Lecture 16/17 Flashcards
what is the function of vergence eye movements?
to shift the gaze from near to far (or far to near) so the image of the target is maintained on both fovea
what is the initiation/transient phase of vergence movement?
open loop, fast/not precise, responds to large changes (1-10PD) and is controlled by coarse disparity detector neurons
what is the completion/sustained phase of vergence movement?
closed loop, slow/negative feedback, responds to small disparity and is controlled by fine disparity detector neurons
what are the 5 types of stimulus to the vergence system?
retinal disparity, retinal blur, proximity of targets, tonicity and voluntary innervation
what is retinal disparity and what does it elicit?
separation of images of a single object so that they fall on non-corresponding points on the retina
elicits = disparity/fusional vergence
what does crossed binocular disparity stimulate?
positive fusional vergence (convergence)
what does un-crossed binocular disparity stimulate?
negative fusional vergence (divergence)
what is the vergence demand (VD) of a target?
the absolute disparity of the fixation target - how much do you need to converge/diverge eyes to maintain foveal fixation
how do you calculate the vergence demand (PD)?
VD = PD/d + prism
PD is in cm, d (target distance) is in m and BO prism (+) and BI prism (-)
what is disparity vergence? how do you calculate it?
amount of disparity vergence (DV) is the difference between the vergence demand for two different targets
DV = B - A
how is fixation disparity different than vergence disparity?
fixation disparity is very small vs. vergence disparity which has a large transient change and then maintained
why is fixation disparity necessary for fixation?
the small binocular disparity stimulates sustained disparity vergence - needed to maintaining the stability of the vergence system
what is the initiation/transient phase for disparity vergence?
responds to larger changes of retinal binocular disparity stimuli (step vergence stimulus) = using a prism bar
what is the completion/sustained phase for disparity vergence?
responds to small changes of retinal binocular disparity stimuli = testing fusional ranges in phoropter
what does the trajectory look like for pure vergence eye movments?
the vergence angles gradually/slowly change (either get larger or smaller)
what is the maximum velocity for vergence?
40 degree/sec
why does vergence have a main sequence relationship for velocity?
(similar to saccades) = the larger the demand - the faster the vergence movement or peak velocity increases with amplitude of vergence
which is usually faster, convergence or divergence?
convergence = there are more neurons stimulating this type of movement
what is the latency for the transient disparity vergence?
160 msec
what is the latency for accommodative vergence?
200 msec
what can help shorten the latency time for vergence movments?
if there are predictable stimuli
what is the duration for a transient disparity vergence?
about 500 msec - the exact time depends on the amount of the vergence change (amplitude)
using a 3BI/12BO flipper for vergence facility - what is the typical cpm?
15 (but different combinations will have different norms) - this one is best to differentiate between symptomatic vs. asymptomatic
why does retinal blur stimulate vergence?
the loss of image sharpness elicits a vergence movement
what is accommodative vergence (AV)?
vergence innervation which is produced by blur accommodation signal (no disparity vergence involved)
how are accommodative vergence and blur accommodation related?
they are directly proportional - for every diopter of BA innervation you will get “x” PD amount of accommodative convergence
what is the dual interaction model?
it characterizes the interaction between accommodation and vergence systems - they innervate each other via cross-talk
what does the AC/A ratio represent?
how much vergence is being stimulated if you change 1D of accommodation
change in vergence angle/change in accommodation
how is the AC/A ratio measured?
by blur-induced changes of heterophoria (alternating cover test or van graefe phoria test = need to break binocular disparity/fusion)
what are the 2 assumptions for the AC/A ratio?
accommodative stimulus equals accommodative response (no errors)
accommodative convergence show linear relationship with accommodative stimulus
how do you find the calculated AC/A?
measure heterophoria at distance and near -
PD (cm) + (near - distance)/amount of accommodation change (2.5D for 40cm)
what are the signs for exophoria and esophoria in the calculated AC/A?
exo deviation = (-)
eso deviation = (+)
how do you find the gradient AC/A?
measure heterophoria a certain distance, then add 1D lens to alter accommodation and re-measure
heterophoria (with lens) - heterophoria (no lens)/lens power (1D)
what is a stimulus AC/A?
used if assuming the accommodative stimulus is equal to accommodative response (no lead or lag) - but patients tend to under-accommodate by 10%
does the gradient AC/A overestimate or underestimate the actual AC/A (response)?
underestimates
based on the dual interaction model, what is the total vergence response?
total = DC + AC DC = disparity convergence AC = accommodative convergence
what will adding a -1D lens do during the gradient AC/A?
it stimulates accommodation and phoria will become more eso or less exo
what will adding a +1D lens do during the gradient AC/A?
it will relax accommodation and phoria will become more exo or less eso
what is a normal AC/A?
