Researchers find retinal rods able to detect photon number distribution

Sep 14, 2012 by Bob Yirka report
(Left) Structure of a rod cell in the retina. Photons absorbed in the outer segment cause a rearrangement of rhodopsin molecules, which initiates a sequence of chemical events resulting in excitation of an optic nerve signal. (Right) Microscope image of the fiber taper and a suction micropipette with the constrained rod cell. The cell response to photons is measured with the pipette, which acts as a micro electrode. Credit: Physics Synopsis, DOI: 10.1103/Physics.5.103

(Phys.org)—The eye, whether in humans or other animals, is truly one of nature's most sophisticated advancements, able to convert light into signals the brain can interpret as imagery, all in real time. Most of the actual work is done at the back of the eye where rods and cones, two types of photoreceptors are located. Cones are primarily responsible for the eye's sensitivity to color, while rods, which are far more numerous (some 120 million exit in one human eye), are more sensitive to light in general. To find out just how sensitive rods are, researchers in Singapore have been studying single rod photoreceptors taken from an African Clawed Frog, and have found, as they describe in their paper published in Physical Review Letters, that such rods are able to discern and count single photons and are also able to determine the coherence of very weak pulses of light.

To study the rods, the team used a tiny pipette that was able to hold a single rod and that also contained a that simulated its natural environment to keep it alive. They then placed a very small laser just in front of it and shot the rod with pulses of light. Because the pipette and solution also served as an electrode allowing for measurements, the researchers were able to measure how much current was produced by the rod in response to the pulses of light.

Each rod has at its tip a segment that contains rhodosin photopigment, a substance that changes chemically in the presence of light. When no light is present, a constant current of ions flows in and out; when light is detected however, some of the current is altered causing the cell to be polarized resulting in the creation of an that is sent via the to the brain.

In the study, the team fired a rapid succession of at the rod and found it able to discern, i.e. measure, individual differences of up to 1000 photons per pulse. They also found that the rods were able to tell the difference between coherent light (the degree to which the waves are in phase) and "pseudothermal" light, where the waves are chopped up by a rotating disk, to such an extent that the researchers believe they will be able to serve as a model for creating highly sensitive artificial detectors.

In the end, the researchers found that single rhodopsin molecules are able to interact with single photons, a finding that demonstrates just how sensitive rods truly are; so much so that further studies by the team will look at their use in quantum optics and communication.

Explore further: Superconducting circuits, simplified

More information: Measurement of Photon Statistics with Live Photoreceptor Cells, Phys. Rev. Lett. 109, 113601 (2012) DOI: 10.1103/PhysRevLett.109.113601

Abstract
We analyzed the electrophysiological response of an isolated rod photoreceptor of Xenopus laevis under stimulation by coherent and pseudothermal light sources. Using the suction-electrode technique for single cell recordings and a fiber optics setup for light delivery allowed measurements of the major statistical characteristics of the rod response. The results indicate differences in average responses of rod cells to coherent and pseudothermal light of the same intensity and also differences in signal-to-noise ratios and second-order intensity correlation functions. These findings should be relevant for interdisciplinary studies seeking applications of quantum optics in biology.

Related Stories

Bright lights, not-so-big pupils

Dec 31, 2008

A team of Johns Hopkins neuroscientists has worked out how some newly discovered light sensors in the eye detect light and communicate with the brain. The report appears online this week in Nature.

Nanosize Rods Light Up Pancreatic Cancer Cells

Apr 18, 2008

Quantum dots have shown promise as ultrabright contrast agents for use in a variety of cancer imaging studies. Now, a team of investigators at the Multifunctional Nanoparticles in Diagnosis and Therapy of Pancreatic Cancer ...

First Look at the 'Birth' of a Retina Cell

May 05, 2006

Scientists at the University of Michigan Kellogg Eye Center have gained new insight into the way an embryonic retina cell develops and then commits itself to a specific role. They have observed a small window of opportunity ...

Why animals don't have infrared vision

Jun 09, 2011

On rare occasion, the light-sensing photoreceptor cells in the eye misfire and signal to the brain as if they have captured photons, when in reality they haven't. For years this phenomenon remained a mystery. Reporting in ...

Recommended for you

Cold Atom Laboratory creates atomic dance

37 minutes ago

Like dancers in a chorus line, atoms' movements become synchronized when lowered to extremely cold temperatures. To study this bizarre phenomenon, called a Bose-Einstein condensate, researchers need to cool ...

Scientists create possible precursor to life

7 hours ago

How did life originate? And can scientists create life? These questions not only occupy the minds of scientists interested in the origin of life, but also researchers working with technology of the future. ...

Superconducting circuits, simplified

Oct 17, 2014

Computer chips with superconducting circuits—circuits with zero electrical resistance—would be 50 to 100 times as energy-efficient as today's chips, an attractive trait given the increasing power consumption ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

MrVibrating
not rated yet Sep 14, 2012
Fantastic work, incidentally however this result is predicted by G. C. Huth's work on the quantum physics of retinal function:

http://www.ghuth....-retina/

And then….it follows from a Poisson calculation of light fluence on the retina in the femtosecond time frame yields the surprising result that the probability of two photons falling on the same site is vanishingly small. The retina is then a 'photon counter' and each individual 'femtosecond frame' in this succession of images consists of millions of single photon* interactions.
ormondotvos
not rated yet Sep 14, 2012
So what explains the patterns you see when you knuckle your eyes -- varying speeds of firing as various portions of your optic nerves interact as they crap out?