How Much the Eye Tells the Brain

In the classic "What the frog's eye tells the frog's brain," Lettvin and colleagues [1] showed that different types of retinal ganglion cell send specific kinds of information. For example, one type responds best to a dark, convex form moving centripetally (a fly). Here we consider a complementary question: how much information does the retina send and how is it apportioned among different cell types? Recording from guinea pig retina on a multi-electrode array and presenting various types of motion in natural scenes, we measured information rates for seven types of ganglion cell. Mean rates varied across cell types (6-13 bits · s^-1) more than across stimuli. Sluggish cells transmitted information at lower rates than brisk cells, but because of trade-offs between noise and temporal correlation, all types had the same coding efficiency. Calculating the proportions of each cell type from receptive field size and coverage factor, we conclude (assuming independence) that the approximately 10⁵ ganglion cells transmit on the order of 875,000 bits · s^-1. Because sluggish cells are equally efficient but more numerous, they account for most of the information. With approximately 10⁶ ganglion cells, the human retina would transmit data at roughly the rate of an Ethernet connection.