For part 3, we evaluate the history of the third kind of evidence in cognitive neuroscience:
A normal subject is asked to do a task. While they are doing the task, a scientist observes their brain in action.
For many readers, this will sound like fMRI, where a person lies down in a huge machine, is instructed to think about something, or view something, and then, VOILA certain parts of their brain light up. But in fact, this kind of logic has been used for much longer than fMRI. Before I launch into a description of fMRI, let's follow the chain of evidence that gets us there. The neuron is the basic unit of the brain, across animals. Some animals have brain systems that are quite similar to human brain systems.
The neuron is the basic unit of the brain. This had to be discovered, and was once the subject of debate. While Schwann and Schleiden proposed that the cell was the basic functional unit of life in 1838, it strangely took some time for this to be applied to the brain. Advances in microscopy, and lenses allowed Purkinje to find the first neurons in 1837... Wait, what's that? Well, even though in retrospect we see that Purkinje discovered the first of many nerve cells, at the time, this was not enough evidence to counter the camp of scientists who thought the brain was fibrous tissue and not cells. Later physiologists, like Otto Dieters, whose work published after his death in 1865 documented other neurons, and Camilo Golgi, who discovered a new method of staining in 1873 and showed the world incredibly clear pictures of the brain.
But Santiago Ramon y Cajal was responsible for breaking through the doubters, by using Golgi's staining methods to show clearly the neurons in the cerebral cortex, in 1888. He shared the Nobel Prize with Golgi in 1906. Except that Golgi still though that the brain was a bunch of undifferentiated tissue, and said so in his half of the lecture, while Ramon y Cajal used his half to try to refute Golgi and show that the brain was made of individual neurons. By the way, the link above is from the blog Neurophilosophy, and is a great synthesis of this story of the discovery of the neuron.
The brains that Golgi and Ramon y Cajal were studying were animal brains, but as they discovered the fundamental unit of the neuron, others began to discover how these neurons worked. In 1921 Otto Loewi discovered that neurons transmit their messages chemically (Nobel Prize, 1936) as well as electrically.
Animals have brain systems, just like us
Why does any of this matter? Well, these discoveries (along with others, and technological development in making really small, narrow glass pipettes) allow for the possibility of recording electrical activity from a single neuron while the animal is awake or active. This allows Eric Kandel (Nobel Prize, 2000) to investigate how memory works at the cellular level, mostly by working with sea slugs (aplysia). It allows Hubel and Weisel (Nobel Prize, 1981) to investigate (using cats) how a certain pattern of light on the retina results in a certain pattern of activity in the brain. These studies are what are called single cell recordings. Basically a very small hollow-tipped needle is placed inside the brain of an animal (inside a single neuron, to record its activity) , then we ask the animal to think something, and observe what makes that particular neuron activate.
With humans, this was not possible. So human live brain scanning began with the EEG, which measured the brain's electrical activity. When a bunch of neurons fire, you can detect electricity by placing detectors on the scalp. This technology had a great advantage of being able to detect neuron's firing at the exact moment that they fired. The disadvantage was that it was not very good at telling where the neurons were firing, because it could only detect broad patterns of electricity, in general brain locations. In technical terms, we say EEG had high temporal resolution (very sensitive to time) but low spatial resolution (could not tell exactly where in the brain the activity was). But despite its lack of localization, EEG is still used today, and has given us much insight into how the brain is connected to the mind.
After EEG, PET scans were an advance, because they could tell with much more accuracy which areas of the brain were active. For PET to work, you need to be injected with radioactivity, which your blood then takes to your brain and makes it glow. When a neuron fires, it needs energy to keep firing (it is actually an electrical signal, which takes energy). It sends a message for more blood, then the blood (ooh, it glows!) gets sent to that area.
PET scans gave way to fMRI, which similarly used the blood flow as a marker, but this time, no radioactive marker. The MRI machine uses an enormous magnet to detect the magnetic fields of oxygen atoms in your blood. In fact, when your brain sends blood to a set of firing neurons, it sends too much. Most of this blood gets used. Some does not. The fMRI detects that extra blood that got sent, using the BOLD (or Blood Oxygen Level Dependent) response.
Despite the fact that fMRI has been in use for twenty years, and the basic principles remain the same, the methods of its use have changed drastically. For one example, consider the computational constraints of taking a scan of the brain. Instead of a 1.5-dimensional graph (on or off) over time for single cell recording, fMRI has full 3-dimensional data, thousands of thousands of voxels (volumetric pixels). As computing power has increased, as the power of the magnets has increased, this has yielded improvements to the techniques in fMRI, which are still imperfect, albeit convincing well beyond their actual scientific utility.
This logic, of observing a brain while the animal is in action, has been around for a long time, depending on whether you start with Galvani's frogs in 1771, or Helmholtz measuring the (non-zero speed) of neural transmission in 1852, or with Kandel's studies of the cellular basis of memory in the 1960's.
Ok, so now I am at the conclusion of my introduction, and it is probably not much better than going to Wikipedia (although after doing this, I started to edit the cognitive neuroscience Wikipedia entry). What was the point?
To return to the main point: cognitive neuroscience is not young. The connection between mind and brain has been a topic of experimental study and debate for at least 200 years. But this 200 years is not simply progress, one scientist taking another's discoveries and adding to them. Golgi and Cajal, despite making amazing contributions, disagreed on whether the brain was made of neurons, in their Nobel Prize acceptance speeches. I have not described developments in whether brain cells can change (neuroplasticity) but that was a lively topic of debate for some time too. If we look closely at the history of most sciences, we see that science progresses in fits and starts. In psychology and cognitive neuroscience, we can see how advances were tied to advances in technology (lenses, magnets, electricity) or advances in other fields (PET depends on safe radioactivity) or even tragedies (head injuries in WWI resulted in many observations). When we look at the history, we see that the history of cognitive neuroscience includes physics, chemistry, biology, and psychology. In other words, despite the apparent immaturity of knowledge at every level, there is still communication and influence. Returning to the exercise that brought all this about (evaluating evolutionary psychology), I am left agreeing with another commenter on TNC's blog, it is relatively pointless to assign maturity or youth to an entire subfield. We should instead consider individual claims, and evaluate the evidence for these claims, in the context of a broad background in related scientific knowledge.