The Brain's Cerebral Cortex (Neocortex)
In Evolving Brains (2000), John Allman explains that cortex means "the outer shell or rind of an object." He explains that the prefix "neo" implies that it is new.
Allman writes: "The neocortex is found only in mammals, although it is related to forebrain structures found in other vertebrate classes. The neocortex has expanded enormously in the brains of humans and other advanced mammals."
Robert M. Sapolsky, in his video course, Biology and Human Behavior: The Neurological Origins of Individuality, 2nd edition, refers to the neocortex as a "primate specialization," terminology which corresponds to Allman's description of an expanded neocortex in "advanced mammals." In the image to the right (image links to source), you can see the medial longitudinal fissure separating the left and right hemispheres of the human neocortex. This image is from BrainMaps.org; the project investigator for this project is Edward G. Jones and other scientists from the Center for Neuroscience, University of California, Davis and University of Wisconsin, Madison.
Regarding the neocortex, Allman writes:
The neocortex, the sheetlike, six-layered structure in the roof of the forebrain that is found in all mammals and only in mammals, was probably present in the earliest true mammals; it is possible that it may actually have evolved earlier, at some point after the separation of the line leading to the mammals from the lines leading to reptiles and birds. The antecedents of the neocortex are present in the telencephalic roof in even the most primitive vertebrates. The neocortex is a specialization in the telencephalon that parallels the formation of the dorsal ventricular ridge and wulst in reptiles and birds. The neocortex is just as much a unique defining feature of mammals as are the mammary glands or the malleus and incus in the middle ear. As with the other distinctive features of mammals, the neocortex probably evolved as a part of a set of adaptations related to temperature homeostasis. The large increases in metabolic expenditure necessary to sustain temperature homeostasis required commensurate increases in the acquisition of food by the early mammals. Since these animals were small and had only a limited capacity to store energy as fat, they were constantly under the threat of starvation. The neocortex stores information about the structure of the environment so that the mammal can readily find food and other resources necessary for its survival (emphasis added).
Jaak Panksepp, in Affective Neuroscience: The Foundations of Human and Animal Emotions (1998), explains that "the modular unit of the neocortex is a column, a vertically oriented functional grouping of about 4,000 interconnected neurons with comparatively weak connections to immediately neighboring modules. These columns are strongly linked to other cortical modules and to lower brain areas by descending and looping connections. Coherently operating groups of neurons are called functional 'networks' or 'cell assemblies.' An especially important point to remember is that even though the human brain has much more neocortex than other animals of comparable size, this is achieved by the addition of more columnar modules and their interconnections rather than by increasing the quality (i.e., complexity) of cortical columns."
Before I discuss the anatomical differentiations referred to as lobes, I would like to discuss a most important recessed area of cortex that traditionally has been considered as part of the limbic system or paleomammalian brain. I am trying to avoid specifying specific components of what has in the past been called the limbic system since a discrete system does not exist. I think the point to remember is that the anterior cingulate cortex is more ancient, from an evolutionary perspective, than outer areas of cortex.
The cingulate cortex shapes itself to the outside curve of the corpus callosum, the thick band of nerve fibers that connects the brain's left and right hemispheres (see The Brain's Two Hemispheres). The term cingulate comes from cingulum, Latin for belt or girdle. This area is sometimes called the cingulate gyrus. The word gyrus means generally circle. Anatomists call each cortical fold a sulcus and the smooth protruding area between folds a gyrus. You can see in the photograph above (image links to source) that the cingulate cortex (all areas labeled 7)
is "girdled" or "belted" around the corpus callosum. MedlinePlus Dictionary defines anterior as "relating to or situated near or toward the head… ." Thus, the anterior cingulate cortex in the image above, outlined in red, bumps up against the inside of our frontal lobe. The above image is from John A. Beal, Department of Cellular Biology and Anatomy, Louisiana State University.
