Perspective on perception
The vast majority of the posts which have appeared on this blog have been illustrated with my own photographs. In this particular instance, however, I have strayed from this convention and borrowed a scanning electron micrograph (of rods, in green, and cones) collected by Chris Ing and presented at Fresh Photons. Although I am not a neuroscientist I have always had an interest in perception. In particular I have enjoyed thinking about how our visual system is able to sense and then represent the world to us. I have come to appreciate our own remarkable sense of vision through my understanding of and appreciation for the myriad other ways in which animals, and plants for that matter, perceive the disparate environments within which they find themselves. One thing that many systems of perception have in common is their reliance on electromagnetic radiation. Perhaps you remember learning about the electromagnetic spectrum which describes various forms of electromagnetic radiation such as X-rays (with high energy and a short wavelength) and radio waves (with low energy and a long wavelength). The visible spectrum, within which we see our world, ranges in wavelength from 400 – 750 nanometers corresponding to violet and red light respectively. Other organisms have specialized senses which utilize other parts of the spectrum, consider the honey bee which is able to sense its world in the ultraviolet (radiation below 400 nm) while snakes utilize energies in the infrared (energies above 750 nm) to sense their prey, for example. And what about the semantics of all of this? What can it mean to say that a snake sees with its tongue? By the same token, what do we mean when we say that an electric eel is able to visualize its environment using electrical impulses? And, when we answer questions like this must we then consider what it is we mean when we say that we, as humans, see our world using our eyes? Consider the somewhat metaphysical issue of what it means to see. Each of our eyes is comprised, in part, by something called a retina which itself is made up of smaller components called rods and cones which are sensitive to shades (of black and white) and to colors, respectively. These elements are able to absorb photos of light and transform the energies contained into chemical signals which are then transmitted to the brain. It is the remarkable interpretation of these signals that is our sense of sight. But the visual system is not unusual in this way for all senses work in a similar manner. Our sense of smell is generated by the brain’s interpretation of electrical signals sent to it by sensory cells in the nose. Our sense of touch relies upon signals from receptors in the skin. And there is always the potential for alternate interpretation of the incoming signals. Have you ever wondered whether other folks see the world as you do? Before I end I would like to quote from someone whom I admire greatly. A zoologist, a thinker, and someone who has never been afraid to speak out in support of reason, Richard Dawkins. I quote from his classic text The Blind Watchmaker. I can imagine some other world in which a conference of learned, and totally blind, bat-like creatures is flabbergasted to be told of animals called humans that are actually capable of using the newly discovered inaudible rays called ‘light,’ for finding their way about. These otherwise humble humans are almost totally deaf (they can hear after a fashion and even utter a few ponderously slow, deep drawling growls, but they can only use these sounds for rudimentary purposes like communicating with each other; they don’t seem to be capable of using them to detect even the most massive objects). They have, instead, highly specialized organs called ‘eyes’ for exploiting ‘light’ rays. The sun is the main source of light rays, and humans, remarkably, manage to exploit the complex echoes that bounce off objects when light rays from the sun hit them. They have an ingenious device called a ‘lens,’ whose shape appears to be mathematically calculated so that it bends these silent rays in such a way that there is an exact one-to-one mapping between objects in the world and an ‘image’ on a sheet of cells called the ‘retina.’ These retinal cells are capable, in some mysterious way, of rendering the light ‘audible’ (one might say), and they relay their information to the brain. Mathematicians have shown that it is theoretically possible, by doing the right complex calculations, to navigate safely through the world using these light rays, just as effectively as one can in the ordinary way using ultrasound. But who would have thought that a humble human could do these calculations. Sensory systems are truly one of nature’s most awesome accomplishments – consider this as you read the words of this post as they scroll across your computer screen.