“…Crosses over? Like in an X shape? Well that seems ridiculous”, my younger sister scoffed. After 10 minutes of me trying to briefly explain our visual system to her over the phone, she still couldn’t believe that sending signals from our eyes to our brain wasn’t a straight “Point A to Point B” system. While any human with common sense would agree with my sister’s logic, transmitting signals from our eyes to our brain is a much more complicated process, involving multiple intermediate functions and convoluted pathways.
When we first detect an image, light shines into our eyes and onto the retina, a layer at the back of our eye that is sensitive to light. As the retinas in each eye initially receive the image, the image is upside down and flipped (See Fig.1). While this seems counterproductive, imagine how we see our reflection backwards in a mirror – the retina works in a very similar manner.
Each eye’s retina sends signals through a collection of neuronal axons known as optic nerves. These nerves serve as the main “connectors” between the retina and the brain. Each optic nerve starts at its corresponding eye. However, midway through their journey to the brain, some of the axons in the optic nerves cross over at a specific location, called the optic chiasm (cue my sister’s confusion). For humans, the axons in the retina towards the outer part of our face don’t cross over. Only the axons in the part of the retina near the center of our face cross over. While this seems absurd, this balance between crossing and not crossing over is advantageous as it allows for greater depth perception.
For the nerves that cross each other at the optic chiasm, they will transmit their information to the side of the brain opposite to the eye they initially received visual input from. So for example, nerves from the right eye cross at the optic chiasm and continue on to the left side of the brain. At this point, each nerve is no longer deemed an optic nerve, but rather known as an optic tract.
As if the process wasn’t already complex enough, these optic tracts still don’t directly transmit information to the brain. Instead, they continue until they reach a part of the brain called the thalamus. In general, the thalamus is responsible for relaying sensory and motor information to and from the cortex. Part of the thalamus includes a structure specialized for vision– the lateral geniculate nucleus (LGN). Once the optic nerves reach the LGN, they terminate, and the LGN becomes responsible for finally relaying the information to its destination.
But the LGN doesn’t just send signals to any random part of the brain. It sends its information to the brain’s occipital (visual) cortex, where the brain can then fully process and correct the images into the desired final image we end up observing.
In hindsight, I understand my sister’s confusion at this entire process – between optic nerves crossing over and all the intermediate structures, it’s a very complicated one. Even today, there is much debate within the scientific community as to why such a complicated process is necessary to transmit visual information from our eyes to our brain. As for me, I’m simply going to appreciate that no matter how convoluted it is, the process still works well enough for me to see Ryan Gosling on screen in all his glory.
All information and images obtained from The Mind’s Machine, Foundations of Brain and Behavior (Second Edition)