Why the eye is better than a camera
May 4, 2011
The retina in vertebrates is lined with a sheet of photoreceptor cells: the cones for day vision and the rods for night vision.
The researchers demonstrated that while rods and cones (light-sensitive nerve cells) in the retina exhibit two types of feedback: they inhibit dozens of their close neighbors to sharpen edges, they also boost the response of the nearest one or two nerve cells.
That extra boost preserves the information in individual light detecting cells (rods and cones) thereby retaining faint detail while still enhancing contrast.
The two types of feedback work by different mechanisms, the researchers found.
The horizontal cells undergo an electrical change when they receive neurotransmitter signals from the photoreceptors, and this negative-feedback voltage change quickly propagates throughout the cell, affecting dozens of nearby photoreceptors to lower their release of the glutamate neurotransmitter.
The positive feedback, however, involves chemical signaling instead of electrical. When a horizontal cell receives glutamate from a photoreceptor, calcium ions flow into the horizontal cell. These ions trigger the horizontal cell to “talk back” to the photoreceptor, the researchers said.
Because calcium doesn’t spread very far within the horizontal cell, the positive feedback signal stays local, affecting only one or two nearby photoreceptors.
In a camera, the sensor contains millions of photosensitive diodes (corresponding to pixels) that behave identically.
Ref: Skyler L. Jackman, Norbert Babai, James J. Chambers, Wallace B. Thoreson, Richard H. Kramer, A Positive Feedback Synapse from Retinal Horizontal Cells to Cone Photoreceptors, PLoS Biology, 2011; 9 (5): e1001057 DOI: 10.1371/journal.pbio.1001057 (open access)