Positive Feedback Mechanism For Eye

This is an unexpected systemworking to sharpen images in the retina, undiscovered until now.  It will certainly inform additional work onimage processing as we learn to emulate the eye.

The achievement of the eye hasalways been amazing as well as out ability to understand it in the details.  Thus adding another remarkable detail is abig surprise to everyone I am sure.

Positive feedback boosts eye's ability to see

May 18, 2011 

Positivefeedback at work in the eye

http://physicsworld.com/cws/article/news/46010

  

Researchers in the UShave discovered a new feedback mechanism that allows the human eye to besensitive enough to see small details in a scene while also being able todetect the large contrast between bright and dark objects. The process involvesboosting the output of certain light receptors in the retina while damping downothers – and could help with the design of digital vision systems.

The retina of the human eye senses light using about 100 millionphotoreceptors and a host of neuronal tricks to convert light signals into auseful picture of the world. One of the most basic processes to occur in theeye is contrast enhancement, which allows the eye to resolve bright and darkobjects. This is done directly in the retina by a negative feedback mechanismbetween neighbouring light-sensitive cells.

The first line of signal processing occurs in the star-shapedhorizontal cells just below the surface of the retina. Each of these cellsreceives input from about 100 photoreceptors (rods and cones) on the retina. Aphotoreceptor responds to darkness with a burst of a neurotransmitter calledglutamate. This chemical causes the horizontal cell to depolarize, and theresulting voltage change shuts off the signalling channels of other nearbyphotoreceptors, preventing them from emitting more glutamate. This has theresult of isolating and sharpening the original signal within a circle of quiet– allowing the eye to see a dark object on a bright background and vice versa.

This process is called "lateral inhibition" and helps the eyeto detect the edges of objects. The downside of this mechanism, however, isthat it also reduces the maximum strength of the optical signal. This shouldtheoretically cause a loss of dynamic range and limit the eye's ability to pickout faint details – something that doesn't happen in a real eye.

Positive feedback at work

Now Richard Kramer, Skyler Jackman at the University California atBerkeley and colleagues at the University of Nebraska and University ofMassachusetts have discovered that the eye is able to pick out fine detailsusing an unexpected positive feedback process. They found that cones exposeddirectly to glutamate, far from reducing their neurotransmitter production,increased their own production of glutamate four-fold.

Upon further investigation, the team discovered that while theglutamate-mediated change in horizontal cell voltage did have a negativefeedback effect, glutamate exposure also caused an additional, more subtlechange in the horizontal cells, increasing the number of calcium ions in nearbyregions. It's thought that this triggers an increase of calcium ions withinthe cones themselves, boosting the production of glutamate very locally: injust the initially firing photoreceptor and perhaps a few of its immediateneighbours. "This recoups the signal strength lost to negative feedback,while preserving edge detection and contrast enhancement," explainedJackman.

The discovery of a secondary signalling system is particularly surprisingbecause the retina has received a lot of attention from researchers. "Thepositive feedback circuit is very susceptible to damage, and disappears whenthe retina is studied using more traditional preparations, involving slices ofthe retina," explains Jackman. "This may explain why positivefeedback has not previously been observed in such a well studied circuit."

Feedback circuits and other processing at a cellular level have severaladvantages for the central nervous system. First, it saves time because pickingout important features helps the brain to interpret what it is seeing quickly,and make appropriate decisions. Second, there is an issue of information flow:100 million photoreceptors are connected to a mere million nerve channels. Thisrequires a slimming-down of data without losing anything important.

Emulating the eye

It is these virtues of the eye that researchers in digital vision wantto copy. Neuroscience and visual prosthetics experts Stephen Hicks at the University of Oxford said, "This finding couldwell have a positive and relatively immediate effect in computer vision forintelligent systems." He added, "In computer vision we perform aprocess similar to the eye's lateral inhibition to identify and enhance theedges in a scene before putting them in context." He believes that thiswork will give researchers "new ideas for implementing a fastapproximation of the boundaries between objects in a video feed, which wouldimprove everything from robot–human interactions to video surveillance".

The work is described in PLoSBiology.