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readerL15 - Visual System I The Eye(1 Insects and other...

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(B) Shown is a scene that a fl y might look at a piece of pizza. Indicated on this scene are the views seen by three of the fl y’s ommatidia. As you can tell, this is a very confusing view of the world. (C) The fl y’s eye sees a fragmentary map, but the fl y’s brain perceives a normal picture. In order to make sense of this confusing view, the fl y’s nervous system connects up receptor cells within several ommatidia which are looking at the same part of the visual scene, that is, cells with the same receptive fi elds . Using this method the fl y’s brain constructs a congruent map of the world out of the fragmentary map of the ommatidia.The fl y’s eye sees a fragmentary map , but the fl y’s brain perceives a normal picture . Visual System I The Eye (1) Insects and other arthropods have compound eyes (A) Compound eyes are made up of many ommatidia which are small individual retinas. Each ommatidium is looking at a slightly different but overlapping region of visual space. Page 15-1
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(2) Human eyes (A) We have two eyes (rather than hundreds of ommatidia) which each give us a different view of the world. Therefore our brain does need to be able to put together the images from our two retinas into a congruent whole so that we do not see double. In the process of merging the two eyes’ views of the world, our brains also use the differences between the views from our two eyes to calculate depth . (B) Notice that because of the optics of lenses objects are projected upside down onto our retina. Our brain is very good at inverting images . (C) Students wearing glasses which invert the visual world saw everything upside-down for a few days, then suddenly saw everything normally again. After taking the glasses off, everything was again upside-down! (D) Our brains have one more problem interpreting the eye’s view of the world. The optic nerve creates a hole (an area lacking photoreceptors) in the retina, and therefore a hole in the eye’s view of the world. One solution to this problem is to put the optic nerve in a different part of the visual fi eld in the two eyes , so that where one eye has a hole in its view, the other eye does not. However, this solution would still mean that we should have a blank spot in our view of the world when we close one eye . We do! Look
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