The brain is the main organ involved in consciousness, thought, emotion, motivation, learning, memory, judgment, control of basic biological functions, and almost everything else that makes a person human. Structurally, it is more complex than any other natural object yet discovered; the difﬁculty of even approximating the complexity and the sheer number of connections in the brain is one of the major obstacles to the creation of artiﬁcial intelligence.
For purposes of discussing structure and function, it may be useful to think of the brain in sections: the hindbrain (or brain stem), the midbrain, and the forebrain. These terms are based on vertebrate biology in general rather than on humans speciﬁcally, which is why the lowest portion is called the hindbrain (because in creatures that do not walk upright, it is located further back than the rest of the brain rather than beneath it).
The Hindbrain (Brain Stem)— The brain stem is the oldest, most primitive part of the brain and is responsible primarily for the body’s automatic survival functions. At its base, it is simply a swelling of the spinal cord as it enters the skull. There are three crucial structures here: the medulla oblongata, the reticular formation, and the cerebellum. The medulla is responsible for what are sometimes called the vegetative functions, involuntary actions required to keep the body alive. The medulla entirely controls heart rate and largely controls breathing, swallowing, and digestion. While it is possible to exert some control over breathing, that control is quite limited. Barring any serious respiratory problem, for example, a person is capable of holding his or her breath, but not capable of holding it indeﬁnitely. Eventually the person will lose consciousness, at which point the medulla will carry on doing its job and start up the breathing again. The medulla is also the place where neurons from the right side of the body cross over to the left side of the brain, in the well-known phenomenon of contra laterality. Because of this crossing over, sensory information from the left side of the body is processed in the right hemisphere of the cerebral cortex (in the forebrain), and vice versa; and the two sides are controlled by contralateral signals from the brain as well.
The reticular formation (or reticular activating system) is a network of neurons that extends from the spinal cord all the way up to the thalamus. Its primary function is the regulation of overall level of brain activity, and so it is also somewhat involved in regulation of sleeping and wakefulness.
Cerebellum means “little brain,” inspired by the cerebellum’s resemblance to the whole brain. Its primary function is the coordination of voluntary movement and gross motor function, especially things that are done automatically. Any ﬁnely coordinated movements that require following a certain sequence with a certain timing, such as playing a sport, dancing, or playing a musical instrument, require the help of the cerebellum. Damage to the cerebellum can result in difﬁculty walking, difﬁculty with balance, and an inability to use the hands without shaking, among other problems.
The Midbrain —The midbrain is the area immediately above the brainstem but below the forebrain, and it is the home of some nuclei, clusters of cell bodies, that govern reﬂexive behaviors. Two important structures are here: the superior and inferior colliculi. There are two of each, one in each hemisphere, with the superior colliculi located on top of the inferior colliculi.
The inferior colliculi are involved in integrating auditory input with automatic movement; they are responsible for the tendency to automatically jerk the head around in the direction of a sudden, unexpected noise.
The superior colliculi serve a similar function, associated with visual input. They are the reason for turning quickly in response to sudden light or movement at the corner of the visual ﬁeld.
The Forebrain —The forebrain includes both the cerebral cortex and a number of very important smaller structures located beneath it, known collectively as the limbic system. The various elements of the limbic system are involved in regulation of emotion, motivation, learning, and memory formation, among other things. The key structures in the limbic system are:
- Hippocampus—was named because an early dissector felt that it resembled a seahorse (the Greek meaning of the name). The hippocampi, like all limbic system structures, come in pairs, one for each hemisphere. They serve a vital function in the consolidation of new memories. Loss of the hippocampus to injury or surgery can result in a permanent case of anterograde amnesia in which the person is unable to form any new long-term memories.
- Amygdala—An almond-shaped cluster of neurons beneath each hemisphere, the amygdala appears to be involved primarily in regulation of anger, aggression, and fear. Early animal studies using cats indicated that electrical stimulation of one area of the amygdala would cause the cat to prepare for attack. Stimulation of a slightly different area of the same structure will cause that same cat to cower in fear when placed in a cage with a mouse.
- Thalamus—Its extremely important primary job involves relaying sensory input to the correct areas of the cortex to be processed. Information from the inner ear, for example, must be sent to the area of the cortex that processes auditory information, whereas visual input must go to the cortical area responsible for visual processing. The importance of this function becomes very clear when it ceases to operate properly, as can sometimes happen under the inﬂuence of psychedelic drugs. It is not unusual for a person who has taken LSD to claim to be “seeing” sounds, or “hearing” colors. Oddly enough, in a sense they really are because the thalamus has relayed the sensory information to the wrong place, a phenomenon known as synesthesia. “Smelling” sounds or colors is never reported because the sense of smell does not pass through the thalamus.
