Imagine with me, for a moment, that the zombie invasion has begun. You try to escape, but the zombies are just too much to handle. You can't run fast enough. They're everywhere. Your favorite science bloggers have been turned into zombies and they're coming for you.
I'm sure you've always wondered what would happen as a zombie ate through your brain. How would it feel? What would the experience be like? Well, dear reader, I'm here to take you on a tour through the brain buffet. It'll be a smorgasbord of white and dark meat (ha!). A cognitive feast. We will dry rub your dendrites and smoke your axons. It shall be an epic meal of neuronal NOMs. Bacon never tasted so good.
First, we need the proper ambiance. One can't properly crunch on cortex or snack on synapses without setting the right mood. Red and white diner tablecloths just won't do. We need white linen, candles, the finest china, expertly polished silverware, and an impeccably dressed waiter who will prepare our meal tableside, as one might a caesar salad or bananas foster, in the finest of fine dining restaurants.
You see, the brain, itself, feels no pain, Clarisse. If that concerns you.
We begin, as Dr. Lecter does, with the dorsolateral prefrontal cortex, or DLPFC. The prefrontal cortex, located at the very front of the brain, is involved in a variety of executive functions. More specifically, the DLPFC is known to be involved in working memory, planning, and decision-making. You would cease being able to speculate about the future; you would be living in the here-and-now. Your short-term memory would be destroyed. The good news, of course, is that without any working memory, you would not be able to transfer the memory of having your brain eaten to long-term memory. You would not be likely to remember this event at all.
The zombies would continue on to the inferior frontal gyrus, which is where you can find Broca's area (on the left hemisphere). Broca's area subserves several functions, but one of the most important is language production. Your language comprehension - the ability to understand others - would be preserved, but your ability to communicate clearly would be lost. You would show the classic symptoms of Broca's aphasia, or expressive aphasia. Speech would be difficult to initiate, and dysfluent. Intonations and patterns of stress, which are important for conveying the emotional content of language, would be lost as well. Most people who are afflicted with Broca's aphasia, after recovery, report that they know what they want to say, but have trouble expressing their thoughts properly. Here is a patient trying to explain how he came to the hospital for dental surgery: "Yes... ah... Monday... er... Dad and Peter H... (his own name), and Dad.... er... hospital... and ah... Wednesday... Wednesday, nine o'clock... and oh... Thursday... ten o'clock, ah doctors... two... an' doctors... and er... teeth... yah." Notice how there is still some meaning to be derived from this, despite the overall structure of the language being lost.
Having completed most of the lateral frontal lobe, this has cleared the way to get farther in, towards the insides of the hemispheres, to the medial surface. The zombies would move on to your ventromedial prefrontal cortex, or VMPFC, also occasionally referred to as the orbitofrontal cortex. As the zombies munch through this area, any semblance of appropriate social behavior would be lost. You would not be able to inhibit your more primal, evolutionarily-ancient emotional responses. Your personality would be forever changed. This area is one of the parts of the brain destroyed in the Phineas Gage accident. In the aftermath of that accident, his doctors remarked that "Gage was no longer Gage." Likewise, you would no longer retain the personality that makes you, you.
With most of the anterior section of your frontal lobe gone, what's left but to eat up the very back of the frontal lobe? I refer, of course, to the motor cortex, which can be found on the precentral gyrus. As you might expect, the motor cortex controls the more than six hundred muscles involved in voluntary movements, and can be found at the rear of the frontal lobe, just in front of the central sulcus.
As we reviewed yesterday, the motor cortex is highly organized, with each area corresponding to a specific set of muscles in the body. The zombies would likely begin on the lateral side, and eat their way up towards the top of the brain, and then down the medial surface. They might begin with the left hemisphere, which would cause paralysis to the right side of your body. You would begin my losing the ability to swallow, chew, or salivate, just before you lose voluntary control over your tongue, jaw, lips, and eventually your face muscles. Next would come your eyebrows, eyelids, and eyeballs. Now you are forced to do nothing but stare straight ahead. The next muscles to go are the ones controlling the neck, before moving on to the limbs: first your thumbs, them the rest of your fingers, the hands, wrists, elbows, and shoulders. Now the zombies eat their way down the medial surface of the motor strip, and you lose the ability to control your torso, hips, knees, ankles, and toes. Moving onto the right hemisphere, the same pattern would occur for the left side of your body. Congratulations! You are now entirely paralyzed.
What's next? Well, the postcentral gyrus of course, on which you can find the somatosensory cortex. Like the motor cortex, the somatosensory cortex is also highly organized. Assuming the zombies follow a similar pattern, you'd now lose sensory feeling in all of those parts of your body, starting with the mouth, moving onto the face, hands, arms, trunk, legs, and feet. You are well on your way to becoming a zombie yourself. Give yourself a pat on the back! Oh wait. You can't. I'd do it, but you wouldn't feel it anyway. Now that you've lost the ability to feel pain, even the most empathetic zombie can continue feasting without worrying for your comfort.
If I was a zombie, the next morsel of delectable delicousness I'd go for would be the superior temporal gyrus, on which Wernicke's area can be found in the left hemisphere. (That's the tastiest part!) Destruction of this area results in Wernicke's aphasia or receptive aphasia, which, contrary to Broca's, impairs language comprehension. Grammar, syntax, intonation are all preserved but the language is meaningless. Patients with this condition generate what is sometimes called "word salad" - intelligible words that appear to be strung together randomly - such as "colorless green ideas sleep furiously." Slightly anterior to Wernicke's area, still on the superior temporal gyrus, is the primary auditory cortex.
At this point, the zombies would probably continue finishing up the temporal lobes. You would certainly lose the rest of your hearing, effectively becoming deaf. Also, since the temporal lobes and occipital lobes are so close, you'd lose all the cortex that binds auditory and visual features together. For example, as the zombies eat their way through the fusiform gyrus (OM NOM NOM!!) of the left hemisphere, you'd lose the ability to bind letter sounds to their visual forms. After eating the right fusiform gyrus, you'd lose the ability to recognize individual faces and become prosopagnosic. Object recognition would still largely be intact - lucky you!
The zombies must be getting full by now, having dined on your frontal and temporal lobes, and even part of your parietal lobes already. But the job of a zombie is never over! They must continue to eat! But first, a musical number.
Alright, let's get back to the brains. The next tasty snack will be the remainder of your parietal lobes. In one big bite, you'll lose your mental number line, spatial orientation, and navigation. (mmmmm...tastes like chicken!) You'll lose your ability to engage in symbolic thought: the ability to represent reality using abstract concepts.
All that's left now, of course, is the occipital lobe. The occipital lobe is almost entirely devoted to vision. With the right occipital lobe, you'll lose your left visual field, and with your left occipital lobe, you'll lose your right visual field. Now you'll be blind.
Is there anything left? Of course there is. There are the subcortical structures - hippocampus, thalamus, amygdala, and so on - and the brainstem and cerebellum. But the zombies are pretty full by now. They'll leave some other zombies to get the the subcortical scraps. Maybe they'll feed the scraps to The Dog Zombie.
Want to explore the brain a little more? Many of the images above come from the Sylvius Neuroanatomical Reference, which I love. It's pretty cool, and they even have an iPhone app!
[UPDATE: PZ is keeping a running list of Scienceblogs zombie sightings here]
you tricked me -- I learned something.