Accomodation, Fusion and Binocularity Amy C. Nau, OD

Outline Accommodation Binocular sensory function Vergence Pathophysiology of common accommodative and binocular anomolies Ken Cuifredda, OD PhD, Glen McCormack, OD PhD in Borish, Clinical Refraction

History of Accomodation Descartes (1677)- proposes lenticular based focusing Thomas Young- (1801)- demonstrates that the lens changes shape Hermann von Helmholtz (1866)- first accurate description of accommodative processes How can we possibly keep everythign we look at clear? Distance, near, changing directions, constant eoms and attentional issues. Foveal and peripheral vision, driving or playing basketball. Not only in one eye but in both. When viewing a far object, the circularly arranged ciliary muscle relaxes causing the lens zonules and suspensory ligaments to pull on the lens, flattening it. The source of the tension is the pressure that the vitreous and aqueous humours exert outwards onto the sclera. When viewing a near object, the ciliary muscles contract (resisting the outward pressure on the sclera) causing the lens zonules to slacken which allows the lens to spring back into a thicker, more convex, form.

Physiology of Accommodation The Eye Ciliary m. relaxed Ciliary m. contracts http://hyperphysics.phy-astr.gsu.edu/hbase/vision/imgvis/accom2.gif

Neural basis for accommodation ewn Magnocellular LGN Area 17 Afferent via EWN to area 17 in occipital cortex. Signal from 17 also goes to parieto-temporal areas for processing Supranuclear signal sends PS motor command via CN3 http://bicmra.usuhs.mil/MRA/Histology/Eye/Images/eye01.jpg http://static.howstuffworks.com/gif/brain-intro.gif

Neural basis for accommodation EWN- CN3 (PS)-CilG- Short Cil N/Long Cil N. Sympathetic: hypothalamus- ciliospinal center of budge-ventral root to superior cervical ganglion- ICA- trigeminal gangion- short and long ciliary nerves. http://info.med.yale.edu/caim/cnerves/cn3/cn3_graphics/cn3_9.jpg

Types of Accommodation Reflex, +/- Vergence, +/- Proximal, +/- Tonic, +/- Automatic adjustment of refractive state. Fxn to maintain a focused retinal image. Automatic up to 2D, over that requires voluntary effort Vergence: Induced by fusional disparity. Gives rise to CA/C ratio. .4D/meter angle. Measure by measuring accommodation wo blur input Proximal: Accommodation d/t percieved nearness within 3m. Normally about 10% of near effort Tonic: No stimulus but baseline neural innervation from midbrain. Range 0-2D , reduces with age.

Systems Analysis of Accommodation Stimulus (D) S Depth of focus gain Cm/lens Reflex Accom Adapt. tonic + - retina Systems diagrams represent physiological processes, not anatomy (which is often unknown) but they have anatomical implications. Depth of focus, however, is a measurement of how much distance exists behind the lens wherein the film plane will remain sharply in focus.

Influences on Blur Accommodation Responses Optical Cues H.O. Aberrations Microfluctuations in EOM and AR Non Optical Cues Size Proximity Apparent Distance Disparate retinal images Monocular depth cues Nonretinal Image Vestibular stimulation Mood Effort Cognitive Demand Instruction set Retinal Image Frequency Contrast Eccentricity Luminance Size Depth of focus

Fusion How do the two eyes, separated in space, give us a single precept? How are the eye movements coordinated with the retinal image to maintain singularity? How does stereo vision enhance our perceptions of our surroundings? The purpose of fusion is to provide stereopsis (seeing solidly). Gives us depth and distance information Accommodation is important because clear bifoveal images are needed for high resolution stereopsis

Stereograms http://www.lri.fr/~marche/images/cameleon-3d.gif It is essential to understnd binocular vision in order to fully grasp the behavior of accommodation during normal viewing or a refractive procedure, or what can go wrong when the Rx is incorrect. http://www.lri.fr/~marche/images/cameleon-3d.gif

