The Frontal Lobe

The largest lobe: motor control, speech production, planning, self-control and personality — from the primary motor strip to the prefrontal cortex.

The frontal lobe is the largest of the brain's lobes, making up roughly a third of the cortex in humans. It extends from behind the forehead to the central sulcus.

Movement

Just in front of the central sulcus is the primary motor cortex, a strip that sends commands to muscles on the opposite side of the body, arranged as a body map (the motor homunculus). In front of it, the premotor cortex and supplementary motor area plan and sequence movements — reaching, grasping, and the sequences of a practised skill.

Speech

In most people, Broca's area in the left inferior frontal gyrus is crucial for producing speech and for aspects of grammar. Damage there causes laboured, halting speech (Broca's aphasia).

The prefrontal cortex

The front part of the frontal lobe, the prefrontal cortex (PFC), is greatly expanded in humans. It supports executive functions: holding information in mind (working memory), planning, switching between tasks, inhibiting impulses and following goals. Its lower and inner regions, the orbitofrontal and ventromedial prefrontal cortex, weigh up rewards and risks and help guide social and emotional behaviour. The anterior cingulate cortex nearby monitors conflict and errors.

When it is damaged

Frontal damage often leaves memory and intelligence test scores intact but changes behaviour: difficulty planning and organising, impulsivity, apathy, poor judgement, socially inappropriate behaviour, or rigid thinking. Phineas Gage is the classic historical case. Some forms of dementia — frontotemporal dementia — affect these regions first, which is why personality changes can be the earliest sign.

The prefrontal cortex is one of the last regions to mature, with myelination and synaptic refinement continuing into the twenties — one reason teenagers can be capable but impulsive.