The ON-OFF Switch

If you were placed in a maze shrouded in near-darkness, with only a black spot signposting the exit, would you be able to escape? This was the task a group of researchers used to evaluate mice on their ability to detect quantal – or extremely dim – shadows (1). Shadow detection may first have been a way for animals to avoid predators at extremely low light levels; though evading prehistoric beasts may no longer be a worry, understanding the processes at work may help us to detect and avert other dangers, such as retinal diseases, instead.

“My lab has been fascinated by the possibility to use extremely low light levels to bridge retinal circuit function to visually guided behavior. Darkness and low light levels critically simplify this extremely difficult task, since only the most sensitive retinal pathways and the most sensitive retinal outputs, retinal ganglion cell types, can contribute to vision,” says research leader Petri Ala-Laurila, Associate Professor of Biophysics at Aalto University in Espoo, Finland. “Our excitement to test quantal shadows as a visual stimulus for the retina and visually guided behavior was motivated by our previous study (2), where we showed that the retinal ON pathway and not the OFF pathway drives behavioral detection of the dimmest light increments in darkness. This happened even when the OFF pathway would have allowed higher sensitivity. As a result, we were questioning what the functional role of the OFF pathway was, hypothesizing that it may be a shadow detector at extremely low light levels.”

Testing this hypothesis required near-complete darkness, which came with a unique set of challenges that forced them to adjust both their mindsets and lab culture. Any exposure to extra light could compromise their results – so retina dissection, electrophysiology, and behavioral work all took place in complete darkness, guided by night vision. Additionally, the researchers needed to know the precise rhodopsin activations required to prevent infrared light exposure (used for visualization) from rising above the intrinsic noise generated by the retinal photoreceptors. Despite these challenges, the team successfully identified the OFF pathway as responsible for shadow detection – and demonstrated its ability to identify shadows as dim as a couple of photons missing from a few thousand rod receptors. Combined with their previous results, Ala-Laurila and his team concluded that the ON and OFF outputs of the retinal rod bipolar pathway mediate behavioral detection of the weakest lights and shadows, respectively.

The ability to correlate performance on a visual task with not just the responsible circuit, but also the specific output, may offer new ways to diagnose retinal diseases. Ala-Laurila explains, “It opens up new doors for tailored psychophysics stimuli dedicated to looking at healthy and diseased visual systems in a cell-specific manner. Single-photon and quantal-shadow behavioral detection paradigms could offer a unique avenue for probing ON and OFF channel function in healthy and diseased visual systems. We are now working on healthy and diseased human patients in collaboration with others to test these visions and are very optimistic.”

But this is not the only step forward Ala-Laurila anticipates. “We currently have several exciting research directions relying on extremely low light levels where we are working across scales, from photons to retinal codes all the way to behavior. We are using various mouse models, as well as healthy and diseased human models, in connection with electrophysiological studies on the primate retina. The development of markerless tracking of mouse behavior in extremely low light levels was the motivation for founding a company called Quantal Vision Technologies, which will take the lead in providing state-of-the-art behavioral tools for mouse and hopefully later also human behavioral studies.”