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Part 11
The system controlling the angle between the two eyes is the vergence
system
It developed over pre-existing conjugate neuronal structures, but made a
mistake
During convergence, we want to increase the contraction of the two
medial rectus muscles
Direct excitatory convergence signals to the medial rectus section of
both oculomotor nuclei
And we want to decrease the contraction of the two lateral rectus muscles
Inhibitory convergence signals to both abducens nuclei
During convergence both MLFs transmit wrong divergence commands to the
oculomotor nuclei
During divergence both MLFs transmit wrong convergence commands to the
oculomotor nuclei
There is a true divergence system, with its own divergence burst and
burst-tonic neurons
What reaches the abducens and oculomotor nuclei is a vergence pulse-step,
obtained by the sum of the vergence (velocity) pulse from the vergence burst
cells and the tonic rate from the vergence neural integrator
For vergence the vergence pulse-step is not achieved in the motorneurons
but in the vergence burst-tonic cells, which then connect to the
motorneurons
The goal of the vergence system is to bring (and maintain) the objects of
interest inside Panum’s fusional area
Vergence system is a cortical multipurpose system
Acts as a fast visual stabilization reflex in depth
Similar to the cortical OKR for conjugate visual stabilization
Acts as a forward/backward translational (or linear) VOR relfelx
Has to be driven by the otoliths
Latency 20 ms
Acts as voluntary transfer of gaze mechanism between two stationary
points at different depth
Much slower than the dynamics of conjugate saccades in response to
conjugate steps
There are NO vergence saccades
Acts as voluntary smooth tracking system in depth
The same voluntary smooth vergence subsystem is responsible for
discrete changes of gaze in depth (steps) and smooth tracking
Step vergence changes are also smooth
For vergence tracking the velocity command is a direct vergence
velocity estimate based on the disparity velocity
For step vergence changes the velocity command is built indirectly
from the depth positional difference between the two targets
Four components of vergence eye movements
Tonic convergence
Responsible for bringing the eyes to a tonic convergence value
Convergence adaptation
Long term changes in tonic convergence
Associated with prolonged wearing of prisms
Accommodative convergence
Convergence responses associated with accommodation changes
Due to the cross-links between the vergence and accommodation
systems
Reflex convergence (fusional or disparity convergence)
Vergence response associated with disparity stimuli
To realign both eyes form the current target o a new target located
at a different depth
Not a reflex----voluntary
To track in depth a moving object
Voluntary tracking
To stabilize images in depth
True reflexive ultra-short latency disparity vergences (85-90 ms)
Functional range of disparities is max 2 deg
Two “reflex convergence” subsystems
The true “fusional disparity system” is the short range disparity
system
Driven by fine disparity mismtahces
Non-fusible objects do not elicit fusional responses
The “long-range disparity system” uses a coarse matching between
“similar” even when non-fusible, images on the retina to align both
eyes on the object
30 deg or more of disparity
Proximal convergence (knowledge of nearness)
Vergence responses observed when moving gaze between objects at
different distances but manipulated in such a way not to have blur or
disparity cues
Vergence is the result of the tonic and dynamic balance of the convergence
and the divergence subsystems
All types of vergence responses are encoded inside the same vergence
system and delivered to the motorneurons by the same vergence cells
All changes in vergence are velocity commands (pulses or bursts) with the
change in the tonic vergence level (steps) obtained by integration of the
pulses in the vergence neural integrator
Step (smooth) vergence responses are much slower than saccades with
durations of 400-600ms and peak velocities of only 40-60 deg/s
When also changes in eccentricity are involved we have combined
conjugate saccades and vergence responses
Inside the saccade there is a very strong acceleration (vergence
enhancement) in the vergence response
The peak of the smooth (saccade-free) vergence velocity is less than 60deg/s
There is a linear relationship between amount of vergence change and
vergence peak velocity ---vergence main sequence
Saccades during vergence are (slightly) slower than conjugate saccades of
the same size
During combined saccade-vergence responses there is a reciprocal
interaction between the (conjugate) saccadic system and the vergence
system
It is a central interaction because the changes are clearly found in the
vergence burst neurons (for vergence) and EBN/IBNs (for saccades)
Voluntary (steps and tracking) vergence responses have a latency of ~160ms
Fast stabilization mechanism has a latency of ~80-90ms has has MST as
the main neural center
Disparity is the main binocular signal driving the vergence responses
Two disparity-driven convergence and divergence subsystems
