Brains, brains, brains!

Readings:

Bear, Connors, Paradiso Chapter 7: The Structure of the Nervous System

Paul Patterson, One World, Many Minds, SciAm Mind

Advanced:

R.G. Northcutt, Understanding Vertebrate Brain Evolution (2002) - This article, while advanced, is not terribly difficult to understand and is a nice explanation of our latest thinking about brain evolution.

Which is more intelligent, a dolphin, a horse or a squirrel? Which has the most evolved brain? How do you determine this? One useful way to better understand the relationship between brain structure and brain function, is to compare the neuroanatomy of different species of vertebrate animals. This type of comparative neuroanatomy is also useful for understanding brain evolution, since the brains of different species will reflect the particular adaptations they have undergone to better survive in their habitats. In this exercise we will compare different brain regions from different species to understand the relative importance of these structure to the organism. From reading the background book chapter you should now be familiar with basic neuroanatomical concepts and anatomical references (i.e. “directions”).

As you learned in your reading, vertebrate brains can be organized into three major subdivisions: forebrain (telencephalon and diencephalon), midbrain (mesencephalon) and hindbrain (rhombencephalon). The primary forebrain structures are the olfactory bulbs, cerebral hemispheres (in mammals containing the cortex) and the thalamus. In the midbrain we find the optic tectum and tegmentum and in the hindbrain the cerebellum, pons and medulla.

Diagrammatic sagittal view of a vertebrate brain showing the

division into primary regions. Source: BCP Chapter. 7

Each of these structures will have evolved to a different degree in the various vertebrate species, based on their individual needs. For example, rodents have relatively more evolved olfactory bulbs (mediating their sense of smell) than humans. However, humans have relatively more developed language areas in their cerebral cortex. So let’s compare three different forebrain brain structures in rodents and humans: the olfactory bulbs, the cerebellum and the cerebral cortex. Let’s first look at the cortex in the images below. The cortex is on the surface of the brain and is the largest structure of both of these brains. The first thing you will notice, however, is that in humans the cortex is wrinkled up, while the cortex of the rat is smooth. The wrinkles in the human cortex increase its surface area, allowing for a bigger cortex to fit inside the skull. Many parts of the human cortex are highly specialized for functions such as cognition and language and it is believe that the reason humans are more intelligent than rats is because of this increased cortical size. Now compare the relative size of the olfactory bulbs in humans and rats. The olfactory bulbs are circled in red. Notice that relative to the overall brain size, rats have a huge olfactory bulb. In contrast, in humans it is puny. That is because rats, unlike humans, primarily use their sense of smell to interact with their world. Thus this brain structure has evolved to be larger and more prominent. Finally, let’s compare the cerebellum. You can find the cerebellum towards the back of the brain, it is the large lobed structure behind the cortex. In both rats and humans the cerebellum is fairly well developed. The cerebellum is important for motor coordination, something which is of critical importance to both species.

Another useful bit of information we can get from comparing brains, is the relative size of an organism’s brain to its body size. If you look at the graph below which plots the relationship between body size and brain size, you will learn that bigger organisms tend to have bigger brains (which is expected). Yet, you will see that for a given body size, there is quite a big range in brain sized for different species.

This graph shows the relationship between body size and brain size for different species.

Source

Based on this variation, neuroscientists have come up with a measure called the Encephalization Quotient (EQ) which describes brains size as a ratio of the expected average brains size for a given body weight. So for example, humans have an EQ of about 7.5. This means that our brains are 7.5 times bigger than what you would expect for animals of our same body size. In contrast, a squirrel has an EQ of 1.1, meaning that their brains are average for animals of their size. While this is somewhat controversial, some have argued that EQ correlates with the intelligence of a species. However this is hard to determine, because there is no good measure of intelligence in animals. Nevertheless, having a larger EQ does roughly correspond to having a more “developed” brain. For example, carnivores tend to have larger EQs than plant eaters, which is consistent with the fact that hunting is a much more complex task than foraging.

Just as a fun fact, here's a list of animals sorted by number of neurons.

In the following virtual lab, you will use several brain databases to gather data about brain regions from different species and then use these data to draw some conclusions about brain function in these organisms.