Researchers from Duke University suggested that direction-selective ganglion cells in retina assist mammals in dark conditions
Mammalian retinas can respond to minute numbers of photons. A light-sensitive cell known as a rod cell in the retina is activated by a single photon. The cell sends a minute electrical signal to the brain through a ganglion cell. Some of these Direction-Selective Ganglion Cells (DSGCs) get excited only when an object is moving upward, whereas others are exited only when objects are moving down, or to the left or right. The researchers at Duke University examined slices of mouse retinas by laying them on tiny glass plates embedded with an electrode array, to identify the adoption mechanism of DSGCs dark conditions. The mouse retinas, bathed in an oxygenated solution, can still function while the array records electrical activity from hundreds of neurons.
In an experiment, the researchers showed the dissected retinas a simple movie that included bands moving across a contrasting background. The light was turned down to a factor of 10,000 that resembled moonlit scene. The researchers observed that three of the four directional DSGCs did not move in their response to the motion when the lights went down. However, the fourth type that generally responds to upward motion, responded to a much broader range of motion that included down and sideways.
The researchers analyzed the unfamiliar behavior of the ‘up’ cells by using a computer model of all four directional cells’ activity. The team concluded that the ‘up’ cells improved the performance of the group as a whole as they sacrificed some of their preference for one direction. This boosted DSGCs’ ability to detect motion in low light. The mice were genetically engineered to lack intracellular connections called gap junctions in their upsensing neurons, to determine the switch in function of ‘up cells’. These protein channels facilitate the transfer of chemical signals from one neuron to another. Moreover, these proteins have previously been linked to night vision. The team found that in retinal tissue of engineered mice, upsensing cells did not adapt to the dark. This suggested that at least some of the ability of ‘up’ cells to boost motion detection in low light depends on gap junctions.