The frontal lobe occupies the front third of each hemisphere. It is the largest of the four lobes in humans and the most recently evolved. It does motor work, planning work, and the executive work that defines higher cognition.
Moving from back to front within the frontal lobe, we encounter functional subdivisions arranged in a meaningful order.
Just anterior to the central sulcus sits the primary motor cortex (M1). Like the somatosensory cortex on the other side of the central sulcus, M1 has its own homunculus — a topographic map of the body, with hands and face disproportionately large because they require fine motor control. M1 sends commands down through the corticospinal tract to the spinal cord. Damage produces contralateral weakness in the body part represented at the lesion site.
Anterior to M1 sit the premotor and supplementary motor areas. These regions plan movement — they generate motor programs that M1 then executes. Premotor cortex handles externally guided movements (reaching for an object you see), while supplementary motor area handles internally generated sequences (the sequence of movements you decide to make on your own). Damage to these areas does not produce frank weakness but produces difficulty initiating or sequencing movements.
Anterior to the premotor and supplementary motor areas sits the prefrontal cortex, the front third or so of the frontal lobe. This is where executive function lives. The prefrontal cortex plans, sequences, inhibits, holds information online in working memory, weighs consequences, and coordinates the rest of the brain in service of goals. We will spend all of Stage 8 inside the prefrontal cortex.
Two more frontal regions deserve mention. Broca's area, in the inferior frontal gyrus of the dominant hemisphere, handles language production. Damage produces Broca's aphasia — effortful, halting, agrammatic speech with preserved comprehension. The frontal eye fields, in middle frontal gyrus, control voluntary saccadic eye movements.
When you assess a patient with a possible frontal lesion, the bedside exam works through these functions systematically — strength testing for M1, motor sequencing tests for premotor and SMA, Luria's three-step task or other executive measures for prefrontal cortex, language assessment for Broca's area. The pattern of preserved and impaired function localizes the lesion within the frontal lobe.
Hold the geography. The frontal lobe is the lobe of doing, planning, and deciding. Damage there does not affect what you perceive — it affects what you can choose to do with what you perceive.