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Fig. 1.2. Spontaneous activity in the retina. Each of the frames shows calcium concentration imaging of approximately 1 mm² of newborn ferret retina; the plots are a measure of how active the retinal cells are. Light gray indicates areas of increased activity. This activity is spontaneous (internally generated), because the photoreceptors have not yet developed at this time. From left to right, the frames on the top row form a 4-second sequence showing the start and expansion of a wave of activity. The bottom row shows a similar wave 30 seconds later. Later chapters will show that this type of correlated activity can explain how orientation selectivity develops before eye opening. Reprinted with permission from Feller et al. (1996), copyright 1996 by the American Association for the Advancement of Science; gray scale reversed.