The Frontal Lobe

The puzzle: one lobe, two extremes

Put your finger on the top of your head, a little forward of the crown, and you are roughly above a strip of cortex whose neurons project, some of them without a single relay in the cerebrum, all the way to the spinal cord. Stimulate that strip electrically and a contralateral muscle contracts. Nothing in the brain is more concrete. The output is a movement, and you can watch it happen.

Now move your finger forward, over the forehead, and you are above cortex whose neurons will not move any muscle today. They are holding a rule, a plan, a promise, an intention that may be discharged in an hour or never. Nothing in the brain is more abstract. There is no output you can watch.

These two territories are in the same lobe. They are continuous with each other. And the crucial observation, the one this page is built on, is that they are not scattered: they lie in a fixed order, with the concrete at the back, hard against the central sulcus, and the abstract at the front, and everything in between arranged in sequence along that axis. Anatomy is offering us a hint of extraordinary strength here, and it would be careless to look at it and simply list the parts.

Where the lobe begins and ends

The frontal lobe occupies everything in front of the central sulcus and above the lateral sulcus, and it runs forward to the front pole of the hemisphere. It is the largest of the four lobes described on the lobes of the brain page, taking up roughly a third of the cortical surface, and it has three surfaces that matter clinically: a lateral surface on the outside of the hemisphere, a medial surface facing its twin across the midline, and an orbital or basal surface on the underside, sitting on the roof of the eye sockets.

The two sulcal boundaries are worth taking seriously rather than memorising. The central sulcus is not an arbitrary line drawn by anatomists. On one side of it is the strip that commands the body, on the other the strip that feels it. The single deepest division in the whole cortex, between doing and sensing, has a groove in the surface marking it, and the lobe boundary was placed there because it was the obvious place to put it.

The orbital surface is the one that will matter later, in the section on vulnerability. It is worth noting now that it does not rest on anything smooth. The floor of the anterior cranial fossa, on which the underside of the frontal lobe sits, is ridged and irregular bone.

Primary motor cortex: the output stage

The primary motor cortex, conventionally called M1 or Brodmann area 4, occupies the precentral gyrus. It is the final cortical staging post for voluntary movement: the place where a command, having been assembled elsewhere, leaves the cortex and heads for the spinal cord.

Its output travels in the corticospinal tract, a great bundle of axons that descends through the internal capsule and the brainstem, crosses the midline in the medullary pyramids, and terminates in the spinal grey matter, where it drives, directly or through interneurons, the motor neurons that innervate muscle. The direct connections onto motor neurons are a comparatively recent evolutionary addition and are best developed for the hand. That fact and the human ability to move one finger independently are not unrelated.

M1 contains, in its fifth layer, some of the largest neurons in the human brain: the Betz cells, giant pyramidal neurons whose cell bodies are visible under low magnification and whose axons contribute to the corticospinal tract. They are outnumbered many times over by smaller corticospinal neurons, so it is wrong to say the Betz cells are the pathway, but they are its most conspicuous constituent and they are found in M1 and essentially nowhere else. A cell type unique to a region is a strong clue about what the region is for.

The body is mapped across M1 in an orderly sequence, medial to lateral: the foot and leg over the top edge and down onto the medial surface, then the trunk, arm, and hand along the lateral convexity, then the face, lips, and tongue near the bottom. This is the motor homunculus, and its proportions are famously grotesque: an enormous hand, an enormous mouth, a shrunken trunk and thigh. The distortion is not a curiosity. Cortical territory is allocated according to the fineness of control required, not according to the size of the body part, so the parts you can move with precision get the acreage. The map was worked out in patients undergoing neurosurgery under local anaesthetic by Wilder Penfield and his colleagues, who stimulated the exposed cortex and recorded what moved; their The Cerebral Cortex of Man (1950) is the classic account.

Why M1 has almost no layer IV

Here is a small anatomical fact that, followed carefully, tells you what M1 is for without anyone having to say so.

The cerebral cortex is built of six layers, and the layers have jobs. Layer IV is the principal input layer: it is where sensory information arriving from the thalamus terminates. Layer V is the principal output layer for subcortical targets: it contains the large pyramidal neurons whose axons leave the cortex and descend to the brainstem and spinal cord. That is the general rule for the whole sheet.

Now look at a stained section of the primary visual cortex, an area whose whole job is to receive. Layer IV is enormous, subdivided, and unmistakable. Look at a section of the primary motor cortex, an area whose whole job is to send. Layer IV is thin to the point of near absence, which is why M1 is described as agranular cortex, while layer V is thick and studded with Betz cells.

