The eye’s retina does more than register images the way film or a digital camera detector does. To allow it to begin analyzing an image, the retina has specialized nerve cells that respond to motion or other important features in the image detected by the light-sensing rod and cone cells.
Scientists discovered the specialized cells that sense motion in the retinas of cats and other mammals more than 40 years ago, but efforts to find similar cells in primate retinas had been unsuccessful. Now, a team of scientists has found motion-sensitive nerve cells in the retinas of macaque monkeys by using a grid of 512 microscopic electrodes to measure how the nerves respond to various patterns of light.
“If you poke around [the retina] with single electrodes, which is the traditional approach in neuroscience, you’re not going to find very many of these [cells],” says Alan M. Litke of the University of California, Santa Cruz. There’s only about one motion-sensing cell, called an upsilon cell, for every 10,000 rods and cones, he says.
This scarcity is the main reason that upsilon cells have been so difficult to discover. A single cell apparently behaving like a cat’s motion-sensing cell could have been the result of experimental error or abnormal behavior rather than evidence of a new cell type. Litke and his colleagues developed an array of electrodes affixed to a glass plate that covered 1.7 square millimeters of the retina. This relatively large area turned out to encompass up to 10 upsilon cells, which provided enough data for the scientists to unambiguously document the cells’ behaviors.
Upsilon cells and detectors of other image features, such as changes in brightness, constitute the forward layer of the retina’s three-tiered structure. Various types of relay cells make up the middle layer, and the back layer contains the rods and cones.
The team used a microscope lens to project video images onto retinas that had been removed from macaques euthanized in HIV-related experiments. One of these movies consisted of a pattern of bright and dark stripes that alternated over time but had a constant overall brightness.
Each feature-recognition cell presides over a small patch of the retina, receiving signals from the 100 or so light-sensing cells in its vicinity and combining these signals to generate an overall response for the region. In the team’s experiments, most feature-recognition cells didn’t react to the changes in the stripe pattern because those cells respond only to the overall brightness in their regions. However, the upsilon cells did respond each time the stripe pattern changed, a defining characteristic of motion-sensing cells in other mammals, the team reports in the Oct. 10 Journal of Neuroscience.
“They’ve provided pretty convincing evidence that these cells are similar to the cells in cats,” comments Jonathan Demb, a retinal-cell expert at the University of Michigan in Ann Arbor.