Fusion Worth Classification • First degree fusion or simultaneous perception • Second degree fusion or flat fusion • Third degree fusion or stereopsis
Theories of Fusion • Alternation or suppression theory: • Fusion theory
Limits of fusion • Panum’s fusional area • Fixation disparity • Panum’s limiting case
Panum’s area • In the fovea Panum’s area measures 5 to 20 minutes of arc. • Panum’s area is larger in the periphery. • Factors influencing Panum’s area • Vergence eye movements • Spatial frequency • Temporal frequency
Binocular Rivalry • What happens when dissimilar images are presented to each eye? • Confusion • Alternating intermittent suppression occurs when dissimilar images are presented to each eye
Binocular Rivalry • First degree fusion • Allows us to monitor suppression • Used in tests of correspondence
Characteristics • Greatest with separation 90 degrees apart • Equal stimulus values
Characteristics • Stimulus strength determines the ability to induce contralateral suppression in the other eye. The strength of the stimulus is related to the amount of contour per area in the pattern and the contrast of the contours.
Characteristics • Local phenomenon • Reduced sensitivity to area suppressed • DaVinci Stereopsis
Color Fusion and Luster • Different luminance values to each eye but contours are the same. • Get a glossy appearance • Colors can fuse under some conditions
Clinical Applications • Confusion in strabismus can be thought of as rivalry • Similar images fall on non-corresponding points on the retina-leads to diplopia • Dissimilar images fall on corresponding points-leads to confusion • Confusion can lead to rivalry where foveating eye becomes dominate • Development of constant suppression and possible amblyopia
Suppression • We do not notice physiological diplopia so we must be suppressing most of time. • Most strabismic patients do not see double, why?
Two processes reduce diplopia • Binocular sensory fusion which operates within Panum’s area • Suppression which is an interocular inhibitory process that reduces visual information from the suppressed eye below the threshold for conscious perception
Adaptations • In normal binocular vision the suppression of physiological diplopia is called physiological suppression or suspension.
Adaptations • When dissimilar images are presented to corresponding retinal points confusion results. Alternating suppression from each results and is called binocular rivalry (see above). Confusion can be eliminated by regional suppression.
Adaptations • Pathological diplopia occurs when the object of regard is imagined on non-corresponding points. This can be eliminated by regional suppression.
Characteristics • Effect of orientation and spatial frequency • Schor (1977) presented sinusoidal gratings at various orientations and spatial frequencies to normal and strabismic subjects.
Characteristics • Strabismic subjects showed normal binocular rivalry when targets of different orientation. Size and spatial frequency did not alter result • When orientation difference reduced to less than 22 degrees there was constant suppression of the deviating eye. In normals a depth effect of was observed resulting from horizontal disparity created by orientation difference.
Suppression in Strabismus • Amblyopia • Reduction in acuity under binocular condition
Suppression in Strabismus • Esotropia • Forms “D” pattern between the fovea and zero measure point Usually confined to one hemiretina and does not extend beyond the naso-temporal decussation line.
Suppression in Strabismus • Exotropia • Usually occurs across the entire temporal hemiretina
Characteristics • Suppression is not uniform across the suppression zone • Most intense at fovea and zero measure point • Can get an inverse suppression when stimulation to the deviating results in suppression of the fixing eye. • Actual suppression areas can vary depending on which area of the fixing eye is stimulated. • Latency of 75 to 125 msec in normal and longer in some cases of strabismus.
Classification of suppression • Central < 5 degrees • Foveal < 1 degree • Parafoveal < 3 degrees (but > 1 degree) • Paramacular < 5 degrees (but > 3 degree) • Peripheral > 5 degrees
What about monovision? • Clear vision to one eye and blurred to the other • Creating binocular rivalry • Clear eye becomes dominate at each distance.
Classification of suppression • Shallow • Most similar to regular viewing conditions • Deep • Abnormal viewing conditions
Red Lens Test • Put red filter of fixing eye. • Can use neutral density filters to measure the depth of the suppression.
Worth 4 Dot • Similar to red lens test • 1 red, 2 green, and 1 white light • Wear red-green glasses • Peripheral target at near and central target at distance.
Tests of Suppression • Worth 4 dot
Tests of Suppression • AO vecto slide
Tests of Suppression • 4 base out test • Put 4 base prism in front of one eye • Displaces image • Eyes should make a version and then vergence eye movement to follow the target. • No eye movement indicates suppresion
Vergence Ranges • Positive Fusional vergence • Introduce prism in front of each eye • What happens if only one eye sees the target. • Central vs. peripheral • Shallow vs. deep
Vision Therapy • First degree targets can take advantage of rivalry • Change target parameters to alter suppression patterns • Use physiological diplopia to create awareness.
Fechner’s Paradox • This occurs when placing a neutral density filter over one eye. When you close the eye with the filter the object looks brighter. The visual system does not add the brightness from the 2 eyes.