This is not a coincidence to be memorised. It is a prediction that falls straight out of the layer logic, and it is confirmed. A region that mainly sends should have a fat output layer and a thin input layer, and M1 does. Beautifully, the same logic then predicts something else: since the prefrontal cortex does not send commands to muscles but does receive vast quantities of processed information from the rest of the brain, it should look the opposite way, granular rather than agranular. And it does. The cellular architecture of the frontal lobe changes as you walk forward through it, and it changes in exactly the direction the function requires.

Premotor cortex and the supplementary motor area

In front of M1, still agranular, lies a band of higher motor cortex, area 6 in Brodmann's scheme. It divides into a lateral part, the premotor cortex, and a medial part, the supplementary motor area or SMA, which continues onto the inner surface of the hemisphere. Both project to M1 and both also send fibres of their own into the corticospinal tract, so neither is merely advisory. But they are engaged by different kinds of movement, and the difference is instructive.

The dissociation is clean enough to be visible in patients. Damage to the SMA can produce a striking reluctance to initiate movement of the opposite limb, and in severe medial frontal lesions, akinetic mutism: a patient who is awake and appears aware but neither speaks nor moves spontaneously. Yet the same patient may respond when prompted from outside. The internal generator is gone; the cue-driven route survives.

Notice, before moving on, what has happened to the level of description. M1 codes the movement being made. Premotor and SMA code a movement about to be made, and they code it in terms of its purpose, its sequence, or the object it is aimed at, rather than in terms of which muscles will contract. We have taken one step forward through the lobe and one step away from the muscle.

The readiness potential, handled carefully

The SMA has an unusual place in the history of the free will debate, and since this page is about anatomy rather than metaphysics, it is worth stating precisely what the anatomy does and does not license.

The observation is real and replicated. If you record the electrical activity of the scalp over the medial frontal cortex while a person makes voluntary, self-paced movements, a slow negative drift, the readiness potential, builds up before the movement begins. It was first described in the 1960s by Kornhuber and Deecke. Benjamin Libet's contribution in the 1980s was to ask participants also to note, using a rapidly moving clock, the moment at which they became aware of the intention to move. The reported moment of conscious intention fell after the onset of the readiness potential and before the movement itself. The finding, that brain activity associated with an action can be detected before the person reports deciding to act, has been reproduced many times.

It belongs on this page for one reason: it is the clearest experimental sighting of the SMA doing its job. A region that starts working before an internally generated movement, and does not do so before an externally cued one, is behaving precisely as the anatomy says it should.

The frontal eye fields

Just in front of the premotor cortex, on the lateral surface, sits a small region devoted entirely to one class of movement: the frontal eye fields. They control voluntary gaze, the deliberate saccade that takes your eyes to a chosen point.

They deserve their place on this page for two reasons. First, the pattern of their damage is a small, exact illustration of everything above. A lesion of one frontal eye field leaves the patient with a transient inability to look voluntarily towards the opposite side, and with the eyes deviated towards the side of the lesion, because the intact field on the other side is now unopposed. Reflexive eye movements, following a moving target, survive. Voluntary gaze is lost while cue-driven gaze persists, the same dissociation that runs through the rest of the frontal lobe.

Second, they show that "motor" in the frontal lobe does not only mean limbs. The lobe contains an output stage for every kind of voluntary action the body can take, including moving the eyes and, as we come to next, moving the mouth in the service of speech. For how gaze is directed and how it interacts with attention, see attention and the brain and the visual system.

Broca's area and the model that no longer holds

Low on the lateral surface of the left frontal lobe, in the inferior frontal gyrus, is the region named after Paul Broca, who in the 1860s reported patients with damage there who had lost the ability to speak fluently while apparently retaining the ability to understand. It sits immediately in front of the part of the motor cortex that controls the lips, tongue, jaw, and larynx, which is a suggestive piece of geography: an area concerned with assembling speech, placed directly upstream of the area that moves the speech apparatus. The gradient again.

Now the correction, which this library makes wherever the classic model appears. The neat story, damage to Broca's area causes Broca's aphasia, does not survive contact with the modern lesion evidence. Damage confined to Broca's area alone rarely produces a lasting Broca's aphasia: it tends to produce a transient disturbance from which patients recover substantially. The enduring syndrome is associated with larger lesions that extend into the surrounding cortex, the underlying white matter, and often the insula and neighbouring structures. What Broca's patients had, on re-examination of the preserved brains, were lesions considerably larger than the area that bears his name.

The wider point is that language is not a chain of two boxes joined by a cable. It depends on distributed dorsal and ventral streams running across left frontal, temporal, and parietal cortex, and on the white matter tracts that connect them, so a lesion's effect depends as much on which connections it severs as on which patch of grey matter it destroys. The language and the brain page sets out what has replaced the classic model.

