ENCYCLOPEDIA BRITANNICA:

The gray matter of the cerebral cortex usually is divided into four lobes, roughly defined by major surface folds; sometimes the limbic system, or limbic lobe, is considered to be a fifth lobe.

The frontal lobe contains control centers for motor activity and speech, the parietal for somatic senses (touch and position), and the temporal for auditory reception and memory. The occipital lobe at the back of the brain holds the brain’s major visual-reception area.

The frontal lobe is concerned with many of the components of intelligence (foresight, planning, and comprehension), with mood, with motor activity on the opposite side of the body, and (in the case of the dominant hemisphere) with speech production.

Frontal lobe damage can affect people in any of several ways, and the results are at once subtle and drastic. On the one hand, they may have difficulty initiating behaviour, in extreme cases being virtually unable to move or speak but more often simply having difficulty in getting started on a task. On the other, they may perseverate, being apparently unable to step a behaviour once started. Rather than appearing apathetic and hypoactive, they are uninhibited, rude, and boorish. Such people may also have difficulty in planning and problem solving and may be incapable of creative thinking.

Irritability and personality change are also frequently seen after frontal lobe damage.