Larry N. Thibos

Larry N. Thibos, PhD, DSc, FAAO, FOSA
Member, Berkeley Optometry Hall of Fame

Larry N. Thibos is Professor Emeritus at the Indiana University School of Optometry. He was born on Christmas Day, 1947, in Detroit, Michigan, and earned BS (1970) and MS (1972) degrees in Electrical Engineering at the University of Michigan. His PhD in Physiological Optics (1975) at the University of California, Berkeley was awarded for research on the neurophysiological mechanisms of sensitivity control in the vertebrate retina.

As a Research Fellow (1975-1983) working with Professsor W.R. Levick at the Australian National University in Canberra, Australia, his neurophysiology research grew to encompass additional aspects of visual information processing. In 1983 Thibos joined the Visual Sciences faculty of the School of Optometry at Indiana University where he taught vision science and investigated the optical and retinal limits to vision until his retirement in 2012.

Professor Thibos served on the editorial boards of Journal of the Optical Society of America (1993-99), Optometry and Vision Science (1993-99), Ophthalmic and Physiological Optics (2007-2015) and Journal of Optometry (2008-present). The American Academy of Optometry awarded Thibos the 1997 Glenn A. Fry Award for Vision Research, and the 2012 Prentice Medal in recognition of contributions to advancement of knowledge in the visual sciences. In 2014 the British College of Optometrists presented Dr. Thibos with the inaugural Presidential Medal for his contributions to optometric research.

Professor Thibos is especially proud of his contribution to the scientific lexicon, including scoton (the elementary particle of darkness, 1990), power vector (a mathematical representation of sphero-cylindrical lenses, 1997), aberrometer (an instrument for measuring optical aberrations of the eye, 1999), and xerop (a hypothetical drop of dryness that causes thinning of the tear film, 2011). His favorite graphical innovations are the Zernike tree (a periodic table of the Zernike polynomials, 2001) and the astigmatism airplane (a whimsical piece of optical origami, 2012).

His research in visual optics with perennial colleagues Arthur Bradley and Raymond Applegate focused on the nature of the eye’s optical flaws and how those flaws affect vision. His team’s development of the Indiana Eye optical model that embodies all of the eye’s major optical aberrations provides a simple framework for computing their effect on the quality of the retinal image.

A popular clinical application of this work was the invention of Power Vectors for the description and statistical analysis of refractive errors. Aberrometry is another clinical application, for which Thibos led the development of ANSI and ISO standards for reporting ocular aberrations. His group was the first to build and use a wavefront aberrometer to quantify the optical defects of a large population of normal eyes, which proved to be a useful standard for assessing abnormal conditions such as dry eye, corneal disease, refractive surgery, and cataract. Dr. Thibos’s work in visual optics culminated in an optical model of the normal human eye that accounts for all of the eye’s optical aberrations across the central visual field and their changes with accommodation.

Prof. Thibos’s research in visual neuroscience began at UC Berkeley with his mentor Prof. Ralph Freeman. Together they gathered the first electrophysiological evidence that abnormal early visual experience (caused by astigmatism) can modify the human brain (to produce meridional amblyopia). For his PhD thesis, guided by Prof. Frank Werblin, Thibos demonstrated quantitatively how the amphibian retina controls its own sensitivity to light by a neural mechanism of lateral antagonism. Later, Thibos and his mentor Prof. William Levick at ANU challenged the prevailing orthodoxy that the cerebral cortex is the first site of complex neural processing in vision. They showed that many aspects of visual processing are already determined in the retina, where there is rich diversity of receptive field classes, temporal and spatial selectivity, and systematic preference for oriented stimulus contours.

Prof. Thibos’s research in visual perception has explored how retinal architecture limits visual capacity throughout the visual field. Initial work with students at Indiana demonstrated for the first time that spatial patterns too fine to be resolved in the peripheral visual field can nevertheless be reliably detected as aliases of the stimulus. Aliasing is an illusory percept that begins at spatial frequencies slightly greater than the classical resolution limit, which proves that peripheral acuity is limited by the coarse spacing of visual neurons rather than by increased size of their receptive fields.

At any given eccentricity, the finest detectable pattern has a much smaller spatial period which approaches the diameter of individual cone photoreceptors. Those initial findings launched a 40 year collaboration with esteemed colleague Arthur Bradley, which included the first systematic survey of sampling-limited resolution acuity across the entire visual field. That survey confirmed that “pixel density” of the discrete neural image carried by the optic nerve limits the spatial bandwidth of veridical perception at all retinal locations.

Moreover, resolution acuity is largely independent of illuminance at any given retinal location. That surprising result suggests that the well-known phenomenon “more light makes better sight” is an eccentricity effect: observers employ a fixation strategy that optimizes resolution for the available level of illumination. Comparison of visual acuity with anatomical sampling density of retinal neurons further suggests that mesopic acuity at all eccentricities, and scotopic acuity for eccentricities beyond 20 degrees, is limited by the spacing between midget ganglion cells.