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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 cellspretectumEdinger-Westphal nucleusciliary ganglionconstrictor 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