Research looks into neural circuitry behind human visual experiences

Researchers have discovered that the dominant neural circuit in the fovea, a structure found only in human and primate retinas, operates effectively independent of any sort of "braking" at the specialized signaling junction called the synapse.

Undertaken by a team of researchers with University of Washington (UW) and Howard Hughes Medical Center, the study published recently in the journal Cell uncovers some of the reasons behind the unusual perceptual properties of the eye's fovea and provides a closer look at the eye's sharp vision spot.

The fovea is a specialized region that dominates our visual perception, said Raunak Sinha of the Department of Physiology and Biophysics at the UW School of Medicine.

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.

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 was quoted as explaining in a news release from UW.

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 circuits.

Nonetheless, Sinha said, little was known about the cellular and circuitry basis of these functional specializations due to a lack of intracellular recordings from foveal neurons.

And the research team recently made the first direct comparisons of the physiological properties of foveal and peripheral retinal neurons and the first correlation between structure and function in the fovea.

Their experiments revealed how differences in the cellular and circuit mechanisms of foveal and peripheral retina can account for the differences in their perceptual sensitivities.

Specifically, they found, among others, that foveal midget ganglion cells, a major class of retinal output neurons, process input and output signals in a manner that shows that they are not smaller versions of their peripheral counterparts.

By comparing the responses of the cone photoreceptors -- the neurons at the frontline of the visual system, the researchers also found that the responses in the fovea are about two-fold slower than those in the periphery, which are nearly identical to the differences between central and peripheral vision in perceiving rapidly changing inputs.

The finding suggests that the perceptual differences originate in the cone photoreceptors themselves.

Figuring out how the fovea functions is essential to the search for strategies to correct central vision loss. "Diseases such as macular degeneration are much more debilitating than deficits in peripheral eyesight because of the importance of the fovea to everyday vision," said Sinha. Endit