How Vision Works

When considering the complexity of vision, the question often arises of where we see- in our eyes or in our mind. The answer to this question is not one or the other, but is actually both. The eye is primarily responsible for capturing light and converting it into an electrical signal. Although some early filtering of this signal takes place within the eye and its optic nerve, the vast majority of the processing of vision occurs in the back of the brain. The basic steps that occur in the eye are outlined below:

Step 1: Light rays, or photons, enter the eye's outer, transparent layer of tissue (the cornea), then pass through the dark, circular opening (the pupil) in the center of the colored iris. The pupil regulates the amount of light that enters the eye.
Step 2: The light rays strike the inner (or crystalline) lens of the eye. This lens focuses the rays on the retina (a layer of light-sensitive cells that line the inside back wall of the eye).
Step 3: The cells in the retina transform the photons into electrical impulses. These are transmitted through the optic nerve to the brain, where complex electrical-chemical interactions give us the sensation of seeing.

The brain then continues to filter the image into areas of color, contrast, location, and direction of movement. These building blocks are then assembled and compared to memory in order to interpret exactly what it is that the eye is seeing. This interpretation of the eyes' vision is analyzed for importance and anything the brain finds interesting is amplified by "telling" the eye to get a better image (this occurs when we "pay attention" to an area of our vision). This amplified signal, or paying attention, makes what we see more likely to be stored in memory.

This complex interaction between the brain and eyes can be summed-up shortly by saying that the eye captures an image, and the brain sorts everything out.