In Descartes' Error: Emotion, Reason, and the Human Brain (1994), Antonio R. Damasio writes: "I would like to propose that there is a particular region in the human brain where the systems concerned with emotion/feeling, attention, and working memory interact so intimately that they constitute the source for the energy of both external action (movement) and internal action (thought animation, reasoning). This fountainhead region is the anterior cingulate cortex, another piece of the limbic system puzzle." Damasio goes on to say: "My idea about this region comes from observing a group of patients with damage in and around it. Their condition is described best as suspended animation, mental and external—the extreme variety of an impairment of reasoning and emotional expression."
Damasio calls attention to an interesting phenomenon that illustrates the function of the anterior cingulate cortex. Damage to the motor cortex will affect a voluntary smile but not a genuine, emotional smile generated from within the anterior cingulate cortex. "When a stroke destroys the motor cortex on the brain's left hemisphere and, as a result, the patient has paralysis on the right side of the face," Damasio notes, "the muscles cannot act and the mouth tends to be pulled toward the normally moving side. Asking the patient to open the mouth and reveal the teeth only heightens the asymmetry. Yet when the patient smiles or laughs spontaneously, in response to a humorous remark, something entirely different happens: the smile is normal, both sides of the face move as they should, and the expression is natural, no different from the usual pre-paralysis smile of that individual. This illustrates that the motor control for an emotion-related movement sequence is not in the same location as the control for a voluntary act. The emotion-related movement is triggered elsewhere in the brain, even if the arena for the movement, the face and its musculature, is the same."
To clarify where emotional expression does originate, Damasio writes: "If you study a patient in whom a stroke has damaged the anterior cingulate in the left hemisphere you will see precisely the opposite result. In repose or in emotion-related movement, the face is asymmetrical, less mobile on the right than on the left. But if the patient tries to contract the facial muscles willfully, the movements are carried out normally and symmetry returns. Emotion-related movement, then, is controlled from the anterior cingulate region, from other limbic cortices (in the medial temporal lobe), and from basal ganglia, regions whose damage or dysfunction yields a so-called reverse or emotional facial paralysis."
Paul MacLean made a very interesting observation relating to the cingulate cortex and the bond between mother and child in an article included in the book The Limbic System and Clinical Disorders (B.K. Doane, and K.E. Livingston, eds., Raven Press, New York). MacLean wrote: "It is beginning to appear on the basis of comparative neurobehavioral studies that the cingulate subdivision of the limbic system is implicated in three forms of behavior that characterize the evolutionary transition from reptiles to mammals—namely (a) nursing, in conjunction with maternal care; (b) audiovocal communication for maintaining maternal-offspring contact; and (c) playful behavior. Significantly, the cingulate gyrus and its subcortical connections appear to have no recognizable counterpart in the reptilian brain."
The anterior cingulate cortex may play an important role in bonds between parent and child, as well as other attachments. We explore this idea in Part 2 of MyBrainNotes.com, in Chronic stress and frustration related to attachment—implications for depression and OCD. If you jump ahead to this link, remember to click the BACK button on your browser to return here.
Wikipedia's entry for the "Human brain" states: "the lobes are named after the bones of the skull that overlie them. There is one exception: the border between the frontal and parietal lobes is shifted backward to the central sulcus, a deep fold that marks the line where primary somatosensory cortex and primary motor cortex come together."
The lobe designations are sometimes used to indicate the relative position of structures. For example, the amygdalae are nestled within the temporal lobes. While we will discuss specialized areas of the neocortex later in this narrative, we will discuss the lobes in brief below. It is important to remember that a lobe does not function independently. Lobe designations are merely an anatomical reference. I will again quote Damasio to forestall the impulse to credit this or that lobe with a discrete process. He points out that "within one second in the life of our minds, the brain produces millions of firing patterns over a large variety of circuits distributed over various brain regions."
The frontal lobe is involved in establishing priorities and planning. Richard M. Restak, in Brainscapes: An Introduction to What Neuroscience Has Learned about the Structure, Function, and Abilities of the Brain (1995), writes: "The frontal lobe makes up 50 percent of the volume of each cerebral hemisphere in humans. It initiates all motor activity, including speech; its most anterior divisions, the prefrontal lobes and supplementary motor cortex, integrate personality with emotion and transform thought into action."
