Vision is one of the main components which is used to control balance. Investigation into effects of different viewing conditions could prove important to understanding more about the interaction between vision and balance. The aim of this study is to assess whether fixating under monocular conditions vs fixating under binocular conditions has an effect on postural control. In this experiment the Modified Clinical Test for Sensory Integration of Balance (mCTSIB) and the NeuroCom® Balance Master® will be used as an assessment tool and the effects of eye dominance will also be investigated.
1.1 ANATOMY OF THE VESTIBULAR SYSTEM
1.1.1OVERVIEW OF THE VESTIBULAR SYSTEM
The human vestibular system involves three components: the peripheral system, a central processor and a device for motor output. (Hain and Helminski, 2007). The role of the peripheral sensory apparatus is to provide information via signals to the central nervous system about head angular velocity and linear acceleration (Hain and Helminski, 2007). The central nervous system is made up of the vestibular nucleus complex and the cerebellum, which combines these signals from the peripheral system with other sensory information and uses the information to make inferences about head and body orientation (Hain and Helminski, 2007).This information is then transmitted to the spinal cord and the ocular muscles in order to aid in three important reflexes: the vestibulo-ocular reflex (VOR), the vestibulocollic reflex (VCR) and the vestibule-spinal reflex (VSR). The functioning of all of these reflexes is checked by the central nervous system (Hain and Helminski, 2007).
1.1.2 VESTIBULAR RECEPTORS
The two vestibular receptor organs are the semicircular canals and the otoliths. The hair cells within the neuroepithelium of each of these organs is connected to a gelatinous structure (HOFFMAN). In the macula of the otoliths the structure is called the otolithic membrane and in in the cristae of the semicircular canals this is called the cupula. (HOFFMAN) The function of these hair cells is to transfer information to the specialised areas of the central nervous system after converting the mechanical information which is generated by head movements. (Hain and Helminski, 2007). In both receptor organs the neuroepithelium is attached rigidly to the head, and thus its movement is equivalent to head movements. However the gelatinous substances are affected by the endolymph which surrounds them, and thus affects the hair cells when the endolymph resists such movements. (HOFFMAN)
Differences in the mechanics of the structures mean that the semi-circular canals respond to angular velocity whereas the otoliths will respond to linear acceleration. (Hain and Helminski, 2007)
1.1.2.1 SEMICIRCULAR CANALS
The semi-circular canals are comprised of three membranous tubes – the cross sectional diameter of each is a maximum of 0.4mm- aligned to form a coordinate system (Hoffman) with the function of sensing rotational movement (Furman). There is one horizontal canal and two vertical canals (anterior and posterior) on each side of the head and these are paired. (Hoffman) Each of the canals are stimulated by rotation on a particular plane, the three planes on which these stimulations occur are approximately perpendicular to each other (Furman). These adaptions mean that movement can be sensed in any direction.
At the openings of each semi- circular canal the tubes enlarge to form an area called the ampulla (Hoffman) which has a role in the conversion of rotational motion into neural activity (Furman). The ampulla is located at the anterior opening of the horizontal and anterior canals and at the inferior opening of the posterior canal (Hoffman). Each ampulla contains a cupula, made of a gelatinous membrane which forms a seal on the canal and is crossed by the crista (Hoffman). The crista contains hair cells, which are positioned in a way which allows all their kinocilia face the same direction. However this direction is different for the horizontal canals than the vertical canals, resulting in nerve fibres in canals being stimulated by different movements. This difference is one function which allows direction to be sensed. (HOFFMAN)
1.1.2.2 Otolith Organs
The function of the two otolith organs: the utricle and the saccule, is to detect linear accelerations of the head (Neuro). Within both organs is the sensory epithelium called the macula, which is a differentiated patch of membrane (Hoffman) made up of hair cells and other associated supporting cells (Neuro). The macula is located in the medial wall of the sacular cavity and near the posterior opening of the horizontal semicircular canal in the utricle. The macula of the saccule is largely in a vertical position whereas the macula of the utricule is mainly in a horizontal position (Hoffman). Like the semi-circular canals, the otoliths are also arranged to allow them to respond to motion across all three dimensions. However, while the semi-circular canals have three sensory organs – one for each axis of angular motion, the otoliths only have two sensory organs for all three axes of linear motion (Hain and Helminski, 2007). The hair cells and their bundles in the macula are covered with a gelatinous layer above which is the otolithic membrane. Embedded within the otolithic membrane are otoconia which are crystals of calcium carbonate (neuro) with a density more than double the density of water (Hoffman). The mass provided by the otoconia makes the otoliths extremely sensitive to movements, such as tilting the head (Hain and Helminski, 2007).
1.1.3 VESTIBULO- OCULAR REFLEX
The vestibulo –ocular reflex (VOR) is a mechanism which ensures the eyes stay on target as any head movement will automatically trigger a conjugate eye movement equal and in the opposite direction to the turn. (Furman 2). The VOR is comprised of two components. One is the angular VOR which compensates for rotation and is mediated by the Semicircular canals and is important for gaze stabilisation. The other is the linear VOR controlled by the otoliths which is important when the head is being moved at high frequencies or close targets are being viewed (Hain and Helminski, 2007). The VOR works because its output neurones are the neurones which control the extraocular muscles.
When the head turns endolymph moves and displaces the cupulae, the discharge rate from the hair cells in the ipsilateral crista increase in proportion to the velocity of the head movement, comparably discharge rate decreases in the contralateral crista. These differences in discharge effect the neurones in the medial and superior vestibular nuclei and cerebellum which then excite the oculomotor nuclei and result in eye movements. (Hain and Helminski, 2007).
The arrangement of the extraocular muscles, which are in pairs, and their proximity to the canals allows a pair of canals to be connected mainly with a single pair of extraocular muscles which results in conjugate eye movements equal to and in the same direction as head motion (Hain and Helminski, 2007).
INTEGRATION OF SENSES TO MAINTAIN BALANCE
A combination of sensory inputs from all sensory systems is required to maintain postural equilibrium. (Horak and Macpherson, 1996) HOFFMAN CENTRAL PROCESSING
Role of Vision
The Romberg quotient has shown that vision is the most important system in maintaining postural control (Edwards 1946). Previous research using Posturography has supported this opinion by finding that when evaluating body-sway in normal subjects the sway area is increased by two or three times for eyes closed conditions compared to eyes open (Henricksson et al, 1966). However in conditions with the eyes open any body oscillation will cause retinal slip. (Paulus et al 1989) Retinal slip is when the image on the retina moves… . This retinal slip results in action from the appropriate muscles which reduce body sway. (Paulus et al 1989) Studies by Kapoula and le 2006 and Strupp 2003 have suggested that certain afferent ocular motor signals also play a major role in the control of body orientation (paper 2) which raises the question of whether it is vision which is resulting in the control or signals from the ocular muscles
1.2 OCULAR DOMINANCE
1.2.1 WHAT IS THE DOMINANT EYE
The idea of the dominant eye was first investigated in the work “De refractione” in 1593 by Giovanni Battista della Porta (FINK, 1938). Since then many investigations have been made into the effects of the dominant eye, and studies have been completed which evaluate what significance the dominant eye has to different functions in everyday life. The dominant eye is recognised as the eye which leads in the function of seeing, and is then assisted by the less important non-dominant eye (FINK, 1938).and can be likened to the way that people will usually have a dominant hand which functions better than the other one- for example in handwriting.