Cerebral cortex · Grey Matter

The cerebral cortex is the folded sheet of grey matter, two to four millimetres thick, that covers both hemispheres and does the brain's perceiving, acting, remembering and deliberating.

Cerebral cortex. The cerebral cortex is the folded sheet of grey matter, two to four millimetres thick, that covers both hemispheres and does the brain's perceiving, acting, remembering and deliberating.

Most of it is neocortex, organised in six layers, and it is folded into ridges (gyri) and grooves (sulci) so that a sheet of well over a thousand square centimetres fits inside the skull. Two deep grooves make the main landmarks: the central sulcus, which separates the frontal lobe in front from the parietal lobe behind, and the lateral (Sylvian) fissure, which sets the temporal lobe below both; the occipital lobe and its visual cortex sit at the back, and the insula lies hidden inside the lateral fissure.

Areas are named by number as well as by place. Korbinian Brodmann divided the cortex in 1909 into 52 areas by how its cells are layered under the microscope, and the shorthand stuck: primary visual cortex is area 17, primary motor cortex area 4, primary somatosensory cortex areas 3, 1 and 2, primary auditory cortex areas 41 and 42.

A rough rule of thumb locates most functions. Senses enter at the primary areas (vision at the back, touch behind the central sulcus, hearing on the upper temporal lobe, taste and the body's inner state in the insula); movement leaves from the front of the central sulcus; meaning gathers in the temporal lobe; space and number in the parietal lobe; rules and control in the prefrontal cortex.

That rule is a map, and every real task recruits a network of several areas with overlap between them. The question cards trace a few: recognising a face, solving a sum, holding back anger.

The cortex is where experience is stored and recombined.

Primary areas are laid out as maps of the senses and the body, and the large association areas between them link those maps into objects, words, plans and memories.

Questions: Is the brain really a reptilian core wrapped in a limbic layer and a rational neocortex? That picture is the triune brain model, proposed by Paul MacLean from the 1960s and popularised by his 1990 book, and it remains a common teaching map. As an account of evolution it is wrong: reptiles, birds and mammals all have homologues of the cortex, the basal ganglia and the limbic structures, which evolved together and were modified rather than stacked as new layers. As a functional map it misleads as well, since emotion and reason are carried by interacting circuits across all levels (the prefrontal cortex is central to emotion, the amygdala to decisions). It can still serve as a mnemonic for where the vital, the emotional and the deliberative tend to be concentrated, provided it is not taken as the brain's history. Is there a simple rule of thumb for where a function lives in the brain? A rough one works: senses arrive at the primary areas (vision at the back, touch behind the central sulcus, hearing on the upper temporal lobe, taste and the body's state in the insula), and movement leaves from in front of the central sulcus, refined by the cerebellum and selected by the basal ganglia. Rules and control sit in the prefrontal cortex, meaning in the temporal lobe, space and number in the parietal lobe, emotion in the amygdala, insula, cingulate and ventromedial prefrontal cortex, and vital functions in the brainstem, hypothalamus and thalamus. Every real task recruits several of these at once with overlap, so the rule says where to look first and never where a function is confined. Why does nearly every sense pass through the thalamus before reaching the cortex? Each sense except smell has a thalamic nucleus that receives it and sends it on to its own area of cortex: the lateral geniculate for vision, the medial geniculate for hearing, the ventral posterior nuclei for touch and taste. Passing everything through one structure lets the brain control the flow, since the cortex sends back more fibres than it receives and the reticular nucleus can mute the relay cells, amplifying what is attended to and damping the rest. The same gate closes in deep sleep, when thalamic cells switch to bursting and the outside world is largely kept out. Why can touching the face of someone who lost an arm be felt in the missing hand? On the somatosensory map the hand area lies next to the face area. After an amputation the hand area stops receiving its own input, and inputs from its neighbour, the face, spread into it. The rest of the brain still reads activity in that patch as coming from the hand, so a touch on the cheek is felt both on the cheek and on the phantom hand, sometimes in a point-by-point layout, as Ramachandran and colleagues reported in 1992. It is one of the clearest demonstrations that adult cortical maps keep reorganising. Why is the primary visual cortex called area 17? In 1909 Korbinian Brodmann stained slices of cortex and found that the thickness and cell makeup of its layers change at sharp borders, and he divided the cortex into 52 numbered areas by that structure alone. The numbers follow the order in which he examined the areas, so neighbours can have distant numbers and the numbers carry no meaning in themselves. They stuck because the borders often coincide with functions: area 17 is the primary visual cortex, 4 the primary motor cortex, 3, 1 and 2 the somatosensory cortex, 41 and 42 the primary auditory cortex, 44 and 45 Broca's area. What does sleep do for the transfer of memories to the cortex? In deep sleep the hippocampus replays the activity of the day: cells that fired together while a rat ran a track fire together again, in the same order and compressed in time, during brief bursts called sharp-wave ripples. These bursts tend to coincide with the slow waves and spindles of the cortex, a timing thought to let the cortex strengthen the links between the pieces the hippocampus is replaying. In rats, disrupting the ripples after learning impairs the memory, and in humans a night of sleep after learning improves retention compared with the same hours awake.