There’s no reason light detection has to be limited to the eyes, as these animals demonstrate.
We humans are uncommonly visual creatures. And those of us endowed with normal sight are used to thinking of our eyes as vital to how we experience the world.
Vision is an advanced form of photoreception – that is, light sensing. But we also experience other more rudimentary forms of photoreception in our daily lives. We all know, for instance, the delight of perceiving the warm sun on our skin, in this case using heat as a substitute for light. No eyes or even special photoreceptor cells are necessary.
But scientists have discovered in recent decades that many animals – including human beings – do have specialized light-detecting molecules in unexpected places, outside of the eyes. These “extraocular photoreceptors” are usually found in the central nervous system or in the skin, but also frequently in internal organs. What are light-sensing molecules doing in places beyond the eyes?
Vision depends on detecting light
All the visual cells identified in animals detect light using a single family of proteins, called the opsins. These proteins grab a light-sensitive molecule – derived from vitamin A – that changes its structure when exposed to light. The opsin in turn changes its own shape and turns on signaling pathways in photoreceptor cells that ultimately send a message to the brain that light has been detected.
Most of our conscious vision stems from photoreceptors in the retina, the light-sensitive layer at the back of our eyeball. In animals with backbones (vertebrates), cells that detect light for vision are vaguely shaped like rods or cones, giving them their familiar names.
We’ve known for a while that other vertebrates have additional photoreceptors in their brains. But scientists had long thought that rods and cones were pretty much the whole story of mammalian vision. Thus, the discovery in the early 2000s by David Berson’s group at Brown University of other cells in a mouse retina that respond to light came as a shock.
Even stranger were associated discoveries in many laboratories demonstrating that these cells contained a new class of opsin proteins called the melanopsins, never before seen in vertebrates (but similar to those of many invertebrates). They seem not to be involved in conscious vision.
We can hardly call them extraocular since they’re right there in the eye. Instead they’re often referred to as “nonvisual” photoreceptors. That’s the term researchers use for all animal photoreceptors that aren’t associated with imaging pathways in nervous systems.
So now we know there are nonvisual photoreceptors in the eyes themselves in many – perhaps most – animals. Where else can we find them throughout body?