The temporal lobe contains a sensory area related to hearing. Nestled within each temporal lobe are the amygdala and the hippocampus, which are, writes Restak in Brainscapes, "involved in learning, memory and the experience and expression of emotion." A portion of the temporal lobe, called the entorhinal cortex, channels cortical inputs to the hippocampus. Restak writes: "Fibers from all four lobes, along with association fibers uniting these separate connections into one unified experience, converge into the hippocampal region." In Evolving Brains, Allman points out that the amygdala—so important to emotional processing—also receives input from a cortical area within the temporal lobe called the inferotemporal visual cortex, which is "greatly expanded in higher primates." Allman notes that "Charles Goss, Robert Desimone, Edmund Rolls, and David Perrett and their colleagues have shown that neurons in part of the inferotemporal cortex are especially sensitive to the images of faces." We will discuss specific areas of the cortex involved in processing different kinds of visual stimuli later in this narrative.
The occipital lobe includes sophisticated topographical maps with complex interconnections required for visual processing, and according to MedlinePlus Dictionary, has "the form of a 3-sided pyramid." The Merck Manuals Online Medical Library states that in addition to processing and interpreting vision, cortical areas in the occipital lobe enable people to form visual memories and integrate visual perceptions with spatial information coming from the adjacent parietal lobes. If a head injury or stroke damages the occipital lobes, visual agnosia sometimes develops. In an entry for brain dysfunction, the Merck Manual home edition defines agnosia as the "loss of the ability to associate objects with their usual role or function." With sufficient damage to the occipital lobe, "People cannot recognize familiar faces or common objects, such as a spoon or a pencil, even though they can see these things."
The parietal lobe contains an area that processes bodily sensations—the somatosensory cortex, which we will discuss further later in this narrative. In Brainscapes, Restak writes: "The parietal lobe is the receiving station for sensory information from the opposite side of the body and is responsible for the integration of what is seen with what is felt via a network of association fibers."
A fifth lobe, the insula, is located "in the center of the cerebral hemisphere that is situated deeply between the lips of the sylvian fissure," according to MedlinePlus Dictionary. To get an idea of where the insula is located, the image to the right (links to source) illustrates the location of the sylvian fissure. This image is from web pages authored by Jody Culhan, University of Western Ontario; the web page is titled "fMRI for Newbies." The insula, hidden inside the sylvian fissure, is sometimes called the central lobe or island of Reil. The insula "integrates sensory and autonomic information from the viscera," according to
Merck Manuals Online Medical Library. This lobe plays a role in certain language functions and when damaged can lead to aphasia, the inability to use or understand spoken and written language. According to the Merck Manuals, the insula also "processes aspects of pain and temperature sensation and possibly taste."
The illustration to the left (image links to source) is a pyramidal cell, which is common in the prefrontal cortex. Bob Jacobs, Laboratory of Quantitative Neuromorphology, Department of Psychology, Colorado College, provides this photograph.
In his video course, Sapolsky explains that the frontal cortex "helps you focus on what the task is right now. Early state dementia patients asked to count backward from 20 will start the task and then revert to reciting the months of the year. This dysfunction is called perseveration and intrusion—reverting to a previous task. Instead of doing the cognitively harder thing via the frontal cortex, the patient reverts to doing something over learned, remembered."
"The frontal cortex is involved in executive control, delayed gratification, long-term planning," writes Robert M. Sapolsky in Monkeyluv and Other Essays on Our Lives as Animals (2005). "It does this by sending inhibitory projections into the limbic system, a deeper, more ancient brain system involved in emotion and impulsivity. Furthermore, the frontal cortex excels at resisting stimulating inputs from the limbic system, ignoring tempting limbic whisperings like 'Screw the studying for the exam, run amok instead.'" Sapolsky later adds that when the frontal cortex is destroyed in a person, "you have a 'frontal' patient—sexually disinhibited, hyperaggressive, socially inappropriate. The frontal cortex is the closest thing we have to a neural basis for the superego."
