Ever wondered why humans can read?
A team led by Kolkata-born scientists has found that a special sweet spot in the eye called 'fovea' plays a crucial role in humans being able to to focus on computer screens and also read, an ability which is unique to Homo sapiens.
The findings decipher the mechanism that lets humans reading this text, recognising faces, enjoying colours, say the scientists.
Raunak Sinha and Mrinalini Hoon describe themselves as a 'scientist couple' who push the frontiers of neuroscience to better understand vision.
Sinha says this "recent breakthrough in understanding how the most important aspects of our vision works 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".
Vision scientists have uncovered some of the reasons behind the unusual perceptual properties of the eye's fovea. Among mammals, only humans and other primates have this dimple-like structure in their retinas.
Owls, some other predatory birds, and some reptiles have a similar structure. The fovea is responsible for our visual experiences that are rich in colourful spatial detail.
Figuring out how the fovea functions is essential to the search for strategies to correct central vision loss, including efforts to design visual prosthetics.
"Diseases such as macular degeneration are much more debilitating than deficits in peripheral eyesight because of the importance of the fovea to everyday vision," says Sinha of the Department of Physiology and Biophysics at the University of Washington's, School of Medicine.
The fovea is a specialised region that dominates our visual perception, he explains.
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," he says, "the fovea subtends a field only about the size of your thumbnail. Our eyes undergo rapid movements to direct the fovea to various parts of the scene."
The absence of a fovea in most mammals, he says, 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.
Using advanced techniques, Sinha helped lead a study that revealed that the 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, he says.
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, Sinha explains.
Past recordings of foveal output signals in the living eye
had demonstrated that the perceptual specialisations of foveal vision originated largely in the retina itself, rather than in subsequent brain circuits.
Nonetheless, Sinha says, little was known about the cellular and circuitry basis of these functional specialisations due to a lack of intracellular recordings from foveal neurons.
The team from 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.
"The novelty of this study is bolstered by a 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," says Hoon, an acting instructor in biological structure at the UW School of Medicine who contributed to the recent research.
These approaches open the door to a wide-range of transient genetic manipulations that will allow scientists to explore properties of other cell types in the fovea.
"Determining the cellular origin of human perception is an important, but rarely realised, goal in neuroscience and biology," Sinha says.
"Our results provide a simple explanation for a salient perceptual observation."
Sinha says the results are important since there is a huge amount of effort underway globally to restore central vision in humans in diseases but our understanding of how the fovea functions is largely missing.
"This is a big step forward in not only our fundamental understanding of foveal function but also for devising therapeutic strategies including designing visual prosthetics to restore deficits in central vision in diseases such as macular degeneration and others.
