normal bsv Flashcards
what is normal bsv
ability to use both eyes simultaneously to contribute to a common single perception
one image from one eye and an image from the other eye (slightly dissimilar images)
what are the 3 components of bsv
- bifoveal fixation - condition where both eyes are simultaneously directed to the same target - the fovea is the centre part of the maccula - if we dont have a squint and we are using our eyes it is the fovea of our eyes that we are using to see - using both foveas to fix on any given object
- fusion - process of 2 images aligning together to form one image
stereopsis- third and highest degree of normal bsv
what are the advantages of normal bsv
allow us to see one image even though we have two eyes
allows us to appreciate depth or 3 dimensional 3d vision
3d vision is known as stereopsis
enlargement of the field of vision
compensation for blind spot and other differences - you have a blind spot in your vision and that is due to the optic disc - the optic disc is a part in your retina that dosnt have any photoreceptors
how does bsv develop
some thought bsv was innate and present from birth
others thought that it was developed after birth - acquired/learnt
current - structures and connections (under normal cirucmstances) are present at birth but the system requires use for normal bsv devlopment
3d vision develops in a child at 3-6 months
what is the critical period
the visual system is developing up to the age of 7 or 8
if they are past that stage and devlopment has occured and they werent able to develop it you can have problems such as amblyopia
what are the benefits of stereopsis
allows us to make fine judgements with resepect to the relative position of objects in space
what are the factors necessary for the development of normal bsv
normal anatomical development of the eyes , associated with connections in the brain and the orbital parts of the skull
overlap of visual fields
nasal retinal fibres must cross at the chiasm - allows you to process the scene in the correct order
fovea (area of most distinct vision) must develop normally
normal (retinal correspondence)
normal (retinal) correspondence
describe the visual pathway
eyeball
retina
optic chiasm
optic tract
lateral geniuclate nucleus of the thalamus
visual cortex
what does their need to be in the visual fields for you to have bsv
- you need to have overlap of the visual fields
- one eye will see a certain amount and then the other eye will seen a certain amount
what happens at the optic chiasm
crossing (decussation) of nasal fibres at the optic chiasm
- fibres from the right nasal and left temporal retina relay imformation to the left visual cortex
fibres from the left nasal and right temporal retina relay imformation to the right visual cortex
why must the fovea develop normally for normal bsv
- the fovea is a depression in the inner retinal surface
1. 55mm wide
the photoreceptor layer = entriely cones - specalised for max visual acuity
within the fovea = an fovea avasualr zone - meaning it has no blood vessles
what is normal correspondence
a binocular conditon in which the fovea and areas on the nasal and temporal side of the one retina correspond to and have common visual direction with the fovea , temporal and nasal retina of the other eye
- basically the fovea of both eyes have corresponding points - they are basically symmetrical in both eyes
describe the fovea in normal correspondence
fovea of left eye corresponds with fovea of right eye
left nasal retina corresponds with right temporal retina
right nasal retina corresponds to left temporal retina
how does normal correspondence lead to bsv
sstimulation of corresponding points in each eye results in a single mental image formation
foveas work together as corresponding retinal points
fusion occurs when then the retinal images fall on corresponding points
if objects fall on corresponnding points they will fuse into a single image
describe projection in normal bsv
the subjective interpretation of the direction of an object
each retinal element projects to a specific point in space
where does the fovea project
the fovea projects straight ahead
the nasal retina projects temporally
the temporal retina projects nasally
the superior retina projects inferioly
the inferior retina projects superiorly
describe monocular projection
-an object stimulates the fovea and is projected straight ahead
something that stimulates the nasal retina is prjected temporally and something that stimulates the temporal retina is projected nasally
in projection diagrams where is the brain represented
in projection diagrams we represent what the brain ‘sees’ by a third central eye called the cerebral or cylopean
what is the horopter
all objects that stimulate corresponding retinal points- i.e. the right nasal retina and the left temporal retina lie on a imaginary line in space called the horopter
if you view a image straight ahead it will stimulate the fovea of the right eye and the fovea of the left eye - these are corresponding retinal points
if you were looking outside the horopter you wouldnt see a single image
therefore if you look at a point on the horopter and you stimulate two corresponding points - you will see the image as single even though you have two eyes
any object lying on or very close to the horopter will be seen singly
does everyone have the same horopter
each person has their own horopter for any fixation distance - the locus of all object points that are imaged upon corresponding retinal elements at a given fixator distance
the locus of all object points that are imaged upon corresponding retinal elements at a given fixator distance
what is the veith muller horopter
a theoretical determined circle passing through the fixation point and nodal points of each eye
what is panums fusional space
a small area around the horopter in which single vision is still present
, so an object may lie slightly ‘’off’’ the horopter but will still be seen singly
panums space is narrow centrally and wider peripherally
if retinal elements stimulated are almost corresponding points the object will still be seen singly as long as it lies within panums fusional space
where is panums area narrower and wider
panums area is narrower at the fovea becoming wider at the periphery
the corresponding part of the retina to panums fusional space is panums fusional area
what is physiological dipolopia
objects not lying on the horopter or in panums fusional space are seen as double
the double vision or diplopia is a normal physiological occurence in normal bsv
what is homonymous or uncrossed diplopia
appreciation that a distant object is double if a near object is fixated
-
what is hetronymous diplopia (crossed)
crossed diplopia
appreciation that a near object is double if a diatnce object ias fixated
why dont we notice physiological diplopia
the brain ignores or supresses the double images - this is called physiological suppression
brain suppresses everything thats out of panums fusional space
descirbe the difference betrween hetronymous and homonymous diplopia
homonymous diplopia - uncrossed dipolpia - seen in esotropia
image falls on the retina nasal to the fovea and projects temporally
hetronymous crossed diplopia
the image falls on the temporal side of the fovea and the image is projected on the temporal retina