In this subsection, we discuss the frontal cortex generally. In the next subsection, we will discuss a particular portion of the frontal cortex, the orbital-frontal cortex, so labeled in the illustration to the right (links to source). In Part 2 of MyBrainNotes.com, as part of a discussion of the VIGILANCE system, we discuss Stress, attention, learning, and memory, including the effects of stress on neurocircuitry in what is called the prefrontal cortex, a larger area also illustrated in the image to the right (links to source).
Temple Grandin and Catherine Johnson make a good point in Animals in Translation: Using the Mysteries of Autism to Decode Animal Behavior (2005). If you damage any part of your brain in an accident or a stroke, they explain, it may appear that you have damage to your frontal lobes, even when your frontal lobes remain perfectly intact. "People always thought this was because the last structure to evolve is the most delicate, while the older structures have been around so long they've become incredibly robust. But a neuropsychologist named Elkhonon Goldberg at New York University School of Medicine, who wrote a fantastic book about frontal lobe functions called The Executive Brain, has a different theory. He thinks that while the frontal lobes may be more fragile, there is another factor involved, which is that every other part of the brain is connected to them. When you damage any part of the brain, you change input to the frontal lobes, and when you change input, you change output. If the frontal lobes aren't getting the right input, they don't produce the right output even though structurally they're fine. So all brain damage ends up looking like frontal lobe damage, whether the frontal lobes were injured or not."
"In primates, the frontal lobe has an important role in establishing priorities and planning," writes Allman in Evolving Brains. "In particular, the lower surface of the frontal lobe, termed the orbital-frontal cortex, is especially important for these functions, as has been shown by an extraordinary series of clinical observations of brain-damaged patients by Antonio Damasio and his team in the Department of Neurology at the University of Iowa College of Medicine."
I will try to use the term orbital-frontal cortex consistently in this narrative. The location of this most important region is highlighted in the image above right (links to source; this image is an MRI of Wikipedia contributor Paul Wicks's brain). In reading, however, you are likely to come across other terminology that designates areas of the brain slightly different from that pictured above. Damasio, for example, clarifies his use of terms in Descartes' Error: "In neuroanatomical terminology, the orbital region is known also as the ventromedial region of the frontal lobe, and this is how I [Damasio] will refer to it throughout the book. 'Ventral' and 'ventro-' come from venter, 'belly' in Latin, and this region is the underbelly of the frontal lobe, so to speak; 'medial' designates proximity to the midline or the inside surface of a structure."
The importance of the brain—especially frontal regions of the neocortex—in shaping behavior and personality began to be realized in the aftermath of an astonishing injury to a man named Phineas Gage in 1848, then a reliable and successful construction foreman working on the railroad in Vermont. Gage was using a tamping iron to compact material—including explosive powder that at some point in time is topped with sand—into a hole drilled into a bed of rock when he accidentally set off an explosion that sent a tamping iron through his head. (Beverly and Jack Wilgus own the original daguerreotype of Gage, pictured at left holding the tamping iron that shot through his head. The image links to their web site, Meet Phineas Gage.) Although Gage was not able to return to work as a foreman, he did pursue and obtain other employment relating to the care and management of horses. After learning something about brain evolution and ethology, I am not surprised that in this subsequent work Gage engaged with animals. But before we get to this topic, lets review a few facts about the accident and Gage's medical treatment, taken from the well researched An Odd Kind of Fame: Stories of Phineas Gage (2002), by Malcolm Macmillan.
Macmillan provides all the unpleasant but extremely interesting details of how physician John Martyn Harlow treated Gage's injury. These details include the following: "During his 1848 examination he [Harlow] had explored the wound by placing one index finger in the opening in the skull until it 'received the other finger in like manner' from the wound in the cheek."
