outline: vergence eye movements: classification i ...schorlab.berkeley.edu/passpro/lecture 3...
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Outline: Vergence Eye Movements: Classification
I. Describe with 3 degrees of freedom- Horiz, Vert, torsion
II. Quantifying units- deg, PD, MA
III. Measurement of Vergence:- Objective & Subjective phoria
IV. Stimuli for Horizontal vergence
Maddox classification for horizontal vergence
Tonic- orthophorization
Proximal- spatiotopic- coarse adjustment
Fusional or disparity- fine adjustment
Cross link- Accommodative convergence- open-loop.
Effects of prisms and lenses on the horizontal phoria:
Concomitant and non-concomitant
V. Stimuli for Vertical vergence
Disparity in tertiary gaze
Cross coupling with gaze direction and distance
VI. Neurological control- Near Response cells-
pre-motor nuclei Mesencephalic Reticular formation.
Burst, tonic and pause cells
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Outline: Vergence Eye Movements
I. Describe with 3 degrees of freedom
II. Quantifying units
III. Measurement
Objective
Subjective
IV. Maddox classification for horizontal vergence
V. Neural control
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Version movement Vergence movement
Same direction Opposite direction
Conjugate rotation Disjunctive rotation
Two classes of binocular eye movements:
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Vergence has three
degrees of freedom
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Units for quantifying vergence:
1) Degrees
2) Prism Diopters ( ) = 100 x tan Degrees
3) Meter Angle (MA) = 1/VDm
= MA x IPD cm
e.g. 15 = 2.5 x 6.0 cm
∆
∆
∆
MA units are quantitatively similar to Diopter units of accommodation.
MA are independent of the inter-pupillary distance.
MA only apply to symmetrical convergence
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p
IPD
VD
? = 100 x tan p
tan p = IPD / VD
by substitution
? = 100 X IPD m / VD m ; IPD m X 100 = IPD cm
? = IPD cm / VD m∆
∆
∆
α
α
α
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Iso-Vergence Circle
Iso-Version Lines
Points A and B have the same
Vergence angle. So do Points
C and D. They lie on an iso-
vergence circle
Points A and C have the same
Version angle. So do Points B
and D. Iso-version lines
This shows the Iso-Vergence
Circle and Iso-Version lines in
the visual plane.
A
B
CD
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Iso-Vergence &
Iso-Accommodation CirclesA
B
CD
Stimuli to accommodation and
convergence are not matched
in eccentric gaze.
Accommodation stimulus
increases faster with azimuth
than does the vergence stimulus
The ideal AC/A
decreases w/
azimuth
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Four Maddox Components of Horizontal Vergence
Maddox proposed that horizontal vergence was
composed of a linear sum of these four components.
1) Tonic convergence-
(Adaptable Resting level)
2) Proximal convergence-
(Spatio-topic Gaze shifter)
3) Disparity (fusional) convergence-
(Retino-topic Gaze refiner & maintainer)
4) Accommodative convergence
(Coordinator of voluntary and involuntary)
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Anatomical Position of rest (APR) = 5 deg divergence
occurs at birth, death, deep anesthesia or deep sleep
Physiological Position of rest (PPR) = approximately zero
vergence with far fixation.
Tonic vergence moves the eyes from APR to PPR
tonic adapts to the PPR during the first 6 weeks of life
Distance Phoria = PPR deviation from zero
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Tonic Vergence Estimates:Anatomical Position of Rest (5 degrees divergence)
Physiological Position of rest ( Approximately zero
vergence with far fixation)
Error of Physiological Position of Rest = Distance Phoria
A n a to m ica lp o s it io n o f re s t
P h ys io lo g ica lp o s itio n of re s t
To n icVe rg e n ce
E rro r =fa r ph o r ia
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Tonic Vergence estimated from
Phoria = Vergence error
Phoria = [Vergence Response – Vergence Stimulus (Demand)]
Convergence Response (CR) = [Phoria + Vergence Stimulus]
5 PD Eso Phoria 5 PD Exo Phoria
Demand = 0 Demand = 15 PD
CR = 5 PD CR = 10 PD
Eso +
Exo -
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Orthophorization:
Distribution of Phorias in the population is not normal, it is
peaked near zero. Peaking (kurtosis) results from adaptation.
TONIC VERGENCE ESTIMATE
Far Phoria
Near Phoria
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Phoria Adaptation: concomitant and non-concomitant
Prism adaptation produces
concomitant change
in phoria
Anisometropic spectacle adaptation
produces non-concomitant change
in phoria. It also compensates for
muscle paresis- Spread of Comitance
Solid lines show orthophoric alignment before adaptation
Dashed lines show the phoria after adaptation, with right eye occluded
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Prism Adaptation after 5 sec and
60 sec
Ver
gen
ce A
ngle
Time -->
Ver
gen
ce A
ngle
Time -->Prism
in
Prism
in
Occluder
in
Occluder
in
Horizontal disparity vergence and Prism Adaptation
Adaptation occurs in about 60 seconds
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Slow Tonic Vergence
Fast Phasic Vergence
Model of temporal interactions between
phasic and tonic components of vergence
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Subjective measures of vergence using diplopia
Red Lens, Maddox Rod, neutralize with prism.
