Is it possible to perceive shadows in the dark? The answer, for some at least, appears to be ‘yes’. A study has pinpointed a hitherto unknown dedicated neural circuit in the retina that can detect shadows even under conditions of near-complete darkness
The research (a joint project between Aalto University and the University of Helsinki) is based on studies using rodents, assessing the ability of mice to register very dim shadows. This led to the identification of a retinal pathway enables mice to have this ability.
What is of wider interest is that this ability to reach the limit of what’s is seemingly physically possible, as neural circuit, exists in the human eye. The data may lead to helping medics in assessing visual diseases.
To undertake the research into shadow detection, the scientists placed mice in a maze with nearly no light. The exit to the maze was marked by a black spot. The spot was just barely distinct from the surrounding darkness.
The researchers looked at how the mice moved through the maze and measuring the activity of neurons at the back of the eye (the retina). Further examination detected a group of retinal cells known as OFF ganglion cells that are responsible for the shadow perception ability.
Assessing the sensitivity of the visual system to the dimmest shadows required high technical demands to ensure the experiments were able to accurately measure responses in low light levels.
Related ON ganglion cells had previously been found to detect a very faint patch of light in darkness. OFF ganglion cells have now been confirmed as carrying out the opposite task – detecting the dimmest shadows, where just a few photons are missing.
Later the researchers calculated the fundamental limit of shadow detection based on the physical properties of the light receptors and neural pathways. This affirmed the sensitive OFF ganglion cells as representing perfect shadow detectors.
From the perspective of evolutionary biology, mice would have developed this need to detect shadows in order to avoid predators at really low light levels.
Following on from the studies with mice, the researchers will seek to detect diseases of the human eye earlier and with greater precision.
The research appears in the journal Current Biology, titled “Retinal OFF ganglion cells allow detection of quantal shadows at starlight.”