Macmillan explains that the tamping iron "was three feet and seven inches long, one and one-quarter inches in diameter at the larger end, tapering over a distance of about twelve inches to a diameter of one-quarter of an inch at the other, and weighed thirteen and one-quarter pounds."
Macmillan presents two versions of the accident—one from Harlow, who tended to Gage's wound and thus saved his life, and the other from
Henry Jacob Bigelow, who presented Gage and his tamping iron at an 1849 meeting of the Boston Society of Medical Improvement. I should note here that Macmillan favors Harlow's version. Macmillan writes:
According to Harlow, Gage was tamping the powder and fuse ("slightly" in the 1848 account) prior to the sand's being poured in when his attention was attracted by his men, who were loading excavated rock onto a platform car in the pit a few feet behind him. With his head still averted and while continuing to look over his right shoulder, Gage dropped the iron onto the powder again where, this time, it hit the rock, struck a spark, ignited the charge, and immediately reversed its initial direction. Bigelow had it differently. According to Bigelow, the powder and fuse had already been "adjusted" in the hole, and Gage had instructed an assistant to pour in the sand. While waiting for this to be done, Gage turned his head away, and after an interval of a few seconds again dropped the iron, this time, as he supposed, onto the sand. However, no sand had been added, and, when the bar struck the rock, the charge exploded, driving it upward.
In terms of his occupation before the accident, Harlow described Gage as a "businessman."
Macmillan explains that in 1848, the term "businessman" was used primarily to describe "one who organized the work of others." Macmillan points out that the most reliable evidence we have about Gage before the accident is that he was a "foreman of a gang of men constructing the bed for the railroad." In his job as foreman, Gage "had to allot tasks to the men in his gang fairly, record accurately the time each man spent on each task, treat the men comprising the gang equally, and pay them properly." After the accident, however, Macmillan reports that when Gage felt well enough to return to work, his employers "would not give him his position back" because the "damage to his brain had changed him too profoundly."
During Gage's recovery period, Macmillan details how Harlow began to notice unusual behavior. Regarding his patient, Harlow wrote: "Does not estimate size or money accurately, though he has memory as perfect as ever. He would not take $1,000 for a few pebbles which he took from an ancient river bed where he was at work." In several references to the recovering Gage, Harlow described his patient as "very childish."
Macmillan explains that what we know of Gage's employment subsequent to the accident comes from Harlow. After spending some time displaying his injury and tamping iron in a museum setting, Harlow wrote that Gage began working with horses. In 1851, Gage "engaged with Mr. Jonathan Currier, of Hanover [New Hampshire], to work in his livery stable." From Harlow we also know that Gage "remained there without any interruption from ill health for nearly or quite a year and a half." Macmillan writes: "We may presume he looked after the horses and that he also drove coaches."
The gold rush and its associated influx of people into California, explains Macmillan, may be responsible for what Gage did in 1852. Harlow wrote that Gage "engaged with a man who was going to Chili, in South America, to establish a line of coaches at Valparaiso." Macmillan writes: "In the 1850s Valparaiso was popular as a first port of call on the western seaboard for ships from the eastern United States going to California via South America. As supplies were being replenished, passengers would take a few days' rest, during which time they often traveled the approximately seventy-five miles to the Chilean capital, Santiago." Harlow wrote that Gage was "occupied in caring for horses, and often driving a coach heavily laden and drawn by six horses."
In detailing how the Gage case shaped thinking about localization of brain function, Macmillan calls attention to the work of David Ferrier (1843-1928). Ferrier's work with monkeys demonstrated that "frontal ablation in the monkey produced behavioral changes that Ferrier came to consider as being exemplified by Gage and that could be explained by the loss of a frontally localized inhibitory-motor function." Ferrier reported that the loss of that function caused, "a form of mental degradation, which may be reduced in ultimate analysis to the loss of the faculty of attention."