Space Perception Object Space “the real world” Visual Space - the world as it is captured by our retina and processed by the visual cortex. Visual space must reproduce object space Applies not only to objects, but to the location of those objects and our reactions to them

The Cyclopean Eye We percieve the world as though we have one eye which exists between our two eyes. “egocenter” Single eye of the mythological cyclops is a useful analogy for the way human binocular vision functions Preschoolers will bring a tube to the bridge of their nose. Illustrates that we behave as though we view the world from one eye existing midway between our two acutal eyes. http://bearah718.tripod.com/sitebuildercontent/sitebuilderpictures/cyclops.jpg

Visual Localization and Visual Direction Egocentric localization of an object Where is the object in 3D space in relation to my own position? Can judge distance in a unit of measurement (i.e., feet or meters) Imaginary arrow from the cyclopean eye to the object (i.e. straight ahead, etc.) Egocentric localization – Answers: where is that object in 3D space, in relation to my position? Gives the abiltiy to judge the distance in units of measurement (i.e. feet or meters)

Egocentric localization The sense of direction from retinal locus is a property of oculo-cortical mapping Each retinal point has a location relative to other retinal points in this map Local sign is the term that refers to the unique direction for each retinal point. Related to va and retinal location (fovea vs periph), amblyopia Perceptual computation begins with the object’s position on the retina. WE figure out where something is and it’s direction by the photoreceptor array that is stimulated. This is called LOCAL Sign. All local signs are not equal. Local sign is messed up in amblyopia and can be perturbed by retinal pathology (macular transolcation!!??)

Principal visual direction Elicits the sensation of “looking” at something Associated with the fixation point /origin Center of anatomic fovea Center of oculocentric direction Guides foveation Registration… In eccentric fixation, the principal visual direction may not originate at the fovea. The ability to sense the direction of gaze is called “registration”, which involves EOM proprioception as well as corollary discharge from brainstem neurons sending eye position information. So visual field position and ocular rotation are summated.

Example: Paretic strabismus Past pointing test Used to detect labyrinth disease but also can be used for paretic eom. Egocentric direction judgement errors when monocularly fixating with the paretic eye Egocentric direction judgement errors when monocularly fixating objects with an eye that has a paretic m. Eye moves to the target with more innervation than usually required,when registered, yields exaggerated egocentric direction for that target

Depth and Distance Perception Distance perception = absolute depth How far any object is from the observer, from another object, able to judge in meters, etc. Depth perception (relative), perception of relative proximity of one object to another or the relative depth between two or more points in space. You cannot get distance information just from depth cues. Depth the coffee cup is 50% farther than the pencil Distance reveals the two objects are separated by 10cm

Depth: coffee cup is 50% farther than pencil Distance: 10 cm

Monocular Cues to Depth Object distance form observer is inversely proportional to retinal image size. Linear/perspective Texture density Luminance variations, shadows, color Atmospheric perspective Overlay cue Parallax Head motion Kinetic depth effect Parallax; when observer moves, we note the apparent relative motion of stationary objects. Closer objects will move faster. Motion: kinetic, things moving closer or farther from you (time to crash distance) Atmospheric: distance is hazy due to blue light scatter

http://www1. cs. columbia http://www1.cs.columbia.edu/~paley/spring03/assignments/HW5/bg2020/size.jpg

Visual cliff experiment. Are cues learned or innate?

Binocular cues to stereopsis Separation of the eyes Accommodation Convergence Herbivores tend to have eyes on the side of their heads and lack depth perception Carnivores/predators, have eyes in the front for enhanced stereo.

Stimulus to Stereopsis Lateral separation of eyes= different views of the world (binocular parallax) Elicits convergence/divergence The magnitude of horizontal disparity for any given point is a function of the lateral separation of the eyes divided by object distance Magnitude of vertical disparity is zero for objects on the median plane, but nonzero for objects to the left or right View the palm on the hand at 25 cm. while hand is held to the left of straight ahead. Look at the had with each eye separately. Hand is slightly larger as seen by the left eye than the right and vice versa.

