Lecture 1 Flashcards
Why do we need 2 eyes?
- spare eye
- larger field of view
- better visual function due to binocular summation
Monocular Field of human
145 degrees
Total Visual Field of human
180+ degrees
Binocular Visual Field of human
130+ degrees
How does stereopsis arise?
subtle difference b/w images in each eye
What is the real advantage of binocularity?
highly accurate depth perception
Clinical binocular problems
- failure to align two eyes
2. two eyes aligned, but system is stressed
Fail to align two eyes:
Strabismus, amblyopia,
diplopia, suppression etc.
two eyes aligned, but system is stressed:
-Incorrect refractive balance
Misconception about binocular vision
The ‘real world’. The 3-D space
around us and all the objects located in it
Physical Space aka object space
Your perception of the physical space in relation to you. It is a construct of the brain.
Visual Space
A goal of vision
is to construct a visual space that looks like the physical space.
What happens when visual space does not match the physical space?
-Optical illusion - normal
Visual Direction under monocular condition
oculocentric
Oculocentric direction
is visual direction relative to where the eye is fixating.
Principal Visual Direction (PVD)
the reference direction, where fovea is fixating
How is visual direction (oculocentric direction) under monocular condition measured?
angle between PVD and the SVD
If SVD is to the right of PVD what is alpha?
positive
if SVD is to the left of PVD what is alpha?
negative
What is the direction signaled by fovea?
PVD
What is the oculocentric sense of “straight ahead”?
PVD
What is zero direction?
PVD
Does oculocentric direction move with the eye?
yes! b/c PVD changes when your eye moves
a unique oculocentric direction associated with a specific retinal point.
local sign
how is the local sign determined?
the retinotopic mapping of the retinal locus in LGN and cortex
what does mechanical visual phosphene show?
the neural basis of local sign
what does local sign size relate to?
retinal eccentricity
what is the basis for local sign size?
directional discrimination
how is local sign mostly determined?
the BRAIN
how big are foveal local signs?
1/7 the size of foveal cone
what is the basis for hyperacuity?
local sign size
How big are peripheral local signs compared to peripheral cones?
much larger
what type of acuity determines what the object is?
resolution acuity
in the demonstration, with the same amount of offset b/w the two lines, why can you resolve the line offset in central vision, but cannot do so in peripheral vision>
local sign size is larger in peripheral retina compared to fovea
what type of acuity determines where the object is?
hyperacuity
a perception of object shape that differs from its true shape
metamorphopsia
Cause of metamorphopsia
– Neuroanatomical:
• Retinal disease—most common and most evident
• Stabismus and amblyopia– very subtle, pt might not be aware of it
– Optical: extreme corneal distortion Example: keratoconus
What do retinal diseases cause regarding metamorphopsia?
displacement of the local signs leads to spatial distortion
different image positions withina local sign region will have the same ____________?
visual direction
PVD become associated with the local sign of peripheral retina other than fovea
eccentric fixation (EF)
EF can be found in:
In eccentric viewing (EV) what is the PVD associated with?
fovea
Regarding EV, is the patient aware of using peripheral vision to see?
yes
Where can EV be found in?
short term macular disease
In low vision rehab, what is the patient trained to use?
EV
visual direction under binocular condition
egocentric direction
The binocular sense of visual direction uses a point in the head called
egocenter (reference point)
represents a single hypothetical eye located approximately midway between the two eyes.
cyclopean eye
why does the hole in hand demonstration work?
oculocentric for left eye: 0
oculocentric for right eye: 0
when both eyes open, brain will unify them with egocentric angle of 0, which is our binocular view and u will see hole in hand.
The brain receives input from the two eyes, and computes the egocentric direction of an object based on two sets of data:
1) The retinal location of the object’s image in each eye (oculocentric direction)
2) Each eye’s orientation, or direction of gaze.
The brain receives direction of gaze information, possibly from proprioception within the extraocular muscles and/or from the oculomotor neurons that drive the muscles.
