It is very difficult to carry out precise science experiments in complete darkness, so the team developed a unique repertoire of state-of-the-art techniques. They had to design ways to measure electrical signals originating from single photons through the neural tissue of the eye – the retina – and linked these signals to mouse behavior in the maze. One of the breakthroughs is that the team can track mice in the dark using night-vision cameras and their deep-learning based software so accurately that they are able to predict with unprecedented resolution where photons land on each mouse’s retinas.
The light the mouse was trying to find was made dimmer each time, to the point that in the last few attempts only a few photons at a time were entering the mouse’s eyes.
The team compared two types of mice. The first group of mice that did the task were ordinary laboratory mice. The second group had been genetically modified so that their most sensitive ON channel needs 10 times more light to send a spike train than the most sensitive OFF channel. These modified mice turned out to be 10 times worse at seeing the light than their unmodified cousins. Therefore, the researchers were able to prove their important discovery: individual spike trains going through the ON channel were responsible for the mouse seeing the light.
Result relevant to all neuroscientists studying perception
This result is the first time anyone has linked visual behavior with this resolution to precise spike-codes coming from the retina. “This is like trying to translate a language,” Professor Petri Ala-Laurila explains. “Previously we were using a phrasebook: we knew what whole sentences meant but not the meaning of individual words. Now that we can link precise codes consisting of individual nerve impulses to behavior, we are getting closer to understanding individual ‘words’.”
The result is highly relevant to researchers working on vision, but also broadly relevant to all neuroscientists working on perception, because of a surprising aspect of the result that overturned previously held beliefs in neurology. For 70 years, researchers have been using information theory to model how the brain handles different signals. One of the assumptions was that if the brain has to choose between two competing codes, it will rely on the signal that contains more information. In the case of the ON and OFF channels in vision in the genetically modified mice, the ON channel – which the team showed was key in controlling behavior – contains less information. The ON channel increases the amount of nerve impulses it sends to the brain when it detects photons, whereas the OFF channel decreases its impulse rate, and the researchers show that behaviour relies only on messages that are encoded in increased impulse rate rather than decreased impulse rate. “This discovery is really exciting for all of neuroscience because it’s experimental proof of the brain prioritizing information encoded in spikes rather than in the absence of spikes” says Lina Smeds, the PhD student at University of Helsinki who is first author of the paper.