Keep the geography, though, because the geography is real and it is doing work. Whatever the precise division of labour, the frontal contribution to language sits in front of the mouth region of the motor cortex, one step further from the muscle, exactly where the gradient predicts an area concerned with what to say rather than how to move the tongue should be found.

The prefrontal cortex, in brief

Everything forward of the motor and premotor areas is the prefrontal cortex. It occupies roughly the front third of the lobe, it is granular where the areas behind it are agranular, and it is the region whose damage changes not what a person can do but who they are. It has its own full treatment on this site, and this page will not duplicate it. Here is the map and the single insight, and then the links.

The insight worth carrying away is this: the prefrontal cortex is defined by its connections, not by what it does with muscles, because it does nothing with muscles. It receives from every sensory modality through association cortex, exchanges signals with the parietal lobe, loops through the basal ganglia and the thalamus, and is reciprocally connected with the limbic system. A region wired to everything and committed to nothing is built to coordinate. That is what it does.

For the subdivisions in detail, the frontal lobe syndromes, the case of Phineas Gage and what it does and does not show, and why prefrontal damage so often spares IQ, see the prefrontal cortex. For the control processes themselves, inhibition, updating, and shifting, see executive function and working memory.

The principle: the lobe is organised by abstraction

We now have the whole series. Walk it from back to front and watch what changes.

1. Primary motor cortex

   Codes this movement, now. The time horizon is tens of milliseconds. The representation is of force and direction of a limb. It is one synapse, in places, from a spinal motor neuron. Damage produces weakness or paralysis of specific body parts on the opposite side.

2. Premotor cortex and SMA

   Code a movement about to be made, and code it in terms of its goal or its place in a sequence rather than its muscles. The time horizon is a second or so. Damage does not paralyse; it disrupts the shaping of movement to objects, or the ability to start a sequence spontaneously.

3. Broca's area and the frontal eye fields

   Code what to say and where to look: outputs still, but outputs specified at the level of the message and the target rather than the muscle. Damage does not paralyse the tongue or the eye. It disturbs the assembly of speech and the voluntary direction of gaze.

4. Prefrontal cortex

   Codes a goal, a rule, a plan, a value. The time horizon is minutes, hours, years. There need be no movement at all today. Damage produces no weakness whatsoever, and instead a change in the conduct of a life.

Two things vary together along that axis, and they vary monotonically. The abstraction of the representation rises: from a muscle force, to a movement, to an action, to a goal. And the time horizon lengthens: from milliseconds, to seconds, to hours. The frontal lobe is a ramp, running from the most concrete function in the brain to the most abstract, and the ramp has a direction: forward is more abstract, backward is more concrete.

Three further facts fall out of the same principle, which is the test of a good one.

A caution to keep the claim honest. The gradient is a strong organising principle and it is well supported by lesion, imaging, and anatomical evidence, but it is a gradient, not a staircase with labelled steps. The boundaries between areas are gradual, the areas overlap in function, and precisely how many levels of abstraction the frontal cortex implements, and whether the hierarchy is strictly ordered along the front-to-back axis, remains actively debated. Take the direction of the ramp as established. Treat any confident claim about the exact number of rungs with more caution.

Why the frontal lobe is so vulnerable

Frontal damage is over-represented in traumatic brain injury out of all proportion to the lobe's share of the brain. This is not because the frontal lobe is delicate. It is because of where it is and what it sits on. Three factors compound.

Size. The frontal lobe is the largest of the four. All else being equal, a lesion landing at random is likelier to land in it.

Position. It is at the front. In a head-on collision, in a fall forward, in an assault to the face, the frontal lobe is where the force arrives. Worse, it is also damaged by blows to the back of the head, through the mechanism known as coup-contrecoup injury: the brain, floating in cerebrospinal fluid, is accelerated by the impact, strikes the skull at the site of the blow, and then rebounds and strikes the opposite inner wall. A blow to the occiput therefore produces a contrecoup injury at the front. The frontal lobe is hit both ways. For the fluid the brain floats in and why it does not fully protect against this, see ventricles and cerebrospinal fluid.

The floor it rests on. This is the factor most often left out, and it is the most important. The underside of the frontal lobe, the orbitofrontal surface, does not lie on a smooth pad. It lies on the floor of the anterior cranial fossa, which is ridged, irregular, and, at the back, terminates in the sharp bony edge of the lesser wing of the sphenoid. When the head decelerates suddenly, the brain slides across that surface. Soft tissue moving over sharp bone tears, and the orbitofrontal cortex is what tears.

The same anatomy explains why frontal syndromes are so often missed. The deficits are of regulation, not of capacity, and a clinic is a structured environment that supplies regulation for free.

Three persistent myths

Related topics