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Does Brain Size Matter?

When I talk about the brain in professional development trainings, I often ask participants to put their two fists together.  That, I tell them, is about the size of their brain.  Typically, everyone starts looking around because everyone’s hands (and, by implication their brains) are different sizes.  We tend to equate size with strength or power (which is why we have the saying “good things come in small packages” to remind ourselves that bigger isn’t always better).  Then, I say, “This is one time that size doesn’t matter. 

So, what does matter when it comes to our brains? 

The Bigger the Animal, the Bigger the Brain 

A fascinating article in Cerebrum from the Dana Foundation makes the case that brain size matters, but there are a lot of other differences as well.  Bigger animals generally have bigger brains.  Mammalian brains can range from the teeny-tiny brain of a shrew (a tiny mammal that resembles a mouse) weighing in a less than a sesame seed, to the 10-pound brain of the massive African Elephant. The size of an animal’s brain generally remains proportionate to its body size.   

In the animal kingdom, a bigger brain is usually correlated with greater cognitive functioning.  So, for example, the cognitive abilities of an African Elephant are superior to those of a shrew.  If the relationship were consistent between overall brain size and cognitive ability, the cognitive ability of a human would fall somewhere between the shrew and the elephant.  But that isn’t the case.  Humans function at a higher cognitive level than species with much larger brains.  

Greater cognitive functioning is seen in other species of mammals and birds than their body size would predict.  In birds, the Corvidae family (jays, ravens, crows, and magpies) all function at an especially high cognitive level for their size, as do parrots! The level of functioning that the brain-body size correlation predicts would certainly not allow Polly to demand a cracker!

Neurons and Brain Regions 

Neurons are the brain cells that communicate with each other and the brains of large animals have more neurons than the brains of small animals.  The average adult human brain weighs about 3 pounds and contains about 85 billion neurons.  The brain of the African Elephant weighs about three times that and contains about 3 times the number of neurons (257 billion or so).   

But the distribution of neurons among different brain regions is dramatically different between humans and elephants.  97.5% of the neurons in an elephant’s brain are in the cerebellum (where voluntary movement is coordinated).  The number of neurons in the cortex of the elephant’s brain is only about one third of the number of neurons in the human cortex.   

While overall brain size is generally correlated with cognitive functioning, the size of various brain regions is actually more important.  Probably the most important is the relative size of the front part of the brain (forebrain).  The forebrain of mammals accounts for a much higher proportion of the brain than other animals.  This larger forebrain yields greater cognitive function consistently across both mammals and birds. And the front part of the forebrain, called the prefrontal cortex is especially important.  The prefrontal cortex is proportionately largest in humans compared to other animal species. 

Animals with greater cognitive capacity also have more specialized areas in the cortex. For example, mammals have two distinct cortical visual systems that function independently. In humans the visual system enables greatly enhanced visual object identificationThis allows us to recognize thousands of faces, a vital ability to promoting complex social systems. 

Neural Networks and Connectivity 

Still, neither the number of neurons in a brain nor where those neurons are tells the whole story of cognitive functioning.  Consider that human babies are born with 85 billion neurons in their brains.  The adult human brain also has about the same number of neurons but is about 3 times the size.  If it isn’t an increase in the number of neurons, what accounts for that growth?   

The answer is it is connections among neurons.  Neurons communicate with each other at junctures called synapses.  Networks or maps of neurons connected together form the basis of all human behavior.  The neurons in our brains make connections as we interact with the environment, so it is the growth in synapses that accounts for the dramatic growth in the human brain in the first few years.   

In addition to creating new neural connections, our brains are constantly refining the networks of connections (a concept referred to as neuroplasticity).  They prune away less useful and unused connections and are constantly making new connections.  The overall efficiency of connections across the brain is correlated with cognitive functioning.    

Even brain functions previously thought to be associated with specific brain regions correlate with connectivity across different areas of the brain, including learning disabilities and conditions like ADHD and Autism. 

Thus, the science is pretty clear that, when it comes to differences in cognitive functioning between or within a species, knowing an animal’s brain size is of less value than knowing how much of the brain is allocated to the frontal lobes of the brain, how brain areas specialize, and how well connected the neurons are. Brain size is just the tip of the iceberg when it comes to cognitive functioning. 



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