Week 4 - Intro to BV Flashcards
What is BV?
• the ability to use both eyes together
• each eye receives a separate retinal image which is interpreted by the brain as a single common perception
• in normal usage the individual sees as one
Advantages of BV:
• main advantage of binocular single vision is stereopsis
- individuals without BSV use monocular cues to appreciate depth, beyond a few metres monocular cues are more important, stereopsis contributing little to depth perception
• spare eye
• large field of vision
• binocular summation (square root)
- visual acuity, contrast sensitivity, motion
• enhanced visual-motor skills
BV: Perception of space
• visual system tells us what and where
- sense of direction
• when we look directly at an object the image falls on the fovea, this enables us to see it clearly and also tells us where the object is located in visual space
BV: Visual direction
• projection
- object location in space depends on the part of the retina being stimulated
- each part of the retina has a given projection for a given distance of fixation
- this is termed visual sign or visual direction
• A system that specifies location/direction needs a reference point
- main reference point for projection is fovea which is described as having the principal visual direction; independent of the direction of gaze
• all other retinal points/areas have a secondary visual direction
What is oculocentric visual direction?
• Visual direction of an obiect can be represented by a line that joins the object and the fovea called the Principal Visual Direction or visual axis
- based on the principal visual direction, the direction of all other objects in the subjects visual field is determined. This is the called oculocentric visual direction
- therefore, each point of the retina can be considered to have it own sense of direction
What are the oculocentric projection directions?
• Nasal retina stimulated projected temporally
• Temporal retina stimulated projected nasally
• Inferior retina stimulated projected superiorly
• Superior retina stimulated projected inferiorly
Egocentric visual direction:
• Egocentric visual direction refers to the direction of an object in space relative to one self rather than the eyes
• Determined by: retinal position, proprioceptive information about the eye, head and body position and the vestibular apparatus
• This information allows us to determine if change in retinal position is due to object movement or due to eye or head movement object has remained stationary
Give an example of egocentric direction
• stationary object imaged on the fovea with head and the body stationary. When the eye moves, the stationary object is then imaged on a new retinal position oculocentric direction has changed but the egocentric direction has not changed as the object has remained stationary
• In another example, the eye tracks a moving object, as the object is imaged on the fovea at all times, the oculocentric direction is the same but the egocentric direction is changing
What is normal binocular viewing?
• certain retinal elements in each eye share a common visual direction
• our fixation object stimulates each fovea simultaneously and each image is projected back to the same location in space
• the 2 foveae are described as corresponding points
What is peripheral retinal correspondence?
• same visual projection in both eyes
• if an object in the right part of the visual field stimulates a temporal point in the left eye it will also fall on a nasal point in the right eye
• these are corresponding points which project back to the same point in space
• this correspondence principle applies to all retinal areas
• this is the basis of normal retinal correspondence
What is cyclopean projection?
• normal retinal correspondence (NRC) is necessary for BSV
• in BSV an object fixated by the two foveae appears to be located centrally by a cyclopean eye
• binocular projection can be considered as if from this imaginary cyclopean single eye
• cyclopean projection diagrams are always used to describe BSV
What is the Horopter?
• Imaginary plane existing in binocular vision
• Represents the locus of all points in space whose images stimulate corresponding retinal points
• The horopter seen in a cyclopean projection is known as the Vieth-Muller circle it passes through the fixation point and nodal point of both eyes
Link between image size and horopter:
• any object placed on the horopter, will have a constant image size on the retina, no matter where it is located on the horopter
• if image size is constant (angular size on the retina), and the binocular disparity remains zero (object stays on the horopter), then any object that is moved along the horopter should always appear to be the same distance from the observer
• 2-dimensional binocular vision i.e. no depth perception
Development of BV depends on:
• Development of Binocular Single Vision depends on the coordination of the Sensory aspects and the Motor responses of the visual processes
• this coordination is not present in the newborn but develop from birth to about 18 months in a series of reflexes called binocular reflexes
Sensory mechanism is comprised of which senses:
• light sense
• form sense … good visual acuity
• colour sense
• spatial relationship….. projection and localisation
Sensory mechanism normal development depends on:
• clear refractive media; normal retina … cones … form and colour rods … light and movement
• good visual acuity in either eye normal
• retinal correspondence
• normal visual pathway from retina to cortex
• normal proprioceptive receptors
• reinforcement of stimulus
• vestibular apparatus otilith and endolymph nerve endings in neck muscles
Motor mechanism purpose:
• to put object of attention onto each fovea …. central fixation
• maintain it there when object moves
• to allow change of fixation from one object of attention to another
• ensure proper alignment of both eyes
Motor mechanism development depends on:
• reinforcement of stimulus
• normal extra-ocular muscles & adnexae
• normal oculo-motor nerve nuclei and pathways
• inter-nuclear and supra-nuclear pathways
• lower motor neuron pathways controlling vestibular mechanisms
How are sensory and motor functions linked?
