Vision without eyes

Vision is in the eye of the beholder, as the saying sort-of goes. But what about when the beholder has no eyes? As strange as it might sound, “eyes” in the traditional sense aren’t always required for vision. For many animals, vision doesn’t come from a pair of obvious eyeballs on their heads, afterall, not all animals even have a head. In this article, I’ll highlight a few animals that do vision in a very different way from us.

It’s important to understand that just as there are many types of eye, there are many types of vision; the word “vision” being used in the broadest sense of the word to mean the detection of light in some form. Broadly speaking there are four types of light detection, which vary in their level of sophistication (figure 1). The most basic form is non directional photoreception which, as the name suggests, just gives an animal an idea of whether their environment is bright or not. Non directional photoreception is used for tasks such as regulating circadian rhythms or mediating colour change in an animal. Next on the ladder is directional photoreception, which as well as detecting the presence of light, gives an animal an indication of the direction it is coming from. This is used for tasks such as finding shelter or controlling vertical migration within the water column. Neither directional nor non directional photoreception require the sophisticated organs that we would think of as eyes. Instead, these sensory tasks can be achieved with simple groups or stacks of photosensitive receptor cells distributed across an animal’s body. These basic forms of light detection are referred to as “photoreception” rather than vision.

The next two forms of light detection involve image formation and are generally what is meant when referring to “vision”. Low resolution spatial vision means that an animal can form an image, or picture, of their world in terms of the light available, albeit in a blurry, general-gist sort of way. Then finally, there is high resolution spatial vision of the type that humans, most mammals and many other animals enjoy. The type of eyes required for spatial vision are more sophisticated than the simple light detectors required for photoreception as spatial vision is far more complicated. It requires that the eye pinpoints the different sources of light in a scene more accurately than just their vague direction. To do this, an eye must effectively be subdivided into smaller units, each of which has its own unique field of view. An image is then formed by connecting together the outputs of this mosaic of smaller light detectors. It’s very similar, in principle, to an image from a digital camera that is made up of individual pixels. Take a pixel on its own, and you can tell if it is light or not (non directional photoreception), take a few pixels scattered about and you might be able to tell which direction the light is coming from (directional photoreception), but to form an actual image you need a vast array of pixels. Even then, to get a really good (i.e. high resolution) image, you need to collect so much light that additional optics, such as a lens, are required.