The Science of Zombies

From the Daily Mail:

Zombie fanatics Timothy Verstynen, an assistant professor in the department of psychology at Carnegie Mellon University in in Pittsburgh, Pennsylvania, and Bradley Voytek, assistant professor of cognitive science and neuroscience at the University of California, San Diego, summarised characteristic zombie behaviour seen in films, which can be explained by looking at the structure of the brain.
They have dubbed the condition 'Conscious Deficit Hypoactivity Disorder', or CDHD, which they describe as an acquired syndrome in which infected people lack control over their actions. In films, the undead display symptoms such as lethargic movement, loss of pleasure, language dysfunction, amnesia and the inability to suppress hunger and aggression.

‘Zombies often have difficulty recognising familiar people and suffer chronic insomnia that results in a delirious state,’ they write.

Undead individuals would also exhibit antisocial behaviour, such as biting and eating people, but they would be friendly to each other and swarm with other infected individuals, according to the scientists’ book, ‘Do zombies dream of undead sheep?’.

The neuroscientists have come up with two subtypes of the syndrome – CDHD-1, which affects ‘slow’ zombies with uncoordinated movements and CDHD-2, which is typical of ‘fast’ zombies that have no problem chasing humans in films.

This may be because at the moment of death, our circulatory systems stop, starving the brain of oxygen and glucose.

In zombie films, they resume at the point of infection - in a limited form - and the brain gets its nutrients by zombies feasting on human flesh.

The longer the brain is starved of oxygen, the more extensive the damage to zombies, the neuroscientists say.

CDHD-1 zombies can be seen in the TV series, The Walking Dead, while the zombies in Waord War Z are faster and typical of CDHD-2.

The brains of both types of zombies would change in different regions, from a combination of decreased activity and altered activity in multiple brain networks.

Lesions to the temporal lobe would partly cause zombies to act like they do in films.

The scientists go on to warn that because they can't feel pain, it does not good to fight a zombie unless you can quickly kill it. Because zombies have poor concentration, you can avoid them by being quiet or distracting them. Don't try to reason with them--it is useless. It may be possible to mimic them, but don't try to outrun the CDHD-2 types.

Why Fast Zombies and Slow Zombies?

Much of zombie behavior is going to be determined by how "fresh" their victim is, and the damage to the victim. A newly zombified human will still have major organs and neurons intact, allowing the zombie parasite to move and react as a normal human. However, as the bodily systems begin to break down, quick movement can become more difficult.

One of the first systems to begin to break down are those highly dependent on an intact neurological system, such as sight and balance. This Wikipedia article explains the fundamentals as well as any article:

Balance is the result of a number of body systems working together: the eyes (visual system), ears (vestibular system) and the body's sense of where it is in space (proprioception) ideally need to be intact. The vestibular system, the region of the inner ear where three semicircular canals converge, works with the visual system to keep objects in focus when the head is moving. This is called the vestibulo-ocular reflex (VOR). The balance system works with the visual and skeletal systems (the muscles and joints and their sensors) to maintain orientation or balance. Visual signals sent to the brain about the body's position in relation to its surroundings are processed by the brain and compared to information from the vestibular, visual and skeletal systems.

From my research, the vestibulo-ocular reflex actually begins to break down quickly. Although the exact progression varies, eyes seem to degenerate fairly quickly in a new zombie. One of the results of this is the well-known tendency of zombies to begin relying heavily on sound and smell over eye-sight. However, as it impacts their ability to balance the body, a zombie's movements also slow and become more herky-jerky as it corrects and over-corrects in an attempt to keep the body upright. A zombie that has started to decay will often fall over if it attempts to run for any distance, although some compensate by scooting along on all fours. There are even accounts of zombies scuttling around in a "crab-race" position. As its condition deteriorates further, the zombie may be forced to crawling or pulling itself along the ground.

As greater and greater decay creeps in, balanced movement becomes more difficult for the zombie. At this stage, the zombie will often become inactive waiting to suddenly lunge at prey. This not only conserves energy, but keeps the zombie from damaging itself or warning off prey due to its more poorly coordinated movements. Zombies in late stages are easily outrun, provided you can avoid the initial and sudden attack.