From The Royal Institution.

Colin Blakemore discusses contrasts and differences we find in the natural world.

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This lecture was recorded at the Ri on 29 December 1982.

Find out more about the CHRISTMAS LECTURES here: https://www.rigb.org/christmas-lectures

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Lecture 5: Vive la difference!

Our senses are fighting a constant battle-against too much information! Because of their great sensitivity and yet the fantastic range of intensity over which they must work, sense organs and the nerves that transmit their messages to the brain have developed clever tricks of detection and coding. Most sensory systems detect changes and differences rather than the absolute value of the physical stimulus. For instance, the retina transmits very little information about the uniform areas of light (like the sky): most messages leaving the eye are concerned with the changes of intensity at the edges of objects. This simple trick frees the optic nerve of irrelevant messages but it can lead to curious illusions and mis-perceptions. It has even been exploited by artists to deceive the eye. The retina is also more interested in signals that change with time. If you look at a uniform area of light that fills the entire visual field, it soon appears to fade to a colourless grey, and you see the light and its colour briefly only when it changes. For this reason your eyes must be constantly in motion, not only to explore the visual sense but also to prevent your view of the world from fading. If an optical instrument is used to hold an image stationary on the retina despite movement of the eyes,,everything seems to disappear in a few seconds. For the same reasons, moving objects are often easier to see than stationary ones and some animals are virtually blind if nothing around them moves. Most sense organs use this strategy of detecting difference and change. Any constant stimulus causes ‘adaptation’ or ‘fatigue’ and this can lead to some amusing sensory illusions. However, sending messages only about difference and change creates problems of interpretation for the brain. The real meaning of such a signal depends on an accurate memory of the preceding signals. This procedure works well enough for senses like vision, touch and hearing, where the physical stimulus usually changes very frequently, but some senses, like those responsible for signalling constant pain, or the direction of gravity or the chemical composition of the blood, have to provide absolute and not just relative information because they deal with stimuli that don’t often change.

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About the 1982 CHRISTMAS LECTURES: Common Sense

Our sense organs are windows on the world. But just like windows, as well as giving us a view of the physical world, the senses also restrict our outlook on the things around us. Philosophers have worried for centuries about the reliability of the human senses and about the relationship between the real world and the world as we see, hear and feel it. Is the world only a creation of our minds? I am no philosopher, so I am happy to accept that there is a real world out there and that our sense organs simply describe it to our brains. But this means that the world we know through our perceptions is created by processes in our brains and the validity of this imagined world depends crucially on the way that our sense organs and our brains work together to perform the magic of perception. My aim in these lectures is to describe the way that the sense organs act as biological instruments of detection, measurement and analysis.

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About Colin Blakemore:

Sir Colin Blakemore (1 June 1944 – 27 June 2022) was a British neurobiologist, specialising in vision and the development of the brain. His own research work was mainly concerned with the mechanisms in the brain for the interpretation of signals from the eyes, and especially in the early development of vision during the first few days and weeks of an animal’s life. He was well known for his work in communicating science to the public and published many popular books.