Ability to 'see' light still important for regulating body clock in some blind people

16 January 2008

People who are profoundly blind may still be able to 'see' light and subconsciously use it to regulate their body clocks, research funded by the Wellcome Trust and National Institutes for Health has shown. The findings have implications for advice and treatment offered to those who lose their vision.

An international team of researchers including scientists from the UK and US found that even if a person's rods and cones - the cells of the eye responsible for vision - have been completely destroyed, the patient may still be able to detect light, even if they are unaware of it.

Researchers worked with two patients, both of whom had developed degenerative blindness and had lost their sight completely decades previously. However, it was noted that neither patient reported disturbed sleep patterns, suggesting that their internal body clocks, known as circadian rhythms, were still operating correctly.

Through a series of experiments1, including asking one of the patients to say whether they thought a light was on or off over a particular interval, researchers were able to show that even though they were not consciously aware of being able to see light, they were able to detect the presence of blue light well-above the level of chance alone.

The results of the study by scientists from Imperial College London, the University of Oxford and City University in the UK, and Harvard University and Thomas Jefferson University in the US, are published in 'Current Biology'.

The retina of the eye hosts a number of photosensitive cells, including cones, which are sensitive to colour but only operate in good light conditions, and rods, which operate at low light levels but only in monochrome. However, before light reaches these cells, it passes through a region containing a small number of photosensitive retinal ganglion cells (pRGCs). The researchers believe that the pRGCs are involved in a number of non-image forming tasks, including regulating production of the sleep hormone melatonin, thus influencing our natural body clock.

"Even people who have been blind for 30 or 40 years may still be able to detect light of a particular wavelength," says Dr Stuart Peirson from the Wellcome Trust Centre for Human Genetics at the University of Oxford. "Although they may have lost all their rod and cone cells, which we traditionally thought solely responsible for detecting light, their ganglion cells still appear to work. While these cells may be of no use for their vision, they play an important role in regulating their body clock."

Tests for diagnosing blindness include using a penlight examination to identify whether or not the pupil responds to light. If the rods and cones are damaged, the pupil does not appear to respond. However, if an intense enough light is used, the pRGCs, which are less sensitive than rod and cone cells, may still detect the light and the pupil will respond. Patients diagnosed completely blind may not appreciate the importance of exposing themselves to a light-dark cycle. In extreme circumstances, doctors may even operate to remove the eye for cosmetic reasons or to prevent risk of complications, such as intractable pain.

"Our research suggests that a regular cycle of light and dark can be important even in some people who are profoundly blind," says Dr Peirson. "Without it, just as with a person whose eyesight is intact, they risk disturbed sleep patterns, which can potentially lead to fatigue, impaired concentration, a weakened immune system and poor mental health."

It is not known why pRGCs are sensitive only to light at the blue end of the spectrum. However, Dr Peirson speculates that the cells may have evolved to detect dawn and dusk.

"There is a relative enrichment of blue light around dawn and dusk", he comments. "These cells may have evolved to detect the dawn/dusk transition to help us lock on to the Earth's cycle. This would be very difficult to test experimentally, however."

Contact

Craig Brierley
Media Officer
Wellcome Trust
T +44 (0)20 7611 7329
E c.brierley@wellcome.ac.uk

Notes for editors

1. Zaidi, F. et al. Short-wavelength light sensitivity of circadian, pupillary, and rudimentary visual awareness in blind humans lacking a functional outer retina. Current Biology, December 2007.

2. The Wellcome Trust is the largest charity in the UK. It funds innovative biomedical research, in the UK and internationally, spending around £500 million each year to support the brightest scientists with the best ideas. The Wellcome Trust supports public debate about biomedical research and its impact on health and wellbeing.

'Sleeping & Dreaming', a groundbreaking exhibition that combines art and medical science to explore sleep - the mysterious state we all inhabit for a third of our lives - is currently on display at Wellcome Collection, the new public venue from the Wellcome Trust.

3. Imperial College London
Rated as the world's fifth best university in the 2007 'Times Higher Education Supplement' university rankings, Imperial College London is a science-based institution with a reputation for excellence in teaching and research that attracts 11 500 students and 6000 staff of the highest international quality.

Innovative research at the College explores the interface between science, medicine, engineering and management, and delivers practical solutions that improve quality of life and the environment - underpinned by a dynamic enterprise culture.

With 66 Fellows of the Royal Society among its current academic staff and distinguished past members of the College including 14 Nobel Laureates and two Fields Medallists, Imperial's contribution to society has been immense. Inventions and innovations include the discovery of penicillin, the development of holography and the foundations of fibre optics. This commitment to the application of our research for the benefit of all continues today with current focuses including interdisciplinary collaborations to tackle climate change and mathematical modelling to predict and control the spread of infectious diseases.

4. Oxford University's Medical Sciences Division is one of the largest biomedical research centres in Europe. It represents almost one-third of Oxford University's income and expenditure, and two-thirds of its external research income. Oxford's world-renowned global health programme is a leader in the fight against infectious diseases (such as malaria, HIV/AIDS, tuberculosis and avian flu) and other prevalent diseases (such as cancer, stroke, heart disease and diabetes). Key to its success is a long-standing network of dedicated Wellcome Trust-funded research units in Asia (Thailand, Laos and Vietnam) and Kenya, and work at the MRC Unit in The Gambia. Long-term studies of patients around the world are supported by basic science at Oxford and have led to many exciting developments, including potential vaccines for TB, malaria and HIV, which are in clinical trials.

5. The Wellcome Trust Centre for Human Genetics was established to undertake research into the genetic basis of common diseases.The scientific objective of the Centre is to explore all aspects of the genetic susceptibility of disease.The Centre houses multidisciplinary research teams in human genetics, functional genomics, bioinformatics, statistical genetics and structural biology.