Visual analgesia: Seeing the body reduces pain

VISION is now well known to modulate the senses of touch and pain. Various studies have shown that looking at oneself being touched can enhance tactile acuity, so that one can discriminate between two pinpoints which would otherwise feel like a single sensation. And last year, researchers from the University of Oxford showed that using binoculars to make a limb look larger or smaller than it actually is can respectively enhance and diminish painful sensations.

These phenomena occur because the brain fuses stimuli from different sensory systems to generate a coherent experience of bodily sensations. The precise mechanisms are unknown, and it is also unclear whether these effects depend upon specific visual stimuli. But according to a new study from University College London, the general “context” of vison is enough to modulate pain. In the current issue of the Journal of Neuroscience, they report that  merely looking at one’s hand can affect the perception of laser-induced pain, and how it is processed in the cerebral cortex. Together with earlier work, these findings point to a simple method for managing acute pain.

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Nature’s fibre optics

THE retina has an inverted structure which seems ill-suited to its function: the rod and cone cells, which are sensitive to light, and which convert light energy into electrical impulses, point backwards and are located at the back of the retina, so that light entering the eye has to pass through several layers of irregularly organized cells before it reaches them. The retina also contains nerve fibres which are positioned perpendicular to the path of light entering the eye, and many of the structures in the upper cell layers have a diameter similar to that of the wavelength of visible light. One would therefore think that light entering the eye would be subjected to a significant amount of reflection and scattering. Yet, nature somehow contrived to overcome this awkward architecture, and the retina performs its function perfectly.

As well as the various types of neurons, the retina contains specialized glial cells called Müller cells, which are arranged in parallel to each other and are oriented in the direction along which light travels through the eye. Müller cells are about 150μm (micrometres, thousandths of a millimetre) in length, and span the entire thickness of the retina, projecting from the vitreous humour (the viscous fluid in the back of the eye) to the back of the retina where light enters the rods and cones. Like other glial cells, Müller cells have been largely ignored until recently: they were thought to do little more than support and nourish retinal neurons. But in recent years it has been determined that glial cells perform other important functions, and Müller cells may overcome the retina’s architectural problem, by functioning as optical fibres which transmit light through it.

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