Visual Direction II Flashcards
Eccentric Fixation Eccentric Viewing Binocular Visual Direction & Corresponding Retinal Points (Veith-Muller Circle) - Ocular Dominance -Binocular Disparity -Diplopia and Confusion -Anomalous Retinal Correspondence
Define Eccentric Fixation
Some abnormalities (like strabismus, amblyopia) of the visual system, a point other than the fovea may be used to determine visual direction
Which disorders can cause eccentric fixation?
- Common consequence of strabismus
- Contributes of vision loss in amblyopia
- Possibly consequence of sensory visual distortions that are most pronounced at the fovea of strabismic eyes
True or False: A pt who fixates with an extrafoveal retinal locus will always have eccentric fixation.

False. A pt who fixates w/ an extrafoveal retinal locus does not necessarily have eccentric fixation
Ex. In low vision pt whose foveal vision is compromised by disease processes an extrafoveal point may be used (trained) for fixating eye to identify objects & reading. this is called eccentric viewing. The pt picks a retinal locus with better resolution than the diseased fovea to serve as a new fixation point.
Name the differences between eccentric fixation vs. eccentric viewing
Eccentric Fixation
- A point other than the fovea may be used to determine visual direction
- Typically develops in childhood on concert with strabismus and amblyopia
- Visuoscopy is used to diagnose. If the foveal reflex does not appear in the center of ophthalmoscopic target (when pt is fixating), eccentric fixation is present
Eccentric Viewing
- Require conscious effort on part of pt (low vision) to utilize the undamaged part of the retina
- Typically occurs after loss of vision from injury or disease & is a normal adaptation to the visual loss rather than the abnormality
- Pt still uses the destroyed fovea as a zero direction reference (the principal visual direction has not changed)
The Hering window experiment
Binocular Visual Direction

Hole in hand demonstration
Binocular visual Direction, Demonstration of egocentric visual direction

Define corresponding retinal points
pair of points, one in each eye that, when stimulated simultaneously or rapidly in succession, are perceived to lie in identical directions

Ocular Dominance
- Monovision CL
- Dominant (preferred) hand vs. Dominant eye
- Preferred eye vs. Dominant eye
- Taken into account when prescribing spectacle or CL corrections to avoid binocular vision problems
Veith-Muller Circle
predicted theoretical location of points in space whose images will be formed on corresponding points in the retina

_____ and _____ are basic building blocks of our pereception of distance & depth
Visual direction & corresponding points
Define corresponding points
can be thought of as a reference point or zero point for stereopsis
- images from corresponding points are perceived as arising from targets at the same distance
The perception of depth arises from stimulation of noncorresponding retinal points
Define Disparity
Images of a single object that do not stimulate corresponding retinal points
If observed monocularly these images would be perceived as lying in different visual directions
The difference in visual direction (of the images) between corresponding points in the 2 eyes is called binocular disparity
Disparity is denoted by n = defined as a difference in binocular subtense angles a1 and aR or n=a1-aR
Define vertical disparity
Vertical differences in the positions of similar images
Define horizontal disparity
Horizontal differences in positions of similar images
allows perception of stereoscopic depth
can be classified as crossed or uncrossed in relation to the fixation point
Images w/ crossed disparity are perceived nearer, wheras those with uncrossed disparity appear to be more distant than the fixation point (within VM circle)

Corresponding retinal points have ____ disparity
Zero, n= a1 - aR = 0
In crossed disparity, angle n, the difference betwen the binocular subtense angles aL and a<span>R </span>is _______.
Positive aL is larger than aR
In uncrossed disparity, angle n, the difference between binocular subtense angles aL and aR is ____
negative such that aR is larger than aL
Diplopia results when….?
binocular disparity is too large
Small disparities
visual system is able to combine them into a single percept
The upper limit of this disparities that can produce the single percept is determined by the _______.
Panum’s area
Small differences in perceived horizontal direction between two eyes give rise to a percept of _______
3-D depth or stereopsis, this principal is used to create stereograms
True or False: Vertical disparities do not directly give rise to percept of depth as do horizontal disparities, but they can still affect our percept of depth directly
False. Indirectly
Diplopia
Images on widely separated corresponding points (that’s beyond Panum’s area), will not be fused as single
This phenomenon is called physiological diplopia because it is NORMAL, pathological diplopia - BV disorders

Object whose image fromed on corresponding retinal points have ____ visual direction
Same

