Early light refines the brain’s circuitry for vision
To assess the effect of light on spike bursts, the researchers used electrodes to record the activity of cells in the inner retinas of newborn mice, first recording in the dark, then in the light, and then again in the dark. In every case, when the retinas were exposed to light, the bursts of spikes lasted about 50 percent longer.
They then tested whether light-sensitive intrinsically photosensitive retinal ganglion cells (ipRGCs) in the inner retina were creating this effect, by repeating the study in “knock-out” mice in which the ability of ipRGCs to sense light had been genetically abolished. With the cells disabled, exposure to light no longer made any difference in the duration of the spike bursts.
The researchers also worked with live tadpoles. They flooded the tadpoles’ tectal neurons with a molecule that makes calcium ions fluoresce. As whole networks of neurons became active, they’d take in the ions and glow. The researchers recorded the tadpoles with a high-resolution, high-speed camera that could capture the millisecond-to-millisecond activity of the neurons.
They found over the course of several experiments that the neural networks in the tectums of tadpoles reared under normal conditions developed a faster, more cohesive, and stronger response (in terms of the number of neurons) to light. However, the tectal neural networks of tadpoles kept in the dark during development failed to progress at all.
The researchers said this evidence indicates that genes provide only a rough wiring plan and then leave it to the developing nervous system to finish work on it own.
Ref.: Jordan M Renna, Shijun Weng, David M Berson, Light acts through melanopsin to alter retinal waves and segregation of retinogeniculate afferents, Nature Neuroscience, 2011; [DOI: 10.1038/nn.2845]