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Learn about the 'men of vision' who sought to explain the mystery of sight with Professor Colin Blakemore.
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Professor Colin Blakemore was awarded his doctorate by an optometry faculty in California. His research has centred upon the development of the visual cortex. He was appointed Waynflete Professor of Physiology at the University of Oxford and is now the Chief Executive of the Medical Research Council and a Fellow of the Royal Society.
This article has been adapted for the web by the museum from a lecture given by Professor Blakemore to mark the 375th anniversary of the Worshipful Company of Spectacle Makers on 14 September 2004.
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The Ancient and Medieval Periods
Our first visionary belongs to the Arab world. Avicenna (980-1037). That's him on the right. He overturned the common idea that somehow the eye emitted rays that bounced off an object being viewed, thus triggering some kind of perception. Avicenna determined that light rays go in to the eye and that the lens of the eye played in a role in the visual process. William Osler described Avicenna's book as the most important work ever...it was still used in medical schools beyond the late 14th century.
"it is not the ray that leaves the eye and meets the object that gives rise to vision, but rather the form of perceived object passes into the eye and is transmuted by the transparent body, that is, the lens."
It is to Galen (c.130-200) that we owe the concept of visual disparity - that the two eyes see differently.
Galen wrote a description of the eyes in the 10th chapter of his work entitled On the Use of the Parts of the Human Body. The story goes that he had intended to omit this chapter because it would contain too many difficult mathematical formulae, but he had a dream which persuaded him to keep it in. Galen was a great anatomist. In the 150s AD he had studied in Alexandria where bodily dissection was allowed. From Galen we all derive our use of many anatomical terms for the parts of the eye such as the retina or the crystalline lens.
Reisch's cell theory (though anatomically inaccurate) recognised that a mechanistic process was at work. In 1503 he suggested that images passed through a hierarchical chain of cells inside the brain. You can see this chain in the diagram to the right. The eyes input information to the first set of cells, the sensus commensis. This information was then relayed to the second and third set of cells for progressively higher thought processing.
The Early Modern Period
Leonardo da Vinci proposed a theory that the inverted retinal image was re-inverted inside the brain. Below you can see two of Leonardo's drawings, dating from 1490 and 1506 respectively. the third image demonstrates his theory of the re-inversion of an inverted retinal image.
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The apple diagram from Descartes' Traité de l'Homme,1664 |
The Seventeenth Century
One of my heroes, Rene Descartes (1596-1650) stripped the sclera and choroid off the eye of an ox to perform a projection experiment onto a piece of paper. This proved that the retina initially sees an inverted image.
This diagram on the right from Descartes' La Dioptrique (1637) shows the great man observing the retinal image.
Later, in his Traité de l'Homme, finished by 1648 but not published until 1664, after his death, Descartes thought of the brain as a machine with fibres running from the eyes like optical cables. These formed a rudimentary visual pathway. The inner soul within the brain then viewed the image transferred to it along the pathway by vibrational forces. You can see the two separate vibrating paths in his drawing on the left which shows the eyes looking at an apple and the image travelling to an artist's representation of the soul.
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| Drawing of the brain by Wren |
Did you Know? The ophthalmic branch of the fifth nerve is called Willis' Nerve.
In 1682 William Briggs (1650-1704) developed the work of Willis when he described optic pathways more thoroughly and hinted at the idea that the retinal nerve fibres from the two eyes may cross over at some point behind the eye. (Newton's Opticks, already written but not yet published, would confirm this in 1705). Briggs recognised that fusion occured in the optic tract, not the soul.
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| Thomas Willis | Diagram (left) of the visual pathway according to William Briggs (1682) bearing the first hint of an understanding of the decussation of fibres at the chiasma | Diagram (right) of the visual pathway according to Isaac Newton produced in 1682 but not publicised at the time. |
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The Eighteenth and Nineteenth Centuries
In 1738 John Taylor produced an improved and very influential pathway diagram.
The crossover of the fibres was now accepted.
The nineteenth century fascination with phrenology proved a false dawn for vision scientists. Amidst the largely discredited pseudo-science of men such as Franz Gall (1757-1828) there was a recognition that specific functions related to localised parts of the brain's cortex (neural localisation). Unfortunately this important point was thrown out with the bath water...a setback for our subject that lasted well into the next century.
Here is a comparison between a standard phrenological head and the head of Gall himself. Beside that we have superimposed a nineteenth century image of the brain showing the visual region marked towards the back of the head.
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The Twentieth Century
Lord Edgar Adrian (1889-1977) recorded sensory nerve impulses from the end of the 1920s and so gave us the notion of a pulse code with a frequency of transmission to the brain that can be measured.
Here are his recordings from the sensory nerves of a cat's toe (1929).
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| Horace Barlow |
In the second half of the 20th century Horace Barlow (b.1921) proved a key figure in interpreting how the eye detects distance in objects. This was research into stereoscopic vision a phenomenon first described by Sir Charles Wheatstone (1802-75). In the 1950s he also studied light and dark 'adaptation'. He described how certain animals learn from experience how to react to 'trigger factors' such a movement, light and outlines.
In the 1960s David Hubel and Torsten Wiesel (b.1924) took this work further when they recorded the visual cortexes of anaesthetised cats and monkeys using micro electrodes - their results showed that the cells in the visual cortex are clearly involved in the recognition of shape and form. The cells show 'selective response' to different visual stimuli.
Wiesel was awarded the 1981 Nobel prize in Medicine and Physiology. His Nobel Lecture was entitled 'The postnatal development of the visual cortex and influence of environment', (Published in Nature 299: 83-592, 1982).
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The pictures illustrate Huber and Wiesel's experiment with a cat's vision and the orientation selectivity results of a neuron in the primary visual cortex of a monkey (1968).
Wiesel recognised that covering one eye of a young animal could cause that eye to lose its connection to he visual cortex (a discovery which proved important to the study of amblyopia).
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Nowadays Functional Magnetic Resonance Imaging can record blood flow in human brains. This is the 3T MRI scanner in Oxford. Richard Gregory did some pioneering work in Cambridge on illusory contours... we have concluded that the human vision system does not work like a wet video camera. Rather, there are cognitive processes at work in addition to the mechanistic processes implemented by the nerves.
Here's an example using a familiar optical illusion:
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Ruben's Vase...What do you see?
Find out more about optical illusions here
Thanks to our visionaries we now know vision is an immensely complicated task - involving great brain capacity ... which is why it is proving so hard to replicate by computers.
We always think that we are almost there in the understanding of vision, but there will be many more visionaries to move our understanding further forward!