What do we see, and how do we see it? How does the world really exist independent of the human gaze? And how much do we shape the visual world around us when we open a trained eye?

The real miracle of all this is that what we want to see in the world are the objects, events, the arrangements in space. But we're going to do so by means of light. So we often say we see light, but we actually use light to see the material world.

To understand how we see, we must first understand what it is we're seeing. Light is actually electromagnetic energy that travels as waves of varying lengths and amplitudes. In fact, there exists a whole electromagnetic spectrum of different types of energy, including radio waves, ultraviolet waves, microwaves, and many others.

Many parts of the electromagnetic spectrum are not visible to the human eye. Other species on Earth can detect ultraviolet light, for example. But the human eye can only see a very narrow slice of the spectrum called the visible spectrum.

Humans are sensitive to a very small range, between 400 and about 700 nanometers, which is a very tiny wavelength. So one of the really interesting riddles of human vision is why are we sensitive in that area where all these other electromagnetic radiations go right by us?

The wavelengths are good for photosynthesis. And when we came along 200 million years later, we used the same part of the spectrum that plants use.

How we perceive this visible spectrum depends on the properties of the light waves. The wavelength, which is the distance from one peak to the next, determines what color we see. Short wavelengths are what we see as blue. Long wavelengths appear red. The amplitude or height of the wave influences what we see as brightness or intensity.

Light's journey into the eye begins when a wavelength of light passes through the outermost layer of the eye called the cornea, which bends the light and sends it through the iris, the part which gives eyes their color. Behind the iris is the pupil, which sits just in front of the lens. Muscles inside the eye control the shape of the lens to bend the light and focus it on the retina, where visual information enters the central nervous system.

At this first stage of processing, the lens causes the image projected on the retina to be upside down. In later stages of processing, images are reoriented to form our final representations. When the light wave enters the eye, it makes first contact with the nervous system at a rod or cone cell, where the packet of energy in the wave triggers a conformation change in the cone or rod receptor, leading to neurotransmitter release and the encoding of the light wave as visual information in neurons downstream of the cone cell.

The cones are specialized for picking up fine detail with very good spatial precision. But because out in the periphery many rods may converge onto one nerve cell, there's a loss of information. But conversely, this is your low light system. This system helps you detect small differences and get around at night. Whereas the cone system needs a certain amount of light that's much higher and is essentially shut off in some dark situations.

Within the neurons packed in the eye, the first stages of color processing take place as color opponent cells react according to the properties of the wavelength and code subtle changes in color by reacting as color opposites.

You can have a reddish yellow and you can have a reddish blue. But you cannot have a reddish green. There are opponent cells which are part of the system that signals red if they're activated a lot and green if they're activated much less than normal.

Blue and yellow are also linked into their own opponent system. So once again, you can have a bluish red or a bluish green. But it's hard to imagine what a bluish yellow would be. And that's because there aren't any the way our nervous system is structured.

Within a large community of neurons, these color opposite cells can code for very subtle shifts in hue. Visual information then travels out of the eye along the optic nerve and arrives at the thalamus, where further processing occurs.

One really interesting fact is that there are many connections from higher parts of the brain coming down to the thalamus. In fact, more than the ones coming up from the eye. So it looks as if the thalamus might be a place where we modulate the visual activity in connection with tasks or attention.

Visual information is already being processed, even before it leaves the eye. Different cells transmit their signal to relevant parts of the thalamus. These signals remain segregated as they travel to the visual cortex. This is called parallel processing, meaning that the brain is doing many things at once.

And each one of those cells sends a different kind of message to the brain about the world. So some of the cells send a color tuned message. Other ones send a message that says there's something moving out there, and don't say anything when there's not anything moving. They're silent.

Others want to see a sharp edge. If there's a sharp edge out there, they fire a lot of spikes. If there's not a sharp edge, they're silent.

These neurons are the first stage of a cortical hierarchy. Neurons further up the hierarchy in different areas of the brain put these features together to recognize things like corners, then color, and finally whole objects. The human brain is especially sensitive to faces, which are processed in the fusiform face area in the temporal lobe.

The world that we see is not the world that's actually out there. I can tell you that.

Consider the way we see color. We're already seeing such a narrow slice of the whole electromagnetic spectrum. And when it comes to hue, we're really only getting the wavelength that any given object is reflecting.

Color is clearly something that's just in the mind.

For example, the chemical composition of the Earth's atmosphere absorbs light from a range of wavelengths that correspond to red, yellow, and green more so than blue. So our eye is struck by the blue section in the spectrum that the sky reflects.

And those experiences of color map on in some ways to wavelength in the physical world. But the blue is a sensation in us. The wavelength is the distance between wave crests in physical theory.

And there are no blues in the physical world in our modern scientific theories. So it's necessarily done through our own coding system. And that mystery of why we have sensations, and what those even are, how out of physical devices you get a conscious experience of a sensory and a perceptual world, these tell us that there are a lot of good problems still to be solved.