Visual acuity is the eye's ability to distinguish two points that are very close to each other. This ability depends on many factors, but especially on the precision of the eye's refraction and the ratio of cones to rods at a given location on the retina. |
Your eye does exactly the same thing, all day long, without your even being aware of it! Your cornea and lens provide the focus, while the iris adjusts to let the optimal amount of light reach your retina. But your retina, with its many layers of neurons, is far more complex and sensitive than any photographic film. The two are similar, however, in that the image focused on both of them is inverted.
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Brodmann Areas Seeing without knowing it : the strange phenomenon of blindsight People whose primary visual cortexes have been damaged consider themselves to be blind and unable to discern anything in their visual environment. But if you ask these people to "take a chance" and point their finger at a dot of light in space, they will point straight at this target. And the data show that this result is not random. This phenomenon is called blindsight. Thus these people are still processing some visual information, even though part of the neural pathways in V1 have been destroyed. The mechanisms by which they do so may involve little understood transfer pathways that bypass V1, as well as certain subcortical visual nuclei. Some researchers also believe that the dorsal visual pathway plays a role in this phenomenon.
| The image captured by each eye is transmitted to the brain by the optic nerve. This nerve terminates on the cells of the lateral geniculate nucleus, the first relay in the brain's visual pathways. The cells of the lateral geniculate nucleus then project to their main target, the primary visual cortex. It is in the primary visual cortex that the brain begins to reconstitute the image from the receptive fields of the cells of the retina. Also known as the striate cortex, or simply V1, the primary visual cortex is located in the most posterior portion of the brain's occipital lobe . In fact, a large part of the primary visual cortex cannot be seen from the outside of the brain, because this cortex lies on either side of the calcarine fissure. This fissure, however, is clearly visible in a sagittal section made between the two cerebral hemispheres. The primary visual cortex, with its distinctive cell architecture, also corresponds to Area 17 described by the anatomist Brodmann in the early 20th century [link to Tool module from the sidebar to the left]. The primary visual cortex sends a large proportion of its connections to the secondary visual cortex [V2], which consists of Brodmann's areas 18 and 19. Though most of the neurons in the secondary visual cortex have properties similar to those of the neurons in the primary visual cortex, many others have the distinctive trait of responding to far more complex shapes.
Similarly to the other sensory systems and the motor system, there is a correspondence or "mapping" between the arrangement of the elements of the visual field as they strike the retina and their arrangement on the surface of the visual cortex. This mapping onto the visual cortex is called retinotopy, because it is the retina that serves as the reference for the cortical maps of the various visual areas.
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