Human Visual Perception

It is another beautiful day today and the start of a brand new week for which I am grateful and I am sure you are also looking forward to more great things to come in the days ahead. As usual, we would be learning about the eye and for today we would be continuing with what we started yesterday.

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Yesterday we started the topic of the visual pathway for which we considered the early stages of how vision starts from the person or object of regard even before it gets to the eye and into the eye for whatever needs to happen. I would do a quick recap before we move on to today's own however I would recommend that you take a read if you haven't before you join us for today's discussion so you do not get lost.

So yesterday we learned that vision starts from the object of regard when light impulses travel from the person or object into the eyes through our cornea, aqueous humor, through the pupil regulated by the iris through the crystallin lens regulated by the ciliary bodies through the vitreous then on to the retina.

The retina then transduces the light energy to electrical energy or impulses which then go through the various 9 layers aside the RPE to be sorted out into parts and functions based on the information carried which then come together at the back of the eye to form the optic nerve which leaves the eye through the orbital fissure into the brain.

The optic nerve then crosses over from the two eyes as the information is summed up from the various field areas of the two eyes to make one. We learned that this helps a lot in binocular vision as flaws in individuals' eyes are overlapped and bettered to help produce a good final result or image...

So after the decussation occurs at the point referred to as the optic chiasma, the nasal nerves cross over whereas the temporal nerves stay put then the opposites join the other side to form the optic tract. The optic tract now joins up with the brain itself to form what we call optic radiation.

These radiations come in the form of parvo and Magno cells which carry specific information regarding object size, shape, and orientation. The optic radiations are grouped into 6, group 1,3, and 5 tend to function similarly whereas group 2,4, and 6 also does a similar thing. From the optic radiations, the next stop is the various brain areas for the various processing that needs to be done.

The majority of the impulses go to the occipital lobe where the primary and secondary visual process begins or occur. These areas are known as the primary visual cortex and the secondary visual cortex respectively. There are also referred to as Brodmann areas 17, 18, and 19 respectively. These areas would help us process who or what we are seeing, as well as the shape color, size, and location of the object in space, etc.

Some of the impulses also travel to Brodmann area 8 which is on the frontal cortex and does have a motor function of helping us track object movement in space. This is known as dynamic visual acuity. From the Optic tract, some of the impulses also go to the midbrain and the pretectal nucleus. This information carried here helps the brain determine whether the light entering the light is too intense or optimal.

The response then goes back for pupillary dilation or constriction to occur. Some of this information may travel back along with the ocular motor nerve to control accommodation concerning the crystalline lens as well as the movement of the extraocular muscles which helps us in moving our eyes up, down left right, and in other gazes.

And so your eye as an organ may be functioning well as it ought to with everything being clear etc and yet still one may not be able to see or appreciate some aspects of vision should there be a lesion in the brain that affects any of the areas responsible for visual interpretation. This is why we have a condition known as cortical blindness, thus blindness stemming from the brain.

This goes to prove the fact that we do not see with our eyes only but mostly with our brains and buttress the point that the eye forms a part of the brain. It is for this reason that 3rd nerve palsy or paresis could have a direct impact on vision, causing blurriness and diplopia. The trigeminal nerve is known to be associated with the herpes simplex virus which causes herpes simplex keratitis which could affect vision. One may even lose some amount of cornea sensation even after recovery.

The facial nerve, CNVI can cause a condition known as lagophthalmos for the eye in the case of palsy or paresis since it controls the circular muscles around the eye that helps in closing the lids and keeping the eyes shut. So now you see how that very simple organ known as the eye can be so complex in its own way?

There's so much that goes on for one to fully appreciate and enjoy vision as we know it, the joint movements of both eyes to see and track a stationary or moving object in space, the pupillary action to protect the eye, the photo-transduction biochemical process, and a whole lot.

This should also tell you that, that very simple discomfort you feel with regards to the eye, or that blurriness you may be experiencing could have diverse causes and thus should be seen by a professional and not be self-treated with over-the-counter drugs until things get out of hand before you visit your Optometrist.

Remember that vision loss is most irreversible and that your eye is your window to the beautiful scenery of this world, cherish it, protect it, love it. I wish you the very best, thank you once again for reading and for your time. Stay safe.