Binocular Contribution to Depth and Distance Perception Horizontal geometric disparity is insufficient to calculate percieved distance. Registered convergence The parallax angles subtended by the pencil at near are greater than at distance. The horizontal disparity cue would suggest that the pencil at near looks larger, but this does not occur, therefore distnace cues must be integrated. The brain can sense the angle of convergence (registered)

Spatial Stereopsis Limits Retinal disparity is not compared with all retinal points Actually any given retinal locus only interacts with a limited area of retinal points in the fellow eye So, disparity is calculated only for a portion of object space at any given time. Centered on the point of fixation

Horopter Center of the region of stereopsis Defined by all those points in object space that stimulate corresponding retinal points (points in each eye that retain the same sense of visual direction). Any object not on the horopter may appear to be in different directions ot the two eyes. It is the center of range of binocularity and region of highest stereoscopic acuity Basis for physiologic diplopia- demonstrate in class.

Horoptor Crossed disparity- closer to observer than the horopter,image is observed on opposite side , (images fall temporal) Uncrossed disparity - object is farther from observer than the horopter (images are nasal from corresponding points ) observed on same side Worth 4 dot.. Etc.

Measuring retinal disparity Maddox rod test Exophoria- streak falls on temporal retina, stimulates crossed disparity Esophoria-streak falls nasally, simulates uncrossed disparity. Anomalous retinal correspondence_ identical sense of visual direction with very dissimilar retinal points Retinal disparity has a “dead zone’ of about 1 prism diopter, where correspondence is fluid. Retinal points that are normally disparate can becume corresponding under situations of fusional stress. In ARC, the eye with a 10p eso and ARC the fovea of the straight eye may retain the same sense of direction as a nasal etianl point in the deviated eye. ARC is also unstable, and seems to spontaneously change. Presence of ARC reduces the prognosis for cure of strabismus. (flom 19580.

Spatial Limits of Fusion Occurs only when corresponding points are stimulated Small to moderate retinal disparities are stimulated. Pannum’s space the space around the current fixation point which can be fused, extents around 15 minarc. Outside this region, some qualitative depth can still be perceived, but stimuli can not be fused. Humans and many other animals sample the surrounding space by constantly changing the fixation point of their eyes. Pannums’ space is the range of depth that is fused without the aid of eye movemet. That portion of the retina wthich optically conjugate to pannums’ space is Pannum’s area. Which is defined as an area of ht retina of one eye, a point of which gives rise to a percept of singlemenss when stimulated simultaneoullsy with a single point on th reitna of the fellow eye.

Nonspatial Limits of Fusion As images become more dissimilar fusion is interrupted (color/shape etc). Diplopia Binocular rivalry Sustained suppression Luminance Luster You can get partial fusion Red green glasses….. Luminance and color averaging- page lighted from oblique source Rivalry is the alternating perception of two objects in the same visual direction Red green stereo- form is the same, color is different. Color rivalry only. Luminance luster: observer sees great interocular luminance difference in one portion of the binocular field and no difference in the remainder of the binocular field. Difference region shimmers. ( diamond reflects light to one eye only)

Anomalous retinal correspondence

Fine and Coarse Stereopsis Fine: parvocellular, higher spatial frequency, smaller retinal disparities, stationary targets. Foveal vision, similar size and shape images necessary. Coarse: magnocellular, lower spatial frequency, larger retinal disparities, moving targets, periphery, similarity of images not such an issue. (motion in depth mechanism) Fine stereopsis (or th eloss of it) seems to be more highly correlated with clinical symptoms.

Local and Global Stereopsis Stereo contributes to pattern recognition Randot: visual system performs interocular image disparity computations across binocular visual field in a process known as global stereopsis Stereograms: When color and contrast reveal the form, dispartiy processing limited to the immediate vicinty of the form is sufficient to reveal the depth = local stereopsis. Fine and coarse = local Fine= global Global- sampling of thousands of corresponding points will cause the image to emerge. Extensive interocular image disparity computations.