4/1 (range is 3/1 to 6/1)
what are some factors that will affect the AC/A (usually stable)?
spectacle lenses if not looking through center, age (increases pre-presbyopia) and CNS and peripheral parasympathetic medications
how do you find the accommodative convergence (AC)?
take the amount of accommodation (usually 2.5) and multiply by AC/A
if they lag +0.5D - then subtract from 2.5 (2.5-0.5)
what binocular anomaly can be seen if there is a low AC/A, normal or exo at distance with greater exo at near?
convergence insufficiency
what binocular anomaly can be seen with a low AC/A and eso at distance but normal at near?
divergence insufficiency
what binocular anomaly can be seen if there is a high AC/A, normal or eso at distance with greater eso at near?
convergence excess
what binocular anomaly can be seen if there is a high AC/A, more exo at distance than exo at near?
divergence excess
if you have a patient with a high AC/A and has esophoria at near with symptoms, what treatment can you do?
plus/minus lenses
if you have a patient with normal to low AC/A and has esophoria at near with symptoms, what treatment can you do?
usually consider prisms or VT or both
what is convergence accommodation (CA)?
accommodation innervation which is produced by binocular disparity vergence
what is the CA/C ratio?
convergence accommodation is quantified by disparity-induced changes of accommodation
what is the normal CA/C ratio? when is it highest?
1/12 (D/PD) - ranges 0 to 1/4 in young adults
highest = age 10 then drops linerally
how does the vergence trajectory change when combined with saccades?
it is sped up
what 3 areas are involved in the control of vergence?
cortical, brainstem and cerebellar control
what are the 4 structures involved in cortical control of vergence?
occipital lobe: primary visual cortex, temporal lobe: MT/MST, parietal lobe: PPC (posterior parietal cortex) and frontal lobe: FEFsem
what are the 3 types of disparity neurons in the primary visual cortex?
tuned-zero/near-zero neurons, tuned-far neurons and tuned-near neurons
what are tuned-zero/near-zero neurons?
respond to small binocular stimuli disparity (fixation disparity) during sustained phase, hold vergence steady and fine-tune precision
what are the tuned-far and tuned-near neurons?
for large binocular disparities (during vergence jump) during initiation phase to stimulate fast response
what 2 areas does the primary visual cortex send information to for vergence control?
the parietal and temporal lobes
what are the 2 areas in the parietal lobe (processes info about spatial location) involved in vergence control?
Lateral intraparietal area (LIP) and parietal eye fields (PEF) within PPC
what does the lateral intraparietal area (LIP) do?
converts oculocentric direction to egocentric direction - location in 3D space
involved in stimulating vergence and accommodation based on perceived distance
what does the parietal eye field (PEF) do?
accommodation, vergence, and pupil constriction (near triad)
what are the 2 areas in the temporal lobe (processing info about motion) are involved in vergence control?
MT and MST
what does MT do?
perception of steroscopic depth
what does MST do?
process egocentric motion - involved in initiation of vergence
what do both MT and MST do?
involved in controlling smooth vergence in space (fixating on a target that moves from far to near)
where do the parietal and temporal lobes send their information?
to the frontal lobe - FEF (frontal eye fields)
what do the FEF do?
involved in initiating voluntary eye movements (saccades, smooth pursuits and vergence)
what is the sub-region in the FEF for controlling vergence?
FEFsem
where do all areas of the cortex send information to the mid-brain?
the supra-oculomotor area (SOA) = for premotor command
what is the supra-oculomotor area (SOA)?
a group of neurons within the MRF, 1-2mm dorsal to oculomotor nucleus and EW nucleus
what structure is specific to accommodation and vergence activity and generates neural innervation similar to the pulse/step of saccades?
supra-oculomotor area (SOA)
what are the 3 types of neurons in the SOA that send pre-motor commands to the CN3 nucleus?
vergence tonic cells, vergence burst cells and vergence burst-tonic cells
which SOA neuron discharges in relation to vergence angle?
vergence tonic cells
which SOA neuron discharges in relation to vergence velocity?
vergence burst cells
which SOA neuron discharges in relation to both vergence angle and velocity?
vergence burst-tonic cells
what is the NRTP in the pons?
it may contain neurons important for vergence integration
receives input from FEF and projects to cerebellum
will a lesion of the MLF affect vergences?
no - vergence innervation bypasses the MLF
what happens if there is a lesion to the NRTP?
leads to impairment of holding the vergence angle
what is the main function of the cerebellum in vergence movements?
to store important constants - AC/A and CA/A
what happens to vergences if there is a lesion to the cerebellum?
affects the accuracy of vergence movements
