The Visual Process
Our perception of the visual world follows from a sequence of events leading from the eye to the brain. Understanding the normal function of the eye enables us to identify where, and how, the eye fails in disease states.
In the eye, the transparent cornea and the lens focus the image of the world outside on light-sensitive elements at the back of the eye. They are called photoreceptors - rods for night and cones for daylight vision. These connect to other nerve cells at the back of the eye in a delicate and thin structure called the retina. The retina is really a special neural circuit that senses and pre-processes the visual image before sending the information of the image to the brain. The information is sent to the brain in a large bundle of nerve fibers leaving through the back of the eye. They leave at the optic disc and form into the optic nerve.
The fibers in the optic nerve pass to the brain where they connect in a special structure called the lateral geniculate nucleus, which in turn sends connections to the visual cortex. Once in the cortex the visual information is processed in parallel through many cortical areas each specializing in particular aspects of the visual world. Through many complex connections between the different visual cortical areas our perception of the visual world is integrated into the image we see.
(Illustration from Hubel & Wiesel, 1979)
A special aspect to emphasize about the visual process is that many things happen in parallel and that separate channels of output from the retina carry different types of information about the visual world. These may be differentially affected by disease processes. One channel, the magnocellular pathway, carries information especially important to the processing of visual motion and another, the parvocellular pathway, carries information that underpins color vision and the fine resolution of form.
Thanks to our understanding of this we can use subtle tests to check perception in patients to provide an invaluable insight into the presence and development of sight threatening diseases. The visual system has a remarkable capacity to compensate for the effects of retinal disease and for this reason people are often not aware that they have sight threatening eye diseases until it is too late to effect treatment. That is why we recommend regular eye exams to check for problems that a patient may not perceive. For more information on common eye problems click here.