The Scarecrow from The Wizard of OzThe cerebellum, the part of the brain that deals with motor learning, is a structure that makes up about 10% of the volume of the brain. It has many, many, tiny neurons in it, so much so that it actually contains more neurons than the rest of the brain. Interestingly, if the cerebellum is removed from someone's head, although they will have motor deficits, their personality and conscious experience of the world will remain unchanged. In contrast, even though the cerebrum has less neurons than the cerebellum, removal of the cerebrum will leave a person in a permanent, vegetative, and unconscious state. Something about the cerebrum that scientists haven't quite pinned down yet is responsible for conscious, subjective experience. I recently watched an excellent discussion among experts from different fields about the nature of consciousness, where they brought up this very point. I highly recommend the video, if only to see neuroscientist Christof Koch's fabulous shirt.

Although consciousness is difficult to define, much less explain, neuroscientists have made a lot of interesting progress in this area in recent years. It makes neuroscience an exciting field to be a part of, and is one of the things that drew me to neuroscience in the first place. What makes it so intriguing is its mystique--consciousness is one of the most scientifically intractable questions of which I know. It is easy enough to explain the neural mechanisms of something such as the tuning of an individual neuron in the visual cortex, but it is another thing entirely to explain what neural basis there is, if any, for the subjective experience of sight.

Neuroscience often teaches us to view the brain in terms of action potentials. Sodium atoms rush into the cell, potassium atoms rush out, causing a transient change in the local voltage. That's what the brain is: different types of atoms moving around in response to different forces. Harmonizing this deterministic, mechanical, soulless view of the brain with the idea of a subjective experience of consciousness is a difficult task. How can a large number of atoms moving around cause consciousness?

Christof Koch put it this way in an interview on NPR:

"So we know the brain is part of the physical universe, just like anything else. But brains - human brains, animal brains, baby brains - brains also exude this stuff, this feeling, like feelings of pain or pleasure, of artistic sensibility, of seeing red.

"And the big mystery has always been, how is it that a physical system that's described by the laws of physics, how can it give rise to conscious sensation? And can other physical systems such as a computer, can they also give rise to physical sensation? Is it something in the structure, is it something in the information, is it something in the complexity of it that gives rise to consciousness?"

He and another scientist, Giulio Tononi, are advocates of the idea that consciousness is a product of something called integrated information. As the name suggests, this theory borrows concepts from information theory and applies them to neuroscience, emphasizing that the integration of information across brain regions and modalities is critical to consciousness. They talk about this theory some in another video here.

Other scientists have recently developed alternative metrics for measuring consciousness. Traditionally, scientists observed electrical patterns recorded from EEG electrodes and then attempted to correlate them with behavioral manifestations of consciousness or unconsciousness, leading to some technologies such as the bispectral index, or BIS monitor used by many anesthesiologists to measure a patient's depth of anesthesia.

Recent studies using EEG have even found heightened signs of consciousness in rats for a short period of time immediately following death. This surprising result came when scientists found high levels of gamma rhythm synchronization between the front and back of the brain. Synchronous activity in the gamma frequency range is thought to be responsible for binding information from different brain regions together to make a coherent experience. However, it is important to remember that these studies do not measure consciousness directly, but rather measure electrical and behavioral correlates of consciousness.

Researchers in Italy and Giulio Tononi also came up with a new metric for measuring consciousness, called the perturbational complexity index, or PCI. Their metric also relies on EEG to measuring different large-scale electrical rhythms in the brain, and was developed by measuring these rhythms in people during a wide variety of different states of consciousness. Although they were able to distinguishing between different states of consciousness in test subjects using only their analysis of the subjects' EEG signals, many more people would need to be tested before this method became clinically relevant.

Yet other researchers have different theories about the nature of consciousness. Some postulate that quantum effects in the brain could explain consciousness. For instance, some have proposed that electrons in structural proteins called microtubules would be a possible candidate for information processing through quantum effects, allowing the brain to perform quantum computer-like calculations. There is even some evidence that microtubules are involved in anesthesia, which could lend support to this hypothesis, but many scientists feel this theory lacks substantial evidence.

Whatever the physical substrates of consciousness are, it will definitely be interesting to see where this field goes in the next few years. What do you think is the basis for consciousness? How should we measure it?

Posted September 17th, 2013 in Neuroscience, Science.