- Hypothalamus—Its name means “below the thalamus,” and that is where it is located. Although tiny, this structure is known for its involvement in a wide range of human activities, partly due to its close proximity to the pituitary gland, which governs the activities of all hormone systems in the body. Through selective lesioning, scientists have identiﬁed neural clusters that inﬂuence or regulate hunger, thirst, body temperature, sexual behavior, and emotional reactions, as well as pain and pleasure.
Above the limbic system is the heavily convoluted outer surface of the brain, known as the cortex. If any area of the brain can be said to be the part that truly distinguishes human brains from all others, it would be the cortex (also called the cerebrum, neocortex, or cerebral cortex). All of what are considered the higher mental functions, reasoning, memory, language processing, musical ability, judgment, etc., emanate from the cortex, which makes up some 80 percent of the total tissue mass of the brain. It is widely believed by the general public that it is the size of the human brain that makes us so advanced, but in fact it is the convolutions themselves that allow our high degree of intelligence. Brain size is closely allied to body size, so certainly there are brains larger than man’s (consider a blue whale, for example); but none are more complex. The convolutions allow a much larger surface area to exist than would otherwise be possible, given the size of a human head. From an evolutionary perspective, as human brains got more complex, there was a ﬁrm upper boundary on how large they could grow, given that a skull still has to ﬁt through a pelvis for birth to occur. Therefore, the cerebral cortex became more wrinkled, allowing more surface area to ﬁt into the same space, in much the same way as a large piece of paper folded up small enough to ﬁt in a shirt pocket still has the same amount of writing on it as it did before.
The cortex is sometimes referred to as gray matter, because the cell bodies that make up most of it are that color. Much of the other brain tissue, as well as nervous system tissue in the body, looks white instead, due to the myelin covering on the axons of neurons.
The gray matter is divided into two separate hemispheres, which communicate with each other via a thick bundle of nerve ﬁbers called the corpus callosum. Each hemisphere is further subdivided into four lobes:
- The frontal lobes appear to be the primary seat of judgment, reasoning, motivation, and decision-making; and they are also the location of the primary motor cortex, or motor strip. This is the cortical area that controls all aspects of ﬁne motor movements.
- The parietal lobe, located directly behind the frontal lobe, is home to the primary somatosensory cortex, which represents touch sensations from all parts of the body. More sensitive areas of the body have a larger amount of cortex devoted to them than less sensitive areas.
- The temporal lobe is located directly beneath the parietal lobe, and is the home of auditory processing, as well as a large proportion of verbal long-term memory storage. The temporal lobe is also responsible for modulation of fear, aggression, and sexuality (along with the limbic system).
- The occipital lobe, behind the parietal lobe, is where visual information is processed, and thus also where visual thoughts and memories occur and are stored. Damage to speciﬁc areas in the occipital lobe can produce visual agnosia, a failure to recognize familiar objects. In some cases a more speciﬁc form, prosopagnosia, can occur, in which the person loses the ability to recognize or distinguish between faces.
There are some clear functional differences between the left and right hemisphere, starting with language functions, which are largely localized to the left hemisphere. Language production (the ability to speak), for example, is controlled by Broca’s area, a small region of the left frontal lobe adjacent to the primary motor cortex. Language comprehension, along with memory of word meanings, is largely conﬁned to the left temporal lobe (see Aphasia). The left hemisphere also appears to be responsible for logical thought, along with singing and writing. Visual and spatial processing, along with perception of rhythm and abstract thought, appear to be primarily right hemisphere functions. Contrary to the spate of popular books arguing that some people primarily use their right hemisphere (“right-brained”) while others rely primarily on the left (“left-brained”), both hemispheres are active in all people at all times, barring serious head injuries. Smooth communication between the hemispheres is a result of the corpus callosum (see Split-Brain Surgery).
The pop-psychology notion that there are gender differences in the brain does have some truth to it. It has long been observed that men’s brains tend to be slightly larger, for example, a difference that a number of nineteenth and twentieth-century thinkers took to indicate the clear intellectual superiority of men. In fact, the difference can be accounted for entirely by the larger average size of men’s bodies. When all other factors are accounted for, however, researchers have found evidence that a small area of the parietal lobe, which is intimately involved in spatial processing and mathematics, tends to be larger in male brains. Other recently documented differences tend to be far subtler, involving different overall response patterns to similar stimuli. When men are shown visual stimuli, for example, emotional centers in the amygdala are activated that are not activated when women view the same images. Pain responses also involve activation of different cortical areas across the two genders. This sort of research, made possible by the availability of PET and MRI scans (see Brain Imaging Techniques), is still in its infancy; there is much more to learn about the brain and its functioning (see also Brainwashing; Chemical Imbalance; Prefrontal Lobotomy).
- Johnson, K. A., and Becker, J. A. (2004). The Whole Brain Atlas. www. med.harvard.edu/AANLIB/home.html, 2004.
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