{"id":129,"date":"2009-10-26T14:44:37","date_gmt":"2009-10-26T14:44:37","guid":{"rendered":"http:\/\/simoningsmirror.wordpress.com\/2009\/10\/26\/vision-in-the-womb"},"modified":"2009-10-26T14:44:37","modified_gmt":"2009-10-26T14:44:37","slug":"vision-in-the-womb-2","status":"publish","type":"post","link":"http:\/\/www.simonings.net\/?p=129","title":{"rendered":"Vision in the womb"},"content":{"rendered":"
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\n\"Rodsandcones1\"\n<\/div>\n<\/div>\n

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By April, Natalie had begun, now and again, to sleep. Every so often she closed her lids and kept them shut, and when she did, her eyes trembled, stirring the oxygen-rich liquid, called aquaeous humour, that lies between the iris and the cornea. (The rapid-eye movement that accompanies dreams has at least one very practical purpose: it feeds the front of the eye.(1)) This trembling woke the light-sensitive cells of Natalie\u2019s retinas. Stimulated, her retinal cells fired at random, preserving and strengthening their connections with each other.(2) Even before she saw, Natalie went through the motions of dreaming, and those motions taught the cells in her retina to hold hands.<\/div>\n
Prepared by dreams, Natalie\u2019s retinal cells took their next lessons from light. Even before birth, the body is no stranger to illumination. Flesh itself lights up a little, every time a nerve fires(3). Perhaps this familiarity with light is why any young nerve cell, transported to the retina \u2013 the body\u2019s most light-sensitive surface \u2013 will learn to see, just as every seeing cell, moved elsewhere, becomes an ordinary nerve(4).<\/div>\n
The womb is not dark: it is easily penetrated by light from the outside world. From November to July of 2003, the month of her birth, Natalie\u2019s retinas grew to seize what news they could from the amniotic murk of her home. They adapted to darkness and to blur. One layer of nerves grew into light-sensitive cells called rods, the better to gather the light. At birth, Natalie was well on the way to acquiring good nocturnal vision. Babies see well at night.<\/div>\n
In the glare of day, though, they are all but blind. (It is one of the ironies of birth that it fills our world with light \u2013 and blinds us in the process.) A sunny day is a million times brighter than a night-time nursery, and a whole other form of vision is needed to handle such a glare \u2013 a form of vision Natalie had not yet got.<\/div>\n
Exposed to the light, Natalie\u2019s eyes had not so much to \u2018adapt\u2019 to the brightness of day, so much as acquire a whole new way of seeing.<\/div>\n
In her retinas lay another set of nerve cells, distinct from the rods and only distantly related to them.(5) At birth, they were little more than ordinary nerve cells. Once exposed to the glare of day, however, they began to change. Natalie will be six before these \u2018cones\u2019 of hers are fully grown, packed as tight as they can be into the fovea \u2013 that tiny circle on the retina where images are focused and light explodes with colour.<\/div>\n

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(1)Maurice DM. 1998. \u2018An ophthalmological explanation of REM sleep.\u2019 Experimental Eye Research 66 pp139-145. This article<\/a> includes some interesting background on Maurice’s work.<\/div>\n
(2)Siegel JM. 2005.<\/a> \u2018Functional Implications of Sleep Development.\u2019 PLoS Biology 3\/5 p178<\/div>\n
(3)Tarusov, BN., Polivoda, AI., Zhuravlev, AI. 1961. \u2018Study of the faint spontaneous luminescence of animal cells.\u2019 Biophysics 6 pp83-85. (This paper is one of the inspirations behind space physiologist Karl Simanonok<\/a>‘s diverting ‘endogenous light theory of conciousness’.)<\/div>\n
(4)Cepko, C. Interview<\/a> with Norman Swan on The health report: the retina and the brain, ABC Radio National, Australian Broadcasting Commission, Monday 13 December 1999.
http:\/\/www.abc.net.au\/rn\/talks\/<\/a>
8.30\/helthrpt\/stories\/s73272.htm<\/div>\n
Livesey FJ, Cepko CL. 2001<\/a>. \u2018Vertebrate neural cell-fate determination: lessons from the retina.\u2019 Nature reviews: neuroscience 2\/2 pp109-18. (PDF)<\/div>\n
(5) The distinction between rods and cones had already arisen in the eyes of jawless fish, swimming in Devonian seas around 400 million years ago. See Bowmaker, J. K. 1991. \u2018Evolution of photoreceptors and visual pigments.\u2019 Pp. 63\u201381 in J. R. Cronly-Dillon and R. L. Gregory, eds. Evolution of the eye and visual pigments. CRC Press, Boca Raton, Fla.<\/div>\n","protected":false},"excerpt":{"rendered":"

By April, Natalie had begun, now and again, to sleep. Every so often she closed her lids and kept them shut, and when she did, her eyes trembled, stirring the oxygen-rich liquid, called aquaeous humour, that lies between the iris … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[64,100,115],"tags":[],"class_list":["post-129","post","type-post","status-publish","format-standard","hentry","category-more-on-the-eye","category-the-eye","category-115"],"_links":{"self":[{"href":"http:\/\/www.simonings.net\/index.php?rest_route=\/wp\/v2\/posts\/129","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.simonings.net\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.simonings.net\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.simonings.net\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.simonings.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=129"}],"version-history":[{"count":0,"href":"http:\/\/www.simonings.net\/index.php?rest_route=\/wp\/v2\/posts\/129\/revisions"}],"wp:attachment":[{"href":"http:\/\/www.simonings.net\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=129"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.simonings.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=129"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.simonings.net\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=129"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}