Image Processing in the Eye: Like "Magic" Evolution News & Views January 16, 2014 5:33 The amount of image processing going on in the eyeball is astounding. Did you know that the signal from your retina splits into some 20 channels that analyze the image before it reaches the brain? A pair of German neuroscientists, writing in Current Biology, describe as "magic" how two of those channels work at the neural level. They begin by commenting on how we enjoy the best of both design constraints: The visual system of primates, including that of humans, famously features both exquisite spatial acuity and a high temporal resolution. This dual focus on both 'sharpness' and 'speed' is made possible through different processing streams set up already in the retina. In a recent study, Puthussery et al. now show that key differences in the processing streams that are thought to underlie these visual abilities are already set up right after the first synapse of the visual system -- in retinal bipolar cells.
The retina breaks the visual world into several parallel representations prior to transmission to the brain. Each representation, or 'channel', is based on a different type of retinal ganglion cell that carries information about specific features of the visual scene -- such as edges, directed motion or 'color'. Of the 20 or so types of ganglion cells that exist in the primate retina, two in particular have attracted considerable attention since they were first described in the 1940s: the parasol and midget cells. (Emphasis added.)
The authors go on to describe how the "parasol ganglion cells" are responsible for "fast, low acuity" while the "midget ganglion cells" provide "slow, high acuity." What makes the difference? At least three factors are involved: (1) the way they are wired in parallel with the bipolar cells (between the photoreceptors and the amacrine cells), (2) the way they are wired in series with the amacrine cells (between the bipolar cells and the ganglion cells), and (3) the waveforms of the electrical output of the bipolar cells.