Michael Gazzaniga was one of the co-authors of the Cognitive Neuroscience book I used my first year of grad school. He has a new popular science book coming out called Human: The Science Behind What Makes Us Unique. I'm not so crazy about the premise, looking at defining features that distinguish humans from other species. You could write a book about any species with nearly the same title. However, it sounds like there will be some interesting stuff in the book.

Here's what I think are excerpts from the book. On the question of whether bigger brains make smarter brains, he says:

Is this assumption about quantity correct? It would appear not. Many mammals have larger brains than humans. This is known as absolute brain size. The blue whale has a brain that is five times larger than a human brain. Is it five times smarter? Doubtful. It has a larger body to control and a simpler brain structure. Although Captain Ahab may have found a whale intellectually stimulating (all be it he was dealing with a sperm whale whose brain is also larger than a human's), it has not been a universal experience. So perhaps proportional (allometric) brain size is important: that is the size of the brain compared to the size of the body. Calculating brain size differences this way puts a whale in his place with a brain size that is only .01% of his body weight as compared to a human brain that is 2%. At the same time consider the pocket mouse’s brain, which is 10% of its body weight. In fact in the early nineteenth century, Georges Cuvier, an anatomist, stated, "All things being equal, the small animals have proportionately larger brains." As it turns out, proportional brain size increases predictably as body size decreases.

Well, there's the volume of the brain, but another obviously important factor is the number of neurons. As I blogged about earlier this year, a study analyzing the number of neurons in a particular whale species' cortex estimated about 12.8 billion neurons. That's compared with the roughly 20 billion neurons in the average human cortex. What makes up the difference in volume? At least in that particular study, the finding was that the whales had many more glial cells, which provide structural support and thermo-regulation.

So Gazzaniga is right on at least one count. You can't just look at the absolute brain size. But there's no doubt that there's a positive correlation between brain size (in terms of volume and neuron count) and cognitive ability.

He also talks about the evolution of the human brain, and rightly points out that going forward, a greater understanding of genetics is going to yield a great deal of information about how the brain is built and how it works.

Anyway, it's fairly long, but an interesting read. Have a look.