Macmillan reports that in 1884, 36 years after Gage's injury, M. Allen Starr published "the first modern comparison of the effects of different kinds of [brain] lesions and tumor" and that Gage figured prominently in the report. Macmillan proposes that Starr also drew on Ferrier's "inhibitory" thesis. Macmillan includes the following from Starr's published comparison:
The mind exercises a constant inhibitory influence upon all action, physical or mental; from the simple restraint upon the lower reflexes, such as the action of the sphincters, to the higher control over the complex reflexes, such as emotional impulses and their manifestation in speech and expression. This action of control implies a recognition of the import of an act in connection with other acts; in a word, it involves judgment and reason, the highest mental qualities. By inhibiting all but one set of impulses it enables one to fix attention upon a subject, and hold it there.
In the twenty-first century, we better understand that the role of specialized areas of the neocortex—the frontal lobes especially including the orbital-frontal cortex—is to modulate or "inhibit" emotion-driven behavior originating in subcortical mammalian brain structures. We share subcortical structures with all mammals including horses. Under optimal circumstances, our large human fontal lobes inspire us to adjust behavior based on environmental circumstances as well as familial, cultural, and societal influences. Although we can survive without certain components of the neocortex, we are subsequently very different in terms of personality, temperament, and judgement. We discuss the emotional systems based in subcortical structures in detail in Part 2 of MyBrainNotes.com
I provide here a link to Macmillan's Gage Page.
In most humans, especially right-handed humans, specialized cortical language areas are located in the brain's left hemisphere. In many left-handed people, however, specialized cortical language areas exist in both hemispheres. A minority of both left- and right-handed people appear to have specialized cortical areas for language in only the right hemisphere. Wikipedia aptly describes the brain's plasticity: "Studies of children have shown that if a child has damage to the left hemisphere, the child may develop language in the right hemisphere instead. The younger the child, the better the recovery. So, although the 'natural' tendency is for language to develop on the left, human brains are capable of adapting to difficult circumstances, if the damage occurs early enough."
The Canadian Institute of Neurosciences, Mental Health and Addiction, and the Canadian Institutes of Health Research, provides the image to the left, which links to an educational module, The Brain from Top to Bottom. The particular module to which this image links provides a good overview of Paul Broca and Carl Wernicke, two nineteenth-century neuroanatomists who, during autopsies, studied the brains of people who had suffered from language disorders during their lives. Broca and Wernicke were thus able to correlate damage to specific areas with language deficits. Two specific areas now bear the scientists' names.
Subsequent brain imaging experiments have revealed a third region of the brain—the inferior parietal lobule—that large bundles of nerve fibers connect to both Broca's area and Wernicke's area. (I should note here that the term lobule generally means a smaller subdivision of the larger lobe.) The inferior parietal lobule, also known as Geschwind's Territory, is named for American neurologist Norman Geschwind (1926-1984) who called attention to its importance. The image to the right links to an intermediate-level module in The Brain from Top to Bottom that sums up the important role of this area: "The inferior parietal lobule of the left hemisphere lies at a key location in the brain, at the junction of the auditory, visual, and somatosensory cortexes, with which it is massively connected. In addition, the neurons in this lobule have the particularity of being multimodal, which means that they can process different kinds of stimuli (auditory, visual, sensorimotor, etc.) simultaneously. This combination of traits makes the inferior parietal lobule an ideal candidate for apprehending the multiple properties of spoken and written words: their sound, their appearance, their function, etc. This lobule may thus help the brain to classify and label things, which is a prerequisite for forming concepts and thinking abstractly."
The image to the left is also taken from the intermediate-level module of The Brain from Top to Bottom (image links to source). This module explains that the left hemisphere controls the "phonological, syntactic, and lexical aspects" of conversation. This explains why the left hemisphere was long considered the dominant hemisphere for language. The left hemisphere is not, however, solely responsible for communication and the module goes on to explain the role of the right hemisphere. "The contributions of the right hemisphere to language behaviour are more subtle and nuanced and were not recognized until much later on. The right hemisphere provides the ability to go beyond the literal meanings of words and employs multiple processes to do so. The new science of communication from the perspective of the 'minor hemisphere' for language is called pragmatics."
The pragmatic function, for example, allