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EYE BRAIN AND VISION - PART 3

Bình luận thứ hai của tôi liên quan đến câu hỏi quan trọng của những gì một dân số của các tế bào như các tế bào sản lượng của võng mạc, làm phản ứng với ánh sáng. Để hiểu những gì các tế bào hạch, hoặc bất kỳ tế bào khác trong một hệ thống cảm giác đang làm | My second comment concerns the important question of what a population of cells such as the output cells of the retina are doing in response to light. To understand what ganglion cells or any other cells in a sensory system are doing we have to go at the problem in two ways. By mapping the receptive field we ask how we need to stimulate to make one cell respond. But we also want to know how some particular retinal stimulus affects the entire population of ganglion cells. To answer the second question we need to begin by asking what two neighboring ganglion cells sitting side by side in the retina have in common. The description I have given so far of ganglion-cell receptive fields could mislead you into thinking of them as forming a mosaic of nonoverlapping little circles on the retina like the tiles on a bathroom floor. Neighboring retinal ganglion cells in fact receive their inputs from richly overlapping and usually only slightly different arrays of receptors as shown in the diagram on this page. This is the equivalent of saying that the receptive fields almost completely overlap You can see why by glancing at the simplified circuit in the diagram above the cell colored purple and the one colored blue receive inputs from the overlapping regions shown in cross section of the appropriate colors. Because of divergence in which one cell makes synapses with many others at each stage one receptor can influence hundreds or thousands of ganglion cells. It will contribute to the receptive-field centers of some cells and to the surrounds of others. It will excite some cells through their centers if they are on-center cells and through their surrounds if they are off-center cells and it will similarly inhibit others through their centers or surrounds. Thus a small spot shining on the retina can stir up a lot of activity in many cells. DIMENSIONS OF RECEPTIVE FIELDS One millimeter on the retina corresponds to 3.5 degrees of visual angle. On a screen1.5 meters away 1 .