A coarse system
Able to macth elements even far apart in terms of disparity (up to 30
deg) and forgiving regarding dissimilarities between the two images
A fusion-lock system
Slower, with a more limited range (2 deg) and more demanding in
terms of similarity between the images
Only fusible object can properly activate
Retinal blur is also a major driving input through cross-links between the
accommodation and disparity-vergence systems
The vergence generated monocularly by accommodation is called
accommodative vergence
The accommodation generated by vergence while the accommodation
system is in open loop is called vergence-driven accommodation or
fusional accommodation
Brain knows that changing depth, in natural conditions, requires both a
vergence and an accommodation response
Standard vergence model proposes that the vergence system is a visual
negative feedback system similar to the smooth pursuit system
Step vergence responses are much slower movements than saccades and
there is plenty of time for the slow negative visual feedback to intervene
Needs a low open loop gain
For vergence tracking responses it needs a copy of the vergence velocity
command inside a positive feedback loop to maintain the tracking when the
disparity velocity (retinal slip) decreases due to the ongoing vergence
tracking
FEF may be the spatio-temporal transformer of the vergence system
NOT an equivalent SC for vergence steps
Some tonic activity but not directly related to the vergence angel
Also code the vergence command during in-depth tracking
Divergence responses are usually slower than convergence responses
Find more convergence cells in FEF and SOA
Vergence commands from the FEF have to reach the SOA where we have
burst vergence cells and burst-tonic vergence cells
The vergence cells in the SOA have a function similar to the saccadic MLBs
and the smooth pursuit burst (velocity) cells
No segregation for the vergence system between the two types of
response
Carry all types of vergence velocity commands
The vergence burst-tonic cells in the SOA carry the combined burst-tonic
signals needed for the movement as well as the maintenance
The signals are directly fed into the motorneurons in the oculomotor
nuclei innervating the medial rectus muscles
Burst-tonic cells do not changes firing for conjugate movements or
conjugate eye positions
Horizontal motoneurons carry both vergence and conjugate burst and
tonic commands
The IP (interposed nucleus) activity is a burst-tonic (velocity + positional
tonic) pattern related to divergence and far accommodation
The NRTP activity is a burst-tonic (velocity + positional tonic) pattern related
to convergence and near accommodation
Stimulation of a NRa cell will generate a PURE accommodative response
Stimulation of a NRv cell will generate a PURE vergence response
An observed change in the vergence angle of 0% and an observed change
in the accommodation of 10% will generate a change of 10% in the firing
of NRa and no change in firing of NRv
An observed change in the accommodation of 05 and an observed change
in the vergence angle of 10% will generate a change of 10% in the firing
of NRv and no change in firing of NRa
AC/A: amount of accommodative convergence in meter angles per spherical
diopter of accommodation
CA/C: amount of convergence driven accommodation in spherical diopters per
meter angle of vergence
The correspondence between degrees and MA depends on the inter-ocular
distance of the subject
VA= 2 x arctan (0.5 x IOD)
Distance
Near Triad
A change of gaze in depth is always associated with the need to adjust
the accommodation level of the lens and the angle between the two eyes
Subcortical vergence cells in the SOA carry both a vergence and an
accommodation sensitivity
Pupil diameter is also temporarily constricted during changes in depth
Pupillary parasympathetic pathway driving the pupil constriction is a 4-neuron
arc
Retina ganglion cellspretectumEdinger-Westphal nucleusciliary
ganglionconstrictor muscle of the iris
The efferent sympathetic pathway which controls the iris dilator muscle is a
3-neuron pathway
Synapse in the cranial cervical ganglion
Minimum latency of the pupil response is 280ms
Damage to the optic nerve: if one pupil does not respond to direct
illumination but it does if the other eye is illuminated
Damage to the motor side: cause the associated pupil to be dilated and to
respond poorly to light, both direct and consensual
AS for the pupil control, there is a dual parasympathetic (excitatory) and
sympathetic (inhibitory) innervation but to the same ciliary muscle
The latency of accommodation responses to small changes in blur is
around 350-450ms and does not change with age
Downward movements are due to the passive stretching of the LPS and only
for blinks the active action of the orbicularis
Downward lid saccades are generated entirely by elastic forces due to an
abrupt relaxation of the LPS motorneurons
Blinks occur ~20 times/min
Blinks are not functionally linked to downward eye saccades
Saccadic omnipause neurons (inhibited during eye saccades) are also
inhibited during blinks
Reflex blinking is extremely fast with a latency of the onset of the eye
closing of ~50 ms