Your Brain, Region by Region

Chapter 1 Cerebral cortex

The lobes of the brain

Four lobes on each side, separated by a few deep grooves. Pull them apart, then see how each lobe got its job description.

12 min read · 5 interactive figures

The left side of the schematic brain with the four lobes in color. Slide Pull the lobes apart to separate them along the central, lateral and parieto-occipital boundaries; drag to turn the brain, and tap a lobe to read what it does.

Each half of the brain has four lobes: the frontal lobe at the front, the parietal lobe on top behind it, the temporal lobe low on the side, and the occipital lobe at the back. Three deep grooves mark most of the borders. Roughly, the frontal lobe plans and moves, the parietal lobe feels, the temporal lobe hears and remembers, and the occipital lobe sees[1][2].

That sentence is the summary everyone memorizes, and it is true enough to be useful. This chapter shows where it comes from: the landmarks that divide the cortex, the evidence behind each job description, and one famous experiment you can rerun in which two physiologists lowered an electrode into the visual cortex and heard a single cell answer a line of light.

The cerebrum and its wrinkled surface

Most of what you see of a brain is the cerebrum, split down the middle by a deep cleft, the longitudinal fissure, into a left and a right hemisphere. A thick band of nerve fibers, the corpus callosum, joins the two halves at the bottom of the cleft[3]. Over all of it lies the cerebral cortex, a thin sheet of gray matter, the cell bodies of neurons, folded into ridges called gyri and grooves called sulci.

The folding is a packing solution. The skull is limited in size, partly by the birth canal, and a crumpled sheet fits far more cortex into it than a smooth one could. OpenStax's anatomy textbook puts it plainly: peeled off and laid flat, the gray matter of the cortex would cover roughly one square meter[3]. Because the brain grows in a regular sequence before birth, everyone ends up with a similar pattern of large folds, and anatomists use the biggest of them as landmarks. established

Where one lobe ends and the next begins

Turn the brain at the top of the page so that you see its left side. Three grooves do most of the dividing[3][2]:

  • The central sulcus runs from the top of the brain down and forward. Everything in front of it is frontal; the lobe behind it is parietal.
  • The lateral sulcus (also called the Sylvian fissure) is the long, deep groove along the side. Below it sits the temporal lobe, a thumb-shaped lobe whose tip points forward.
  • The parieto-occipital sulcus is clearest on the inner, midline surface of each hemisphere. Behind it lies the occipital lobe.

On the outer surface there is no clean groove between the occipital lobe and its neighbors. Anatomists draw an imaginary line from the parieto-occipital sulcus down to a small notch on the lower edge of the hemisphere, and that is what the model does too. OpenStax notes that the lack of an obvious border fits the way these regions work together[3]. Slide Pull the lobes apart in the first figure to see the four pieces separate along exactly these lines.

Lobes are a map of geography, not of function. The Queensland Brain Institute's summary is a good one to keep in mind for the rest of this chapter: most brain functions rely on many regions working together across the whole brain, but each lobe still carries out the bulk of certain functions[2]. established

Frontal lobe: moving, planning, speaking

The frontal lobe is the largest of the four. Its back edge, the strip of cortex just in front of the central sulcus, is the precentral gyrus, the primary motor cortex. Its neurons send the commands that move your skeletal muscles, and they are laid out in order as a map of the body[4]. In front of it lie the premotor area, which plans movements, and the frontal eye fields, which steer your gaze[3].

Low on the frontal lobe, in most people only on the left, is Broca's area, needed to produce speech[3]. People with damage there usually understand what is said to them but speak with great difficulty[1]. The front of the lobe, the prefrontal cortex, supports the functions that are hardest to pin down: working memory, planning, judgment, the regulation of emotion, and the things we call personality[3][2]. The most famous evidence for that last claim is a railroad foreman who survived an iron bar through his left frontal lobe in 1848; his case has its own chapter.

How do we know the motor strip is a body map? In the 1930s the Montreal neurosurgeon Wilder Penfield operated on people with epilepsy while they were awake under local anesthesia, and touched points on the exposed cortex with a weak electric current. Stimulating the precentral gyrus made a part of the body move; stimulating the gyrus just behind the central sulcus made the patient feel a tingle in a part of the body[5][4]. Try it.

Figure 1 A section through one hemisphere along the central sulcus, seen from the front with the midline on the left. Drag the electrode along the strip, or use the slider: each body part is drawn beside the strip at the size Penfield and Rasmussen's 1950 drawings give it, so the figure is a small homunculus. Switch between the motor strip (frontal lobe) and the sensory strip just behind it (parietal lobe). The sizes are a reading of the drawing, not measurements.#

Two things stand out. The map runs upside down and inside out: the feet are deep in the cleft between the hemispheres, and the face is low on the side. And the body is drawn at the wrong scale: the hand and the mouth take up far more cortex than the trunk or the legs, because they are moved and felt with far more precision[4]. The homunculus page has the full story, including recent evidence that the strip is not one continuous map after all.

Parietal lobe: touch and the map of space

Just behind the central sulcus, on the postcentral gyrus, lies the primary somatosensory cortex (Brodmann's areas 1, 2 and 3). It is where touch, pressure, pain, itch, vibration and the sense of where your limbs are (proprioception) first reach the cortex[3]. It carries its own body map, a mirror of the motor map in front of it: genitals, feet and lower legs on the inner face of the gyrus, then hips, trunk, arms and hands, then the face, low on the side[6].

