Brain Anatomy

The frontal lobe is one of the four lobes of the human brain. It is the most anterior part of the brain, located above the eye sockets. It is located anterior to the central sulcus, and superior to the lateral fissure. The frontal lobe is responsible for a number of higher-order functions, such as planning and inhibition, and is considered to be the seat of working memory. It is the most recently evolved part of the brain. The human frontal lobe is far more developed than in other animals, even more than the great apes, our closest living relatives. In addition, the frontal lobe is the last part of the brain to mature developmentally. The frontal lobes of human children do not mature fully until around age four. The prefrontal cortex, the most anterior section of the frontal lobes, is the most evolutionarily and developmentally recent part of the frontal lobes. Its major function seems to be inhibition, although there are clues that it may also be involved in memory.

As discussed in the section on methodology, one way to learn about the function of a particular area is to study people who have damage in that area of their brains. In the case of the frontal lobe, one of the most famous brain-damage cases is that of Phineas Gage. By studying what he lost after his brain injury, early neuropsychologists learned a great deal about what the frontal lobe does.

Phineas Gage was a construction worker in the nineteenth century. He was known as a kind, even-tempered man, and beloved by his friends and family. In a freak accident, a steel rod was propelled by an explosion into Gage's cheek, through the frontal area of his brain, and out through the top of his skull, landing several yards away. Amazingly, Gage remained conscious, and doctors examining him could find nothing immediately wrong with him. However, over the following days and weeks, distinct changes became evident in his personality. This previously gentle man became quick to anger and unpredictable. He used profane and vulgar language for the first time and frequently made inappropriate comments. In addition, he seemed unable to make coherent plans. From this evidence, doctors and psychologists deduced that the damaged area, the frontal lobe, must have been responsible for inhibiting socially inappropriate behavior and violent emotions, as well as forming plans for future actions. Indeed, clinical studies of modern patients with damage to the frontal lobes provide evidence that memories concerning social context and future plans reside in the frontal lobe. Gage's skull now rests in a museum at Harvard Medical School, and he remains one of neuropsychology's most studied subjects.

One neuropsychological test often used to assess frontal lobe function is the Wisconsin Card Sort task. In this task, the subject is asked to sort a number of cards according to a rule. Each card typically contains three aspects, such as color, number, and shape. The subject must sort the cards according to one of those aspects, but he is not told which one; he must discover it through trial and error. Eventually, the subject might figure out that the rule is to sort by color, so he would place all the red cards in one pile, all the yellow cards in another pile, and all the blue cards in a third. After the subject has sorted several cards correctly, the experimenter changes the rule, without telling the subject. Now, the rule might be to sort by shape, so all the circles, squares, and stars must be separated into three piles. This requires a shift in the subject's understanding of the task; he must inhibit the old "color" rule and switch to the new "shape" rule. Subjects with damage to their frontal lobes have difficulty figuring out the sorting rule, because they are unable to use their memories of previous right or wrong guesses to guide their present behavior. They also show difficulty in switching rules, since they cannot inhibit the previously correct "color" rule. Instead, they perseverate on the old rule, continuing to sort by color regardless of the experimenter's feedback. This failure of normal inhibition is the primary finding of this task. Children, schizophrenics, patients with organic frontal lobe damage, and monkeys with ablations of their frontal lobes all perform poorly on the Wisconsin Card Sort. Specifically, the prefrontal area (the most anterior part of the frontal lobe) seems to be crucial to inhibition.

In addition to their role in inhibition and planning, the frontal lobes also mediate emotion. Evidence suggests an asymmetry between the hemispheres of the frontal lobes; the left lobe seems to process more positive emotions, while the right lobe appears to process more negative ones. People who have suffered damage to only the left frontal lobe report fewer positive emotions since their injury. In contrast, people with damage to the right frontal lobe report a decrease in negative emotions. Using an EEG to record activity in each lobe, researchers have found that people show more left-frontal activity when watching happy movies, such as a puppy playing with flowers, and more right- frontal activity when watching emotionally negative movies, such as medical pictures of an amputated limb. In addition, ten-month-old babies show more left-frontal activity when approached by their mother, and more right- frontal activity when approached by a stranger.

The role of the frontal lobes in memory is not yet conclusive, but emerging evidence suggests the presence of another asymmetry between the hemispheres. An fMRI study examining the activity of the frontal lobes during encoding of new memories found increased activity in the left frontal lobe but not the right (specifically, the activity was located in the left prefrontal area). In addition, the study found increased activity in areas of the temporal lobe, which is not surprising since the hippocampus, a structure well-known for its involvement in memory, and its adjacent pathways are located in the temporal lobe.

In addition to all the complex, high-level functions described above, the frontal lobe contains one crucial area that is functionally separate from the rest of the lobe: the primary motor cortex. The motor cortex lies in the most posterior section of the frontal lobe, adjacent to the central sulcus. Nerves from the motor cortex carry signals to the brainstem and spinal cord, controlling and directing complex movements. Motor neurons have their somas in the motor cortex, but their axons can be very long, reaching down into the spinal cord. Nerves going to similar places tend to lie near each other in the cortex, so that different regions of the cortex represent different parts of the body. The motor cortex is plastic, meaning that it can change as a result of experience; depending on how much each body part is used, the region representing that part can grow larger or smaller. Because the hemispheres control the body in a contralateral fashion, as seen in the section on two hemispheres, nerves from the right motor cortex send signals to the left side of the body, while nerves from the left motor cortex send signals to the right side. Damage to the motor cortex can cause paralysis in parts of the body corresponding to the affected areas in the cortex. The secondary motor cortex, sometimes called the premotor cortex, is also located in the frontal lobe. It is activated when we mentally rehearse actions before actually doing them.