Particle-physics technology meets the eye

The retina is the layered, paper-thin “wallpaper” on the back of the eye that senses incoming light using the rods and cones of the input layer and then electrically encodes that information in the middle layers. The output layer of the retina has ganglion cells (about 1 for every 100 rods and cones) to gather and collate the encoded electrical signals and send them through the optic nerve to the brain. Although about 22 different morphological types of retinal ganglions are currently known to exist in primates, biologists know in detail how only a handful of them respond to visual images. Enter Alan Litke (University of California, Santa Cruz) and his familiarity with the silicon-microstrip detector technology used in experiments at SLAC and CERN. Along with circuit designer Władysław Dabrowski (AGH University of Science and Technology, Krakow, Poland) and other colleagues, Litke developed a detector array (see the figure) that packs 512 microelectrodes into a mere 1.7 mm². In collaboration with E. J. Chichilnisky at the Salk Institute in La Jolla, California, the team began to study the retina in monkeys. The scientists have now identified and characterized the behavior of a new ganglion cell type that they call the upsilon cell. For each of several in vitro preparations, they focused dynamic visual stimuli onto the retina and simultaneously recorded the electrical activity of about 250 ganglion cells, 5–10 of which turned out to be upsilons. The upsilons’ sparseness was offset by their large area of sensitivity; their collective detection mosaic covers the retina nicely. The researchers also found that the new cell type sums signals nonlinearly and responds sharply to highly transient stimuli; that finding led the team to speculate that upsilons are important for the visual detection of motion. (D. Petrusca et al., J. Neurosci. 27 , 11019, 2007 .)