understand the world around us. The vestibular system of the inner ear plays a major role in integrating the information from these other senses and putting it all together into a meaningful whole. Dr. A. A. Tomatis described this role as the vestibular integrator. Good sensory perception is important in the development of accurate neuronal body models or rather body schema.
D istance Receptors: Seeing (vision) and Hearing (audition) So far in discussing the vestibular integrator we have been talking about experiences that are either within our body, in contact with our body, or have to do with how our body relates to the environment. We feel something when it touches us and we are constantly adjusting our bodies to the force of gravity. When it comes to vision and hearing we have a different experience. We see and hear things outside our bodies, and we refer to these sensory systems as external or distance receptors. We hear or see things from a distance; it does not have to come in contact with the body. Sensory information from the visual and auditory systems has to be integrated with the other senses and this is again accomplished through the vestibular system.
Visual Integrator Vision involves the mechanical reception of light, and visual perception is how we interpret that information. Dr. A. A. Tomatis referred to the visual integrator as the mechanism for integrating visual information from the eyes with vestibular information from the body. The vestibular system has a direct and very fast connection with the eyes. This allows the individual to quickly detect whether he or the environment is moving. Have you ever had the experience of sitting on a train stopped in the station next to another train? As the other train starts to pull out, you experience a moment of anxiety as your body tells you that you are motionless, yet your eyes tell you that you are moving. In that instance you have a sensory mismatch, and it is a good example for how these two systems are always referencing back and forth between each other.
This vestibular-visual integration is very important developmentally because a baby starts to attach meaning to his visual environment via this double-checking with the vestibular system. The baby starts to recognize that it is the same rattle whether he is lying down or sitting up. He recognizes that objects are the same no matter which way they are flipped. He starts to know if he is to the left, right, over, under, in front of, or behind an object, as well as how these objects relate to him, long before he knows the words for these orientations. In fact it is hard to learn the words for these prepositions if you don't 'get' the physical experience of these positions through good vestibular-visual integration. The vestibular system provides the foundation for accurately interpreting information from our visual field. Therefore it has a major impact on the development of visual perception. People who have been blind since birth and regain their vision, are completely overwhelmed by what they see because their brain doesn't know how to make sense out of it. Developmentally the visual system depends on the vestibular system to make sense out of what one sees. Space perception (where we are in space/directionality), visual perception (spatial orientation of object and symbols such as letters), and even linguistic concepts of prepositions, are end products of sensory integration that are dependent upon good vestibular-visual integration.
Cochlear (Linguistic) Integrator Dr. A. A. Tomatis distinguished between hearing, which he described as the passive reception of sound, and listening, which he described as the active ability, intention, and desire to focus on sounds. This is similar to how we distinguished between sight and visual perception. It is possible, and even likely to have normal hearing, yet have poor listening. A child may be able to hear a pin drop from across the room, yet not be able to attend and listen to what is being said to him. The vestibular system and the cochlear, the part of the ear that analyses sound, are anatomically joined and form what we call the inner ear. The VIII cranial nerve carries sensory information to the brain from both the balance and hearing parts of the inner ear. The auditory and vestibular systems lie closely together throughout the nervous system. This allows for much opportunity for sensory integration between the vestibular and auditory systems. Sensory integration disorders that involve vestibular processing can impact the area of speech and language development. Research has found that therapy aimed to improve the function of the vestibular system can also result in improved language.
Dr. A. A. Tomatis discovered that faulty sensory information from the ear could affect vocal output. The concept that the voice can only produce what the ear can hear was known as the 'Tomatis Effect'. When children mishear sounds, they will misarticulate them also. This can have a significant impact on speech development. Faulty sensory information can also affect auditory perception. The auditory system is required to interpret all the sounds of spoken language and attach linguistic meaning to them. For example, a dog is able to hear as well or better than humans; however the dog's ear isn't able to separate the speech stream into meaningful words that he can understand. This requires auditory perception and auditory processing. Together they provide the foundation for understanding language, spoken or written. When we mishear sound through faulty perception and processing, we have difficulty attaching these sounds to the visual symbols for them (letters). Because we mishear the sounds we then misspell them. So problems with reading and writing can be associated with an auditory problem, not just a visual problem. Although we separate auditory perception and processing for diagnostic reasons we often refer to difficulties with them under the single title of 'auditory processing disorders.'
Auditory processing disorders are often related to a disorder of processing within the vestibular system and to difficulties in integrating sensory information between the vestibular and auditory systems. The auditory system needs the stable base provided by the vestibular system in order to process information. Much like the visual system, which has to reference what it sees through the vestibular system, the auditory system also must perform a similar reference. Without stability from the vestibular system, it is difficult for the auditory system to accurately interpret the sound stream.