The Frontal Lobe

It is the largest brain structure in humans: the frontal lobe. It occupies the entire front part of the cortex up to the central fissure. Many experts locate the higher mental functions of humans here, some even describe it as the director of the brain, the bearer of our culture, and shower it with further superlatives. And indeed, although large areas of the frontal lobe have motor tasks, its foremost area, the prefrontal cortex, or PFC for short, is repeatedly mentioned in connection with attention, thinking, decision-making and planning, and is regarded as the seat of personality. With such important functions, it is not surprising that the PFC is the structure in the human brain that takes the longest to develop: It takes up to 25 years for the frontal lobe to fully mature.

Let's look at the frontal lobe from back to front, starting just before the central fissure of the cerebrum: this is where the primary motor cortex (the gyrus praencentralis, Brodmann area 4), which plays a key role in voluntary movement, is located. It is organised somatotopically, according to a kind of map that reflects the body. Interestingly, the face – and especially lips and tongue – as well as the hand occupy a strikingly large space, while the body is represented comparatively small. At second glance, it becomes clear why: the muscles in the back or stomach have fairly simple tasks, while the facial expressions, the production of sounds via lips and tongue – speech – or the sophisticated motor skills of the hands are highly demanding. This is reflected in the proportions of the map. Incidentally, it also has a nice detail: the knee, which so often bends the leg by 90 degrees in real, seated life, also does this in the brain: it is represented directly on the medial edge, precisely where the cortex folds inwards.

The primary motor cortex (M1) is considered to be the origin of voluntary motor skills, particularly with regard to speed, direction and strength development. However, it is important to bear in mind that although M1 is the primary motor cortex according to its title, it is integrated into a comprehensive network for controlling movement. The axons of its pyramidal cells form the pyramidal tract. It runs to the spinal cord, where the movement impulses are interconnected to the muscles.

Directly in front, towards the forehead, is the premotor cortex (according to Brodmann area 6 and partly 8), which is involved in complex movement sequences. Like M1 it is organized in the form of a body map, although a special feature is suspected here: the medial – inner – premotor cortex appears to be more involved in planned movements, while the lateral, outer area reacts more to sensory signals – i.e., to what is happening in the outside world. The comparatively small supplementary motor area is located directly in front of the premotor cortex on the surface. The neurons here are involved in the development of the movement plan.

In addition to this whole-body motor area, there are two other specialists, the frontal eye area (Brodmann area 8) and Broca's area (Brodmann area 44): the former controls conscious eye movements, while Broca's area is responsible for the motor skills of speech production. It occurs in one hemisphere only and is usually located on the left in right-handed people.

Depending on their areas of responsibility, disorders become apparent when one of the motor areas is damaged: For example, a stroke in the primary motor cortex can lead to a weakening or a paralysis of the opposite side of the body. If it affects the supplementary motor cortex, movements are no longer initiated - those affected then suffer from a lack of movement: hypokinesia.

As already mentioned, when it comes to what makes a person human, the prefrontal cortex is one of the most promising structures in the brain. It all starts quite mundanely: the olfactory bulb is located on its underside – which rests on the eye socket, the orbit, from which this area is called the orbitofrontal cortex. However, if we look at the PFC from the point of view of its inputs and outputs, its significance can be surmised. The PFC is informed by almost all sensory association cortices, as well as by the hypothalamus, the raphe nuclei and the ventral tegmentum. It is reciprocally connected to the septum, the amygdala, the caudate nucleus and the pons. A prominent fiber connection also leads to the primary motor cortex. Perhaps it was this striking interconnection that led the great British neurologist  John Hughlings Jackson  to assume at the end of the 19th century that all regions of the cerebral cortex must be represented somewhere in the frontal lobe.

The PFC can be roughly divided into the dorsolateral prefrontal cortex (according to Brodmann 9/46) and the orbitofrontal cortex (according to Brodmann areas 10, 11, 47/12, 13, 14 and parts of 45). Both play a special role in the so-called executive functions. These include directed attention – which can also suppress disturbing factors – the organization of complex actions as a sequence of individual steps, planning – including time planning in the form of a meaningful sequence – continuous monitoring, and working memory functions. All of these aspects primarily affect the dorsolateral PFC orbitofrontal, and the emotional and motivational aspects of a decision are also negotiated. And all of these individual points are highly interconnected in terms of their influences and dependencies.

In view of these responsible tasks, the symptoms of damage to the PFC should be massive. This is confirmed by one of the best-known patients in brain research – the American Phineas Gage. He was a railway foreman when he suffered a terrible accident in 1848: a five-centimetre-thick iron bar shot through his left cheek and exited at the top right of his skull. The entire PFC was badly damaged, and it was basically a miracle that Gage survived not only the accident but also the infections that followed. But his personality had changed dramatically, as his doctor at the time, John D. Harlow, noted: Whereas he had previously been known as friendly and reliable, after the accident Gage became opinionated, impulsive and sometimes foul-mouthed. He was also unable to plan properly and was therefore very unreliable.

The clinic is now familiar with several cases like Gage's. For example, damage to the orbitofrontal region can lead to pseudo-depressive disorders. In this case, patients are listless to the point of apathy, reduced in sexual behavior and show little emotion. The pseudo-psychopathic disorder is almost the opposite. These patients show motor restlessness, are distant and uninhibited. Particularly in the sexual sphere, they lose their sense of social conventions and show excessive desire. Today, symptoms such as these are categorized as frontal lobe syndrome, which has different aspects depending on the diverse tasks of the frontal cortex and the high degree of its interconnectedness. The PFC also appears to be affected in schizophrenia.

First published on September 8, 2011
Last updated on October 28, 2024