The sensory and motor functions are linked by POSTURAL and PSYCHO-OPTICAL (binocular) reflexes
What do Postural reflexes do?
• innate controlled by lower motor neuron pathways, independent of visual stimuli (occur in the blind)
•maintain posture despite changes in the position of the head in relation to space and the trunk
•maintain orientation of the eyes despite changes in position of the head
• Present at birth but as visual function develops the control of posture becomes more dependent on the eyes
What are the two postural reflexes?
• Static postural reflex
• Stato-kinetic postural reflexes
Static postural reflex:
• tonic labyrinthine … stimulation of otolith organs when head falls forward eyes move up
• tonic neck …. stimulation of proprioceptive nerve endings in the neck muscles when head is inclined to right shoulder eyes make a compensatory movement to the left
Stato-kinetic postural reflex:
• stimulation of endolymph in semi-circular canals when head and body are rotated results in physiological vestibular nystagmus ….. the slow movement of which is to maintain the eyes as long as possible in the same position in relation to the visual field
Binocular reflexes and types:
• the stimulus is visual but
- interest and attention needed
- these reflexes are conditioned and are basically involuntary (in some instances can be voluntary inhibited)
• Fixation reflex
• Re-fixation reflex
• Conjuncate fixation reflex
• Fusional vergence reflex
• Vergence reflex
• Near reflex
Fixation reflex and re-fixation reflex:
• fixation reflex
…. places object of attention on fovea (present very soon after birth)
• re-fixation reflex
.. passive enables eye to maintain fixation on a moving object ‘smooth pursuit’
….active enables eye to change fixation from one object to another ‘saccadic’
Conjuncate fixation reflex and fusional vergence reflex:
• conjuncate fixation reflex
… this reflex links the first two in binocular fixation demonstrable at 6 months
• fusional vergence reflex
… this reflex maintains bifoveal fixation in the presence of a heterophoria demonstrable at 6 weeks established by 6 months
Vergence reflex and near reflex:
• vergence reflex
this reflex maintains bi-foveal fixation on an object as it approaches the eyes demonstrable at 1 month well established by 6 months
• near reflex
… or near synkinesis this links vergence reflex with pupillary reflex (unconditioned and accommodation reflex
- NB. they are linked but can be demonstrated separately under certain conditions
Characteristics of binocular single vision
• fusion Vision
• retinal rivarly
• stereopsis
• physiological diplopia
Binocular visual Fusion components:
Fusion has two components
• sensory fusion
• motor fusion
Sensory fusion:
• Sensory fusion
- is the ability to perceive 2 similar images, one formed on each retina and interpret them as one
- images must be located on corresponding retinal areas
- similar in size, brightness, sharpness
Motor fusion:
• is the ability to maintain sensory fusion through a range of vergence movements
which may be:
- horizontal, vertical or cyclovergence
Binocular vision retinal rivarly:
• When dissimilar images are presented to corresponding retinal areas fusion becomes impossible
• Dissimilar images localized in the same place give rise to conflict and confusion one or the other is temporarily suppressed
- luminance
- colour
- contour
Binocular vision stereopsis:
• occurs when similar images are presented to non-corresponding retinal areas
• and are situated close to the horopter in Panum’s fusional space they will be fused with the appreciation of depth
• Panum’s fusional space is a narrow band around the horopter within which object points give rise to binocular single vision
Panums fusional space:
• is elliptical in shape narrowest at fixation and increasing in depth towards the periphery this is consistent with larger receptive fields and poorer visual acuity in the periphery
• 6-10 seconds of arc at the center and 30-40 seconds of arc at the periphery
• does not have a fixed size, but varies depending on stimulus conditions e.g. Panum’s areas in the fovea are larger with low spatial and temporal frequencies
Panums fusional area:
• the area surrounding corresponding retinal points within which disparity of correspondence can occur whilst maintaining BSV
• in the presence of binocular vision objects outside of Panum’s fusional space cause very large disparities on the retinas, and they cannot be fused.
- They are seen in diplopia
