Hack8.Tour the Cortex and the Four Lobes

Hack 8. Tour the Cortex and the Four Lobes

The forebrain, the classic image of the brain we know from pictures, is the part of the brain that defines human uniqueness. It consists of four lobes and a thin layer on the surface called the cortex.

When you look at pictures of the human brain, the main thing you see is the rounded, wrinkled bulk of the brain. This is the cerebrum, and it caps off the rest of the brain and central nervous system [Hack #7].

To find your way around the cerebrum, you need to know only a few things. It's divided into two hemispheres, left and right. It's also divided into four lobes (large areas demarcated by particularly deep wrinkles). The wrinkles you can see on the outside are actually folds: the cerebrum is a very large folded-up surface, which is why it's so deep. Unfolded, this surfacethe cerebral cortexwould be about 1.5 m² (a square roughly 50 inches on the side), and between 2 and 4 mm deep. It's not thick, but there's a lot of it and this is where all the work takes place. The outermost part, the top of the surface, is gray matter, the actual neurons themselves. Under a few layers of these is the white matter, the fibers connecting the neurons together. The cortex is special because it's mainly where our high-level, human functions take place. It's here that information is integrated and combined from the other regions of the brain and used to modulate more basic functions elsewhere in the brain. The folds exist to allow many more neurons and connections than other animals have in a similar size area.

1.9.1. Cerebral Lobes

The four cerebral lobes generally perform certain classes of function.

You can cover the frontal lobe if you put your palms on your forehead with your fingers pointing up. It's heavily involved in planning, socializing, language, and general control and supervision of the rest of the brain.

The parietal lobe is at the top and back of your head, and if you lock your fingers together and hook your hands over the top back, that's it covered there. It deals a lot with your senses, combining information and representing your body and movements. The object recognition module for visual processing [Hack #13] is located here.

You can put your hands on only the ends of the temporal lobeit's right behind the ears. It sits behind the frontal lobe and underneath the parietal lobe and curls up the underside of the cerebrum. Unsurprisingly, auditory processing occurs here. It deals with language too (like verbal memory), and the left hemisphere is specialized for this (non-linguistic sound is on the right). The curled-up ends of the temporal lobe join into the limbic system at the hippocampus and are involved in long-term memory formation.

Finally, there's the occipital lobe, right at the back of the brain, about midway down your head. This is the smallest lobe of the cerebrum and is where the visual cortex is located.

The two hemispheres are joined together by another structure buried underneath the lobes, called the corpus callosum. It's the largest bundle of nerve fibers in the whole nervous system. While sensory information, such as vision, is divided across the two hemispheres of the brain, the corpus callosum brings the sides back together. It's heavily coated in a fatty substance called myelin, which speeds electrical conduction along nerve cells and is so efficient that the two sides of the visual cortex (for example) operate together almost as if they're adjacent. Not bad considering the corpus callosum is connecting together brain areas a few inches apart when the cells are usually separated by only a millimeter or two.

1.9.2. Cerebral Cortex

The cortex, the surface of these lobes, is divided into areas performing different functions. This isn't exact, of course, and they're highly interconnected and draw information from one another, but more or less there are small areas of the surface that perform edge detection for visual information or detect tools as opposed to animate objects in much higher-level areas of the brain.

How these areas are identified is covered in the various brain imaging and methods hacks earlier in this chapter.

The sensory areas of the cortex are characterized by maps, representations of the information that comes in from the senses. It's called a map because continous variations in the value of inputs are represented by continuous shifts in distance between where they are processed in the cortical space. In the visual cortex, visual space is preserved on the retina. This spatial map is retained for each stage of early visual processing. This means that if two things are next to each other out there in the world they will, at least initially, be processed by contiguous areas of the visual cortex. This is just like when a visual image is stored on photographic negative but unlike when a visual image is stored in a JPEG image file. You can't automatically point to two adjoining parts of the JPEG file and be certain that they will appear next to each other in the image. With a photographic film and with the visual cortex, you can. Similarly, the auditory cortex creates maps of what you're hearing, but as well as organizing things according to where they appear in space, it also has maps that use frequency of the sound as the coordinate frame (i.e., they are tonotopic). And there's an actual map in physical space, on the cortex, of the whole body surface too, called the sensory homunculus [Hack #12] . You can tell how much importance the brain gives to areas of the map, comparatively, by looking at how large they are. The middle of the map of the primary visual cortex corresponds with the fovea in the retina, which is extremely high resolution. It's as large as the rest of the visual map put together.

When the cortex is discussed, that means the function in question is highly integrated with the rest of the brain. When we consider what really makes us human and where consciousness is, it isn't solely the cortex: the rest of the brain has changed function in humans, we have human bodies and nervous systems, and we exist within environments that our brains reflect in their adaptations. But it's definitely mostly the cortex. You are here.