[To bully myself into actually writing the lecture on visual perception I’m due to deliver in January, I’ve decided to try drafting the wretched thing as a series of blog posts. This is probably a very bad idea. It’s a two hour lecture, so there could potentially be quite a few of these posts, but also the chance of bailing is pretty high. Whatevs. Here goes nothin’.]
1. Preamble
To start with the bloomin’ obvious: vision is the process — the collection of processes — by which we gather information about the external world through the medium of light. To be precise, like Thomson and Thompson, that’s the medium of visible light — a relatively narrow band of electromagnetic radiation of wavelengths roughly 400 to 700 nm.
Light is a pretty great medium for gathering information — almost the definitive one. It propagates really fast over long distances, even through a vacuum. It travels, at least to a reasonable approximation, in straight lines. It interacts with matter and participates in chemical reactions, making it possible for biology to come up with mechanisms for detecting it. And it’s plentiful. There’s a lot of it about, at least during the day, thanks to an enormous nuclear furnace burning in the sky.
Vision is so useful that it has evolved numerous times over the history of life on Earth, with a fair bit of variation in the implementation details. There are a bunch of different structures and configurations of optical sense organs — the eyes of a fly or a flatworm or a scallop are quite different from our own — although some of the underlying biochemical components are pretty well conserved. We’ll touch on those in a little while.
Vision is also so useful that it’s a bit overpowering. It’s the dominant sense for most humans, one on which we rely heavily, one that radically shapes our understanding of the world, both literally and metaphorically.
The vocabulary of vision pervades our language, encompassing much more than mere optical detection: we say “oh, I see” meaning “I understand“, or “let’s see” meaning “we want to find out” or “we’ll see about that” meaning “I will act to prevent it.” The word vision doesn’t just refer to the sense of sight, but extends to intellectual brilliance, to clarity and drive, to divine revelation. We describe a great creator as a visionary, someone who predicts the future as a seer, or a clairvoyant — literally one who sees clearly.
Much of human culture revolves around the visual — painting and drawing, photography and movies, games. Practical necessities like food and clothing are wrapped up in visual aesthetics. The written word is primarily mediated — read — through our eyes. We have all manner of advanced technology dedicated to the enhancement of vision — microscopes and cameras and telescopes — and to the production of visual stimuli — from paper and pencils to printing and screens to HUDs and holograms and VR headsets. It is one of the major ways we interact with computers and software, often the main component of a user interface.
At the same time, vision is complex and fragile and lots of things can go wrong with it. Visual impairments are extremely common. People may have difficulty with focus or resolution close up or far away or — especially when you get to be ancient like me — both. People — particularly men — may be unable to distinguish some or all variations in colour. People may be unable to see in dim or bright conditions. They may lose parts or all of their visual field to obstructions in the eyes or degeneration of the sensory tissues or loss or damage to perceptual pathways. Some of these problems can be readily mitigated with technology — very many of us wear glasses or contact lenses, whether all the time or for specific tasks — other problems not so much.
So, given both vision’s centrality to human endeavours and its frequent failings, it is important to understand how it works and how it goes wrong, and try to find ways we can make the most of it while also maximising accessibility.