ChuckleVision

Photo credit: comedy_nose

Let's face it: we humans have pretty ordinary vision. Unlike birds of prey, we can't see tiny mice running around several metres below us, and we can't detect the ultraviolet 'signature' they leave behind by their urine trails (as in kestrels; Viitala et al., 1995). Unlike snakes, we don't find our dinner by detecting combined movement and heat (infrared) signals. I could go on, but you get my point.

In particular, our colour vision is rather inferior. I'll come onto that in a moment. But first, how do we and other animals see colour? All around us, we see light bouncing off objects, which we perceive as different colours: blues, greens, reds, yellows, purples, pinks... But how is this possible? How does the light give us such rich information about the world?

In a nutshell: The light is made up of different properties ('wavelengths' measured in nanometers [nm]) across a spectrum of different wavelengths (shown below). The eye has a specialised internal tissue called the 'retina' which contains cells that are sensitive to different wavelengths of light ('photoreceptors' or 'cone cells') (see cross-section of human eye below). When light hits the retina, certain photoreceptors are stimulated (depending on the wavelength of the light) and if enough are triggered, then they fire signals or nerve impulses to the brain's visual cortex where it is recognised as a specific colour. 

For instance, in humans, we have cone cells that are sensitive to three different types of wavelength: short wavelengths (≈400-500nm; 'blue light'), medium wavelengths (≈500-600nm; 'green light') and long wavelengths (≈600-750nm; 'red light'). In other words, we have 'trichromatic' vision, as we have three types of cone cell. 

Look at the blue chair below. It is reflecting light of short wavelengths (≈400-500nm), which is then stimulating enough of our short-wavelength-sensitive photoreceptors for them to fire a signal to our brain which recognises the light as a 'blue' colour. All in milliseconds. Pretty amazing eh.

Now, yes, this is pretty amazing, but not as amazing as some other animals' colour vision. Not only can other animals see that elusive 'ultraviolet' part of the spectrum on the left, but they can also see the very-long-wavelength infrared part of the spectrum on the right. And some animals possess not just three cones, but four, five, six, and up .... to even 16. And the last one is just a mantis shrimp. Why do we see such variation in vision, and what are the consequences? And why do I care? I'll have to save these questions for next time...