India Born Vision Scientist Push Frontiers of Neuroscience to Better Understand Vision

Last Updated on April 27, 2017 by Bharat Saini

Kolkata- India -born vision scientists Raunak Sinha and Mrinalini Hoon, who call themselves as a ‘scientist couple’, recently made comparisons of the physiological properties of  fovea and peripheral retina and pushed the frontiers of neuroscience to better understand vision. This is a recent breakthrough in understanding how the most important aspects of our vision work at a cellular level. This work illustrates the physiological basis of how our central vision, mediated by the region in the eye called fovea, works at a cellular level and how it differs in its operation from the region that mediates our peripheral vision. Their work has been published in the journal CELL.

The word fovea, with origins in Latin, means pit or pitfall. The fovea centralis, a small central pit in the eye, composed of closely packed cones is responsible for sharp central vision. Among mammals only humans and other primates have this sweet spot fovea, the dimple –like structure in their retina. Fovea is responsible for visual experiences that are rich in colorful spatial detail and plays a crucial role in humans by enabling reading and recognizing colors, images and faces. Fovea is the only part of the retina that allows 100% visual sharpness. The clarity of vision is the best when the object is in the line of sight, an imaginary line connecting fovea and a fixation point. The credit of identifying the line of sight goes to Leonardo da Vinci. Fovea decides our visual perception and passes on major portion of the input from the eyes to visual cortex of the brain. Hence, central vision has better quality than peripheral vision. Fovea performs tasks like reading in a much better manner compared to other parts of retina. However, the fovea is a poor performer in processing rapidly changing visual signals.

According to Dr. Raunak Sinha of the Department of Physiology and Biophysics at the University of Washington, School of Medicine and Mrinalini Hoon, an acting instructor in biological structure at the UW School of Medicine:

• Diseases such as macular degeneration are much more debilitating than deficits in peripheral eyesight because of the importance of the fovea to everyday vision.
• The fovea is a specialized region that dominates our visual perception. It provides more than half of the input from the eyes to the visual cortex of the brain.
• When you look at a scene an arm’s length away, the fovea subtends a field only about the size of your thumbnail.  Your eyes undergo rapid movements to direct the fovea to various parts of the scene.
• The absence of a fovea in most mammals and technical challenges associated with recording from the primate fovea, led to a paucity of information about how the fovea operates at the level of cellular circuits.
• Advanced techniques now show that the fovea’s computational architecture and basic visual processing of the fovea are distinct from other regions of the retina.The results help explain why central and peripheral vision have different qualities.
• Located near the optic nerve, the fovea is at its best for fine tasks like reading. Compared to the peripheral retina, however, the fovea is less able to process rapidly changing visual signals.
• This low sensitivity is what makes us see motion in flipbooks and movies. It’s also what prevents us from seeing flicker when a computer or TV screen refreshes, unless we glance at the screen (especially the old-fashioned CRT monitors) from the corner of our eye.
  • Past recordings of foveal output signals in the living eye had demonstrated that the perceptual specializations of foveal vision originated largely in the retina itself, rather than in subsequent brain
  • Nonetheless, Sinha said, little was known about the cellularand circuitry basis of these functional specializations due to a lack of intracellular recordings from foveal neurons.
  • The Howard Hughes Medical Center research team recently made one of the first direct comparisons of the physiological properties of foveal and peripheral retinal neurons and among the first correlations between structure and function in the fovea.
  • Publishing their work in the journal CELL, their experiments revealed how differences in the cellular and circuit mechanisms of foveal and peripheral retina can account for the well-established differences in their perceptual sensitivities.
  • The latest study provides one of the first glimpses into how the fovea works at a cellular and circuit level. It turns out to be very different from how other regions of the retina operate.
  • Returning to the issue of sensitivity to rapidly changing inputs, Sinha and colleagues compared the responses of the cone photoreceptors—the neurons that are the frontline of the visual system. They found that the responses of cone photoreceptors in the fovea are about two-fold slower than those in the periphery.

• This is nearly identical to the differences between central and peripheral vision in the sensitivity to rapidly changing inputs.
• The finding suggests that the perceptual differences originate in the cone photoreceptors themselves.
• According to Hoon, the novelty of this study is bolstered by comprehensive structure-function analyses, lacking in previous work on the fovea, using techniques such as particle-mediated gene transfer to study protein expression in a diverse array of ganglion cells.
• Determining the cellular origin of human perception is an important, but rarely realized, goal in neuroscience and biology.

The research by Dr. Raunak Sinha and Mrinalini Hoon provide simple explanation for a salient perceptual observation and the results are important since there is a huge amount of effort underway globally to restore central vision in humans in diseases but their understanding of how the fovea functions is largely missing. This revelation is an important move towards devising therapeutic strategies including designing visual prosthetics to restore deficits in central vision in diseases such as macular degeneration and others as it has given significant explanation of the fundamental understanding of foveal function.

 A micrograph created by Dr. Mrinalini Hoon shows the light receptors as cones in the fovea of retina.