Dyslexia has been defined as, "a disorder in children who, despite conventional classroom experience, fail to attain the language skills of reading, writing and spelling commensurate with their intellectual abilities" (World Federation of Neurology 1968). More recently this definition has been expanded and described as, "a complex neurological condition which is constitutional in origin.
The symptoms may affect many areas of learning and function and may be described as a specific difficulty in reading, spelling and written language. One or more of these areas may be affected. Numeracy, notational skills (music), motor function and organisational skills. However, it is particularly related to mastering written language, although oral language may be affected to some degree (British Dyslexia Association, 1998).
In addition to problems with reading, spelling and written language expression, dyslexic children often manifest problems with motor skills such as hopping and skipping, catching and throwing a ball; learning to ride a bicycle, coordination at gym and sometimes swimming; problems with directionality, such as telling left from right, laying a table correctly and telling the time from an analogue clock.
Problems with fine muscle skills may include difficulties tying shoelaces, doing buttons up and manipulating a writing instrument. Sequencing, visual memory and auditory perception may also be affected and there may be ambiguity of laterality (Ott, 1997). Performance in these areas is dependent upon the maturity of the reflex system which underlies motor learning, vestibular functioning and kinesthetic integration.
Reflex abnormalities affect performance in automatic balance, posture and motor skills. For example, a retained Tonic Labyrinthine Reflex will affect balance and distribution of muscle tone depending on head position in relation to the midplane; If the Asymmetrical Tonic Neck Reflex is retained it will affect hand-eye coordination, ability of the hand and sometimes the eyes to cross the vertical midline and the development of cross pattern movements.
A retained Symmetrical Tonic Neck Reflex can affect both sitting posture and coordination because it creates a horizontal midline barrier within the body which interferes with integration between upper and lower sections of the body. If the Palmar Reflex remains, it will affect the development of fine finger movements, particularly the ability to hold a writing instrument in a tripod grip. These basic motor abilities are fundamental to many of the skills required for academic learning.
Eye movements may also be affected as a result of the links between the labyrinths and the eyes via the vestibular-ocular-reflex arc (VOR). Neurological connections also exist between the vestibular and the auditory systems.
Although these two systems are considered separately, the cochlear and the semi-circular canals of the vestibular apparatus adjoin within the inner ear, they share the same fluid and a common pathway for the transmission of signals to the brain along the 8th cranial nerve. Dysfunction in one system can affect the processing and perception of information from the other.