My regular readers, if I have any, are likely wondering what the hell Debra has been smoking recently but, rest assured, these things go together like peas and carrots.
I am going to discuss visual pathways and prosopagnosia at the Neuroscience Journal Club at West Chester and I haven’t looked at my notes for a good week so constructing this post will help. Note: I have been unsuccessful getting the photos where I want them so if the post looks good, it will be a bit more of an accomplishment than you might expect.
Where to begin? The man on the moon! Humans are born with a preference for looking at a human face and will choose a face over other shapes. In fact, humans will see a face at the most basic pretense this explains why most of us see a human face on the moon. Many were convinced the planet Mars has statues of humanoid aliens when it was nothing more than opportune shadows on the red planet. The need to see a human face is reflected in the newborn visual field being about the distance between where the baby is when he or she is nursing and his or her mother’s face.
Isn’t that cool?
Faces are so important to humans that a special region of the brain, namely the Fusiform Gyrus, is dedicated to facial recognition.
To appreciate this region of the brain that is located at the bottom of the temporal lobes, I need to take the reader through the visual pathways. When an image hits the retinas, the light is transformed to an electrical impulse that runs down the optic nerve to the place where the optic nerve synapses with the next nerve. This part of the brain, found on the bottom of the thalamus, and is called the Lateral geniculate nucleus otherwise known as the LGN. The LGN has six distinct layers of neurons and these layers correspond wih different tyypes of neurons and different purposes.
The image is carried back to the visual cortex that is found at the back of the brain in the occipital region.
If someone has damage up untiol this point, they are blind.
From the visual cortex, the image gets transmitted to various regions of the brain to determine (in general) where, when, and what the image is.
We are looking at what and the image will go to the facial recognition area on the Fusiform Gyrus.
If there is damage between the visual cortex and the fusiform gyrus, the person cannot recognize the face. This is called Prosopagnosia.
Things get even more interesting as the image is identified as a face gets transmitted over to the limbic system and the amygdala senses whether there is an emotional response to the image. If there is damage the person recognizes the face but thinks it is an imposter because they aren’t feeling the emotional response that is normal and the most logical conclusion for these people is to think their loved one has been replaced with a stunt double or alien (like in the movie Invasion of the Body Snatchers which was better in the remake.)