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Clifton M. Schor, OD, PhD
Member, Berkeley Optometry Hall of Fame

Clifton M. Schor

Clifton M. Schor was born in Denver Colorado in 1943 into an optometric family including his father and two uncles. He is a grandchild of Russian, Austrian and Romanian immigrants. He received his BS, M. Opt, OD and PhD in physiological optics from the University of California at Berkeley. His PhD thesis was the first application of the linear systems analysis with the modulation transfer function to quantify visual loss in amblyopia, and the first to link arrested developmental components of oculomotor functions with fixational eye movements in amblyopia. In 1971 he was Assistant Professor of Optometry at Pacific University. Cliff joined Berkeley Optometry until his retirement in 2014 when he became Professor Emeritus of the graduate division at UC Berkeley. He was a member of the Vision Science and Bioengineering graduate studies groups. His research was centered around the topic of motor-sensory aspects of binocular vision.

Professor Schor was a leading researcher in the application of linear systems analysis to binocular oculomotor functions, including vergence and accommodation. His modeling efforts elucidated how these two motor systems interact in the near response to changes in viewing distance though cross-coupling interactions between accommodation and convergence. The models also illustrate how binocular eye alignment is adjusted with sustained vergence and accommodation responses by an adaption mechanism. He showed that adaptation was versatile and could simultaneously achieve multiple states that depended on changes in binocular disparity with context specific viewing conditions, such as direction and distance of gaze and with head orientation.

The models incorporate oculomotor neural plasticity, controlled by the cerebellum, with a negative feedback servo control system to illustrate how context specific adaptation of horizontal, vertical and cyclo vergence movements underlie the rehabilitation of motor anomalies including non-concomitant strabismus and non-concomitant vergence biases, such as phorias, produced, for example, by optical prismatic distortions in anisometropic spectacles. The models were adopted by clinicians to serve as a framework for understanding disorders such as non-comitant strabismus, accommodative esotropia, presbyopia, convergence and accommodative excess and insufficiency, and by researchers to understand how stimulus conflicts in virtual and augmented reality displays disrupted the normal interactions between accommodation and convergence and resulted in visual discomfort.

Dr. Schor developed a biomechanical model of the accommodation plant, (i.e., lens, capsule, ciliary zonules and muscle), to be used by industry in the development of accommodating intraocular lenses (for example, AIOL), that are dynamic intraocular implant replacements for cataracts. The model illustrates the dynamic accommodation response to focus changes based upon the same biomechanical properties as the natural lens. The goal was to match the biomechanical properties of the AIOL with the force programs applied by the ciliary muscle to achieve normal stable dynamics of accommodation.

Professor Schor was a pioneer in the study of abnormal monocular eye movements in amblyopia. He investigated vertical and horizontal directional biases of fixational eye movements (i.e. jerk nystagmus) found in amblyopia, latent nystagmus, and dissociated vertical deviation (DVD), that were related to a postnatal developmental biases of a reflex optokinetic stabilization reflex (i.e. Optokinetic Nystagmus), that failed to develop responses to all motion-stimulus directions when binocular vision was disrupted, for example by strabismus or extreme anisometropia. These nystagmus disorders were the result of a deficit of binocular cortical neurons that relayed motion signals to the midbrain centers, such as nucleus of the optic tract, that control reflex following and fixation eye movements. The presence of nystagmus is used by clinicians as a time marker for diagnosing early-onset-strabismic amblyopia and to form a prognosis for functional correction with patching and orthoptics. The later the onset of amblyopia, the higher its functional prognosis.

Professor Schor also investigated how the two eyes combine monocular visual directions to yield single vision (fusion), and binocular perception of direction and depth (stereopsis). Behavioral correlates of the spatial selectivity of binocular cortical neurons that underlie fusion and stereopsis were demonstrated by comparing lower and upper limits of binocularly fused and stereo vision with spatially filtered narrow-band bar stimuli. Perceptual measures demonstrated a link between the size (spatial frequency) and disparity range of the optimal stimulus for binocular fusion and stereopsis. Modeling showed how a cross-correlation between monocular inputs, sensitive to a band-limited size range, could constrain the disparity tuning range of a binocular cortical cell. The studies reinforce Kepler’s theory of an array of multiple neural populations of disparity sensitive cells that are tuned to different ranges of disparity and spatial frequency.

Professor Schor was a leader in understanding how effectively binocular fusion and stereopsis operate in the presence of monocular blur or anisometropia during binocular fusion. Traditionally clinicians anticipated that patients would suppress the monocular out-of-focus fused image and experience stereo blindness. However, the visual system is capable of both suppressing the blurred image (interocular blur suppression), while retaining stereopsis of the binocularly fused image, presumably using the common low frequency information in both images to derive disparity. This phenomenon is coined monovision suppression and is widely used in the correction of presbyopia with spectacles, contact lenses and IOL implants.

Professor Schor demonstrated stereo depth perception without binocular retinal disparity in normal binocular vision – head-centric disparity, using distortions of perceived direction that occur immediately before a saccade. Head-centric disparity is also used in strabismus diagnosed with anomalous binocular correspondence (ABC). These strabismic patients binocularly fuse images with the two eyes when the eyes are misaligned under conditions when normal eyes would perceive double vision. Unlike normal vision, each eye senses direction monocularly or independently of the other eye, and then the two eye’s directions are compared to form binocular disparity. Normally visual direction is computed after retinal images are combined into a cyclopean percept. Patients with ABC lack binocular cortical cells and use a dormant head-centric mechanism to fuse, that is used by lateral eyed animals such as the chameleon. This theory displaces traditional clinical views of an adaptation of binocular cortical connections or anomalous retinal correspondence in strabismus. The head-centric model justifies anti-suppression forms of treatment for anomalous correspondence, i.e. Walraven technique, that often complicates functional correction of eye alignment in strabismus.

The Schor Lab was a place that many students from different backgrounds and nationalities interacted and learned together. The purpose of the lab was to financially support students, to train them as researchers with technical and analytical skills, develop critical thinking and to be innovative, self sufficient, and ultimately to place them in a position that fulfilled their career goals. Everyone brought their own background expertise to the lab and learned from one another.