tailieunhanh - EYE BRAIN AND VISION - PART 4

Số có thể giúp chuyển tải một ấn tượng về sự rộng lớn của cấu trúc này so với geniculate, trong đó có 1,5 triệu tế bào, vỏ não striate chứa một cái gì đó như 200 triệu tế bào. Cấu trúc của nó là phức tạp và hấp dẫn, nhưng chúng ta không cần biết các chi tiết để đánh giá cao này là một phần của bộ não | This Golgi-stained section of the primary visual cortex shows over a dozen pyramidal cells still just a tiny fraction of the total number in such a section. The height of the section is about 1 millimeter. The long trunk near the right edge is a blood vessel. The primary visual or striate cortex is a plate of cells 2 millimeters thick with a surface area of a few square inches. Numbers may help to convey an impression of the vastness of this structure compared with the geniculate which has million cells the striate cortex contains something like 200 million cells. Its structure is intricate and fascinating but we don t need to know the details to appreciate how this part of the brain transforms the incoming visual information. We will look at the anatomy more closely when I discuss functional architecture in the next chapter. I have already mentioned that the flow of information in the cortex takes place over several loosely defined stages. At the first stage most cells respond like geniculate cells. Their receptive fields have circular symmetry which means that a line or edge produces the same response regardless of how it is oriented. The tiny closely packed cells at this stage are not easy to record from and it is still unclear whether their responses differ at all from the responses of geniculate cells just as it is unclear whether the responses of retinal ganglion cells and geniculate cells differ. The complexity of the histology the microscopic anatomy of both geniculate and cortex certainly leads you to expect differences if you compare the right things but it can be hard to know just what the right things are. This point is even more important when it comes to the responses of the cells at the next stage in the cortex which presumably get their input from the center-surround cortical cells in the first stage. At first it was not at all easy to know what these second-stage cells responded to. By the late 1950s very few scientists had attempted to record