The rest of the parietal lobe combines the senses into a sense of space: where things are relative to your body, and where your body is. A simple clinical test of parietal function is two-point discrimination: with the eyes closed, can you tell whether one point or two is pressing on your skin? Fingertips can resolve points that are close together; the back cannot[2].

Temporal lobe: hearing, language and memory

The temporal lobe holds the primary auditory cortex (Brodmann's areas 41 and 42), on the upper (superior) part of the lobe, next to the lateral sulcus[3]. Around it lie areas that make sense of what you hear: Wernicke's area, usually on the left, is needed to understand speech. People with damage there can still speak fluently, but what they say makes little sense and they cannot follow what others say[1][7].

The lower surface of the temporal lobe helps recognize complex visual things such as faces and scenes, and its inner (medial) part contains the hippocampus, essential for forming new long-term memories[2][3]. That is why some memory disorders follow damage to the medial temporal lobe.

Where vision lands

Your eyes are at the front of your head. Where do their signals first reach the cortex?

Question 1 Prediction: where vision reaches the cortex#

The signals travel the whole length of the brain. The primary visual cortex, also called V1 or area 17, sits at the back of the occipital lobe and wraps onto its inner face along the calcarine sulcus. Areas 17 and 18 are responsible for primary visual perception[3]; from V1, the information goes to further visual areas that work out depth, distance, location and the identity of what you are looking at[2].

Figure 2 The same model with the functional areas named in this chapter: primary motor and somatosensory cortex on either side of the central sulcus, the prefrontal cortex, Broca's and Wernicke's areas on the left, the auditory cortex and V1. It opens with V1 highlighted; turn the brain to Back or Inside to follow V1 onto the inner face of the occipital lobe, or pick another area under Highlight.#

Listening to a cell in the visual cortex

Knowing that vision lands in the occipital lobe says nothing about what the cortex does with it. The answer started to come in 1958, in a laboratory at Johns Hopkins University in Baltimore, where David Hubel and Torsten Wiesel were recording from single neurons in the visual cortex of anesthetized cats[8]. A fine tungsten electrode picked up each nerve impulse of the cell nearest its tip, and an audio monitor turned the impulses into clicks.

For weeks the cells ignored the spots of light they projected onto the cat's retina. Then, during one especially stable recording, a cell suddenly "went off like a machine gun." It had nothing to do with the spot. As Hubel put it in his Nobel lecture, the edge of the glass slide they were inserting cast "a faint but sharp shadow, a straight dark line on a light background. That was what the cell wanted, and it wanted it, moreover, in just one narrow range of orientations"[8]. Cells in the cat's striate cortex (V1) respond to lines and edges at particular orientations, not to spots or diffuse light[9][10]. established

The next figure puts you at the bench. The bar on the dark screen is what the cat sees; the electrode advances at a slant through the cortex, meeting a new cell every 50 micrometers.

Question 2 Prediction: the next cell down the track#
Figure 3 Rerun the experiment. Turn the bar by dragging its round handle or with the first slider; the trace and (if you switch on Sound) the clicks show how fast the cell at the electrode tip is firing. When a cell fires hard, its favorite orientation is drawn on the track. Advance the electrode with the depth slider and find the next cell's favorite.#

Two facts carry most of the explanation. Each cell responds to a narrow range of orientations: a line at right angles to the best one has no effect at all, and neither does light spread evenly over the whole screen[8]. And neighboring cells prefer neighboring orientations. In 1961, recording from the visual cortex of a spider monkey along a slanting track 2.5 millimeters long, Hubel and Wiesel found the preferred orientation shifting "about 10° for every advance of 50 µm"; across many penetrations, in cats and monkeys, the direction of the shift reversed "on the average about once every millimeter"[8]. The cortex is organized into orientation columns, so that every small patch of the visual field is examined at every orientation by a neighborhood of cells. The figure's track borrows that monkey track's length and its 10° step, and has two reversals.

Hubel and Wiesel shared the 1981 Nobel Prize in Physiology or Medicine with Roger Sperry, whose split-brain studies are the subject of Chapter 3[11].

Below the cortex

The lobes are only the outer layer. Under the cerebrum, the thalamus relays every sense except smell on its way to the cortex: the visual signals that reach V1 pass through it first[3]. Below and behind it are the brainstem, which connects the brain to the spinal cord and keeps breathing and heartbeat going, and the cerebellum, which corrects movements while they happen. Both are the subject of Chapter 2.

How the job descriptions were written

Every job in this chapter was first assigned by switching something off or on and watching what changed. Damage was the oldest method: a stroke, a tumor or an injury in one place, and a specific loss that followed (speech after damage to Broca's area, comprehension after damage to Wernicke's area)[1]. Stimulation was the second: Penfield's electrodes made a thumb twitch or a cheek tingle[5]. Recording was the third: Hubel and Wiesel heard what single cells responded to[9]. Imaging, which watches the whole brain while a person does a task, came much later.

Each method has the same limit. A region being necessary for a function does not mean the function lives there alone. Speech needs Broca's area, but it also needs the motor cortex, the auditory cortex, the cerebellum and the fibers that connect them. "The occipital lobe sees" is shorthand for "the occipital lobe is where seeing starts and where it is most easily broken." With that caveat, the four-lobe map is one of the most reliable things neuroscience has, and the rest of this site is built on it. established

This page describes normal anatomy and classic experiments for education. It is not medical advice; anyone with symptoms such as changes in movement, speech, vision or memory should see a doctor.