LOCAL STEREOPSIS http://archive.museophile.org/3d/pub/tunnel.gif

Stereo Acuity Ability to discriminate very fine differences in depth from geometric disparity Minimum geometric disparity that elicits a sensation of depth. Fine- 2 sec arc hyperacuity Image displacemnts smaller than the diameter of foveal cones

Binocular Sensory Fusion Summary Single precept from two ocular images Ensures visual space represents object space. (no diplopia) How can the visual system integrate two separte images into the cyclopean eye? How does this contribute to our sense of placement in our surroundings? How can we look about in a coordinated fashion without getting diplopia. Not only are we maintaining fusion, but the images are clear in the absence of pathology. We must use EOMS to foveate the image so we get fusion, but the image must be clear before tracking can occur. This is why vergence and accommodation are so intimately linked and why a problem in one of the systems can generate problems in the other and vice versa.

Binocular Motor Function Eye Movements support foveal vision Saccades/smooth pursuits/vergence support stable retinal imagery Vestibular/optokinetic Vertical eye movements Supranuclear level- different from horizontal Torsional eye movements Compensate for head tilts Hering’s law- motor embodiment of cyclopean vision VOR- vestibulo-ocular reflex responds to unintended head ovements when walking/running- maintains fixation that might otherwise be lost. , suppressed during saccades. OKN/Vestibular nystagmus strive to maintain retinal stability either d/t head motion or object motion. Vertical have lower gain, velocity and range of movement Basis for Bielschowsky head tilt, cylcoversions actually do not “right” the vertical meridians of the retina, so the percpetion of verticality during head tilt must arise from perceptual mechanisms. Hering- a single motor command that alters the cyclopean direction of gaze.

Horizontal Vergence Movements Tonic, +/- Accommodative, +/- Proximal, +/- Fusional, +/- Purpose of vergence movements is to put fixation targets on the horopter and keep them there. Tonic: steady innervaiton, about 3-5prism diopters convergent. Resting anatomic position is 17 divergent… Accomomdative: adds additional vergence innervation for near viewing when accommodation responds to blur Proximal: adds additional vergence innervation for when object appear to be close Fusional: adds innervation needed to attain single binocular vision Maddox, 1893

Disparity Vergence (allows stereo) Vergence innervation is stimulated by retinal disparity Corresponding retinal points are not stimulated Purpose is to place targets of interest on the horopter to maximize steropsis Coarse –v- Fine innervation Diplopia is not the stimulus, it will occur when images do not fall within Pannum’s area. Positive- convergence; negative is divergence. Separate brainstem cellular groups are responsible. There are less divergence cells.

Feedback Control of Disparity Vergence and Fixation Disparity (MA) S Pannum’s Area gain EOMs Verg. Resp. Adapt. tonic + - retina Gain: how efficiently the fine dv mechanism converts retinal disparity into vergence innervation. Coarse dv is not controlled by continuous negative feedbback- it is only calculaed intermittently.

Convergence Accommodation Stimulus (D) S Depth of focus gain Cm/lens Reflex Accom Adapt. tonic + - retina Synkinetic (single stimulus generates multiple responses) Helps to clear the fixation target as disparity vergence aligns the eyes. Helps out RA. (bias) It is the ratio of cA to convergence stimulated by the retinal disparity. Averages about 1D/10prism at aget 20. So if observer with 1/10 CA/C ratio exerts 5prism of DVI, then .5D of convergence accommodation will be generated. Declines with presbyopia Main link between binocular vision and accommodation and refraction. If CA/C abnormal, then reflex accommodation does not receive appropriate assistance. CA/C Disparity (MA) S Pannum’s Area gain EOMs Verg. Resp. Adapt. tonic + - retina

Accommodative Convergence Stimulus (D) S Depth of focus gain Cm/lens Reflex Accom Adapt. tonic + - retina AC/A Retinal blur also produces synkinetic accommodative convergence innervation that is summed to produce the correct vergence response. Helps to align the eyes as reflex accomodation works to clear the images. Serves to minimize retinal disparity and optimize stereopsis. Not inverse of the CA/C ratio- it is a different neurological entity Stimulus: measured convergence/accommodative stimulus value without regard to accommodative response. – this is more often used clinically. Avg is 3.5prism diopters of convergence per diopter of accommodation. Response: measured convergence/measured accommodation CA/C Disparity (MA) S Pannum’s Area gain EOMs Verg. Resp. Adapt. tonic + - retina

Proximal Vergence Stimulus (D) S Depth of focus gain Cm/lens Reflex Accom Adapt. tonic + - retina CA/C AC/A Disparity (MA) Pannum’s Area EOMs Verg. Resp. Percieved Nearness Prox Accom gain Prox Vergence gain

Pathophysiology of common binocular anomolies Accommodative insufficiency Accommodative excess Accommodative infacility Convergence Insufficiency Convergence Excess

Accommodative Insufficiency Accomodation is about 2D lower than age expected norms Ill sustained/fatigue Paralysis or paresis Unequal Paralysis and Unequal ususally d/t head trauma. Organic causes. Unequal is the same but also can result from amblyopia

Accommodative Excess Accommodative response is greater than expected Accommodative spasm Usually presents with convergence insufficiency Use reflex accommodation and accomodative convergence to compensate. Will have low AC/A ratio.

Accommodative Infacility Dynamics of latency/ velocity/ time constant are slower than normal Accommodative amplitudes are normal Difficulty changing focus from far to near and back. flippers

Convergence Insufficiency High near exophoria, low AC/A ratio, low positive relative convergence, excess accommodation at near Poor or nonexistant convergent adaptation Reduced accommodative vergence=low ac/a Abnormallly high convergence accommodation Negative reflex accommodation offsets the excess convergence accommodation, but is overwhelmed leading to accommodative excess in near vision. VT- improving positive vergence adaptation

Convergence Insufficiency Stimulus (D) S Depth of focus gain Cm/lens Reflex Accom Adapt. tonic + - retina CA/C AC/A Disparity (MA) Pannum’s Area EOMs Verg. Resp. Percieved Nearness Prox Accom gain Prox Vergence gain x Very high accommodation at near because you cannot converge well. Convergence accomodation kicks in to compensate and you are overminuesed at close range. x

Convergence Excess High esophoria at near, high AC/A ratio, lag of accommodation Caused by excessive sustained accommodative vergence innervation. Accommodative adaptation mechanism does not adapt to near vision, so reflex accommodation carries more of the burden. Higher than normal reflex accommodation causes excessive accommodative vergence by way of AC/A synkinesis. Negative disparity vergence is used to maintain binocularity. This causes negative convergence accommodation that increases the lag of accommodation. This increases the deman=d for positive accommodation from the already taxed RA. VT- not very successful at changing the Ac/A ratio. Use plus lenses for near to reduce reflex accommodation and excess accommodative vergence.

Convergence Excess x x accom lag x Stimulus (D) S Depth of focus gain Cm/lens Reflex Accom Adapt. tonic + - retina CA/C AC/A Disparity (MA) Pannum’s Area EOMs Verg. Resp. Percieved Nearness Prox Accom gain Prox Vergence gain x accom lag Caused by excessive sustained accommodative vergence innervation (high AC/A). x

Effect of blur on binocular vision BI prism and minus lenses: demand increases of negative disparity vergence innervation and positive reflex accomodation innervation BO prism and plus lenses: demand increases of positive dispartiy vergence innervaiton and negative reflex accommodation innervaiton.

Summary- it is all connected.. Aniseikonia, anisometropia, prism can challenge binocular motor and binocular sensory function Bo prism makes flat wall appear concave because of asymmetical image magnification, worse in myopia. Anisophoria is a change in the phoria with gaze, common with anisometropic glasses Plus lenses cause prism effects that increase the ocualr rotation required to fixate the limits of an object, and minus lenses decresae the rotation needed. Accommodative spasm, use binoclar refraction http://www.mabot.com/images/2001/w2/brain-render.jpg