Visual perception is just one great example of how our brains infer and synthesize information based on ambiguous and limited input.
When we look at the world around us, rarely are we seeing objects in their entirety. Usually we only see parts of them. This is because some sections may be obscured by other objects.
For instance, you can readily figure out that what looks like unconnected wooden pillars underneath a table is actually a chair. This is true even if you have never seen that particular type of chair before.
Another example is we are looking at them at a less than optimal angle, or they are at the periphery of our field of vision.
In the latter, case, you can experience a phenomenon called boundary extension. This is something that happens when you take a look at a something, then look away and have the impression of seeing a more extended angle view than was in fact there.
But where the visual perception algorithms in the brain get really interesting, at least to me, is in the realm of color.
Your eyes have three kinds of color receptors, red, green and blue. So how do we see all the amazing pallet of other colors that make up our world?
Check out this video explaining how humans can see every color from Amethyst to Yellow.
And what about black and white photographs and films? Do we have a separate area in the visual cortex for them or what? Probably not.
Color processing starts at a very early level in the visual system, within the retina, through initial color opponent mechanisms.
Trichromacy is driven at the level of the receptors, and opponent processes arise at the level of retinal ganglion cells and beyond. In the visual system, it is the activity of the different receptor types that are opposed.
Visual information is then sent to the brain from retinal ganglion cells via the optic nerve to the optic chiasma: a point where the two optic nerves meet and information from the temporal (contralateral) visual field crosses to the other side of the brain.
What is Color?
Color is actually not a property of the spectrum of electromagnetic radiation, but a feature of visual perception by an observer.
Not only that, but there is an arbitrary mapping between wavelengths of light in the visual spectrum and human experiences of color. Although most people are assumed to have the same mapping, the philosopher John Locke recognized that alternatives are possible, and described one such hypothetical case with the “inverted spectrum” thought experiment.
For example, someone with an inverted spectrum might experience green while seeing ‘red’ (700 nm) light, and experience red while seeing ‘green’ (530 nm) light.
Synesthesia provides some atypical but illuminating examples of subjective color experience triggered by input that is not even light, such as sounds or shapes. The possibility of a clean dissociation between color experience from properties of the world reveals that color is a subjective psychological phenomenon.
Neuroscientist David Eagleman explains some of the known facts about Synesthesia in the video below.