Normally, disparity produces diplopia
which is nulled by fusional vergence. Disparity equals
the unfused vergence error (phoria).
Quantify the phoria by measuring diplopia. Disrupt fusion with
a red lens or vertical prism over one eye to produce diplopia.
Esophoria is produced by uncrossed disparity
Exophoria is produced by crossed disparity
Divergence corrects esophoria
Convergence corrects exophoria
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Near crossed
diplopia- Exo error
Far uncrossed
diplopia- Eso error
Illustration of crossed and uncrossed diplopia
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Un-crossed
disparity behind
the fixation plane
Crossed disparity in
front of the fixation
plane
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Diplopia used for subjective measures of vergence error:
Fixate the black spot and view the open spot in crossed
diplopia. Close left eye and right spot disappears.
Crossed diplopia
Fixation point
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Proximal Vergence-Perceptual distance cues for gaze shifts
Dynamic gaze-shifting response (analogous to a saccade)
Proximal vergence is a voluntary gaze shift from one target
distance to another in response to perceived distance.
Horizontal proximal vergence is under voluntary control.
Vertical and cyclo-vergence are not controlled
voluntarily. Instead they are cross-coupled with voluntary
horizontal proximal vergence.
And all three vergence directions respond to disparity.
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Three components of disparity vergence
stimulated by retinal (disparity) cues.
Horizontal vergence – horizontal disparity
Vertical vergence – vertical disparity
Cyclo vergence- cyclo disparity
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Vertical disparities are produced by near tertiary points in space
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left eye right eye
pupil
visual
target
tertiary
gaze
position
Spatial geometry produces vertical disparity in tertiary directions.
Vertical vergence responds even when one eye is occluded.
It is cross-coupled with gaze direction and convergence
and is part of the near response.
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Two Classes of Dynamic Vergence:
1) Gaze shifting responses to proximal stimuli
(saccade like coarse adjustment to perceived
distance, refined by disparity vergence)
2) Smooth tracking (pursuit like fine adjustment
to perceived motion in depth. It is refined
by disparity vergence).
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Disparity vergence response to small step disparity
150 ms latency (half the latency as for accommodation)
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Peak velocity increases
proportionally with
response size.
Velocity is highest at the
beginning of the vergence
response and it decelerates
to the end point.
Vergence step response
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Response velocity is proportional to step disparity stimulus
Slope = 5 deg/sec/deg
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Disparity Vergence along the line of sight
(Asymmetric vergence)
Can we move one eye while the other eye
remains still?A
B
Hering’s law says we can not.
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Velocity interactions between
saccades and vergence
A C
D
C->A D -> A A->C A->D
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NoSaccade
AC Stim = -2.0 D
-0.2 0 .2 0.4 0 .6 0 .8 1.0 1 .2 1.4 1 .8 2.00 1.6
1 deg
0.5 D
LE
RE
10
0
D/S5
VRG
ACC
ACV
AC Stim = +2.0 D
LE
RE
VRG
ACC
ACV
-0 .2 0.2 0 .4 0 .6 0.8 1 .0 1 .2 1 .4 1.8 2 .00 1.6-10
0
D/S-5
1 deg
0.5 D
Rightw
ard
Saccade
Time (seconds)
-0 .2 0.2 0 .4 0 .6 0 .8 1 .0 1 .2 1 .4 1.8 2 .00 1 .6
LE
RE
VRG
ACC
ACV
10
0
D/S5
1 deg
0.5 D
-10
0
D/S-5
LE
RE
VRG
ACC
ACV
-0 .2 0.2 0 .4 0 .6 0.8 1 .0 1 .2 1 .4 1.8 2 .00 1.6
1 deg
0.5 D
Saccades increase the velocity of accommodation and shorten latency
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Smooth vergence pursuit tracking with prediction
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Brainstem nuclei for the near response:
Supra-oculomotor nucleus
Vergence-related
Near-Response
cells found here
III Nucleus
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Types of vergence cells in Pre-motor nuclei:
Superior Oculomotor Nucleus
Tonic– Position cells
Phasic– Velocity cells
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Phasic or burst (B)
and Tonic (T) cells
in superior
oculomotor nucleus
Final common pathway
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Tonic cell coding
vergence position activity
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Phasic Velocity Cell:
Medial Rectus Motoneuron activity during Convergence
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Lunch time
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Dynamics of Accommodation:
Latency (300 ms) & response time (1 sec)