Binocular Vision Fusion and Accommodation Fusion Development in

Binocular Vision, Fusion, and Accommodation

Fusion Development in the Infant § Birth – fixation reflex poor with random eye movements § 2 -3 weeks – follows light monocularly § 6 weeks to 6 months – follows light binocularly § 4 weeks to 6 months – convergence developing § By 6 months – accommodation developing – lags behind convergence

Fusion § Sensory fusion – the cortical process of blending the images from each eye into a single binocular stereoscopic image § Motor fusion – the mechanism that allows fine-tuning of eye position to maintain eye alignment – this is known as fusional vergence

§ Approximately 70% of the cells in the striate cortex are binocular cells § Retinal areas from each eye that project to the same binocular cortical cells are called corresponding retinal points.

Worth’s Three Degrees of Fusion § Simultaneous perception § Flat fusion § Stereopsis

Simultaneous perception

Flat Fusion § Worth Four Dot Test

Stereopsis § Titmus Stereo Test

Binocular Single Vision Separate and slightly dissimilar images arising in each eye are fused together as a single image.

Advantages of Binocular Vision § Single Vision § Stereopsis § Enlargement of visual field § Compensation for blind spot and other differences in the field of vision

Normal Binocular Single Vision Requirements § Clear visual axis § Sensory fusion § Motor fusion

Sensory Obstacles § Dioptric obstacles – refractive errors, anisometropia § Prolonged monocular activity – severe ptosis, media opacities § Retinoneural – lesions of retina, optic nerve § Proprioceptive

Motor Obstacles § Congenital craniofacial malformations § Conditions affecting extraocular muscles § CNS lesions – involving upper motor neurons

The Perception of Depth § Stereopsis § Monocular clues to depth

Monocular Clues to Depth § § § Relative size Interposition Linear perspective Aerial perspective Light and shade Motion parallax

Visual Direction § Local sign – positional information transferred from the retina to the motor system – the fovea has a retinomotor value of zero § Retinal correspondence = single vision § Retinal disparity = diplopia

Oculocentric Visual Direction

Egocentric Visual Direction

Cyclopean Eye

The Perception of Space § § § Retinal corresponding points Horopter Retinal disparity Panum’s fusional area Physiological diplopia

The Horopter § Vieth-Muller Circle § Empirical Horopter

The Horopter

§ Normal binocular single vision – the images of the fixated object are bifoveal with no manifest deviation of the eyes § Anomalous binocular single vision – the images of the fixated object are projected from the fovea of one eye and an extrafoveal position in the other eye – there is a small manifest deviation of the eyes

§ Misalignment of the two eyes can lead to diplopia and confusion § Diplopia – occurs when one object is seen in two places § Confusion – occurs when two dissimilar objects are seen in the same place

Suppression and Retinal Rivalry § Suppression § Normal – physiological diplopia – also known as suspension § Anomalous – pathological diplopia and confusion – cortical inhibition of suppressed eye § Retinal or binocular rivalry – rapid alternation of dissimilar images

Testing Retinal Correspondence § § § Bagolini lenses Red filter test Worth four dot test Hering-Bielschowsky afterimage test Foveo-foveal test of Cuppers

Worth Four Dot Test

Testing Suppression § § § Worth four dot test Polaroid vis-à-vis test Amsler grid 4 prism diopter base out test Red filter test Bagolini lenses

Polaroid Tests

Amsler Grid

§ Versions – conjunctive eye movements – the two eyes move in the same direction § Pursuits § Saccades

§ Vergences – disconjunctive eye movements – the eyes move in opposite directions § § § Convergence Divergence Vertical vergence

Vergence § § Tonic Proximal Fusional Accommodative

Accommodation § Tonic – in the absence of a visual stimulus, the accommodation adopts an intermediate position of 0. 5 -1. 0 D § Proximal – resulting from apparent nearness of an object § Convergence – accommodation linked to convergence § Reflex – involuntary response to blur to maintain a clear image § Voluntary – not dependent on a stimulus