Having previously discussed how the electrical impulses triggered by the photoreceptors in the retina reach the area of the brain that is devoted to visual processing, we will now discuss further details how exactly the visual processing in the brain supervenes.
Primary Visual Cortex: Brain and vision
The processing of signals ensues in the primary visual cortex, an area of the brain cortex that is located at the very back of our sculls in the occipital lobe of the brain. The neurons in this area, which is often referred to as area V1, are very specifically oriented. Generally speaking, there are 7 layers of neurons that are located at the specific areas of the cortex in regard to the area of the retina where the signals are coming from. This is called retinotopy or retinotopic map. Basically, this area just filters the impulses that are coming there by the optic radiation and sends them further to the adjacent areas, V2 or secondary visual cortex in particular.
From there then, the neurons disperse literally all over the brain (or at least over half the brain), which makes further processes very hard to understand not only for laymen but also for people who have spent a lot of their lives studying that. You have probably guessed it; we are not going into further details on that. It also goes very much beyond ophthalmology and into the area of neurology from then on. Well, it is perhaps not even a matter of neurology but just a matter of basic neuroscience.
Dorsal and Ventral Pathway
Nonetheless, I will try to summarize all that in a few sentences. In short, visual processing then follows 2 distinct pathways, referred to as the dorsal pathway and the ventral pathway. They deal with recognition where the objects are and what the objects are, respectively.
The recognition of shapes, color, form, pattern, depth, and movement are all functions of the brain. In other words, should there be lesions in any of the specific areas responsible for different aspects of sensations one would not be able to recognize them? For instance, people with lesions in one area will not be able to recognize colors but will recognize shapes and form perfectly, and some with lesions in other areas will experience completely the opposite. In reality, however, isolated damage to a particular visual processing aspect is unlikely, as lesions due to trauma, vascular events or tumors usually affect multiple areas and are not confined to a particular anatomical region of the brain, the one that is responsible for color recognition for instance. Or shape or form or pattern for that matter.
Still a land of the unknown
All in all, the processing of vision in the brain is an immensely complicated process. As of today, we still do not understand it all and perhaps we might never will. Which is just as important, as similar applies to amblyopia as well. We might still do not know many things about amblyopia, but what we do know, is that there are certain disturbances in the primary visual cortex as well as adjacent cortices, some of which we are trying to address with therapies such as patching and our AmblyoPlay solution.
So then, we have reached our finish line now. We have started 8 blogs ago and 100 million miles away. Firstly, we have followed the light through the optical parts of the eye, explained the conversion of light to electricity in the photoreceptors of the retina, then pursued the electrical impulses further into the brain and eventually concluded scattered all over our brain.
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P.S. There will be an extra content of the current blog series, in which we will connect all the dots from Part 1 through Part 8 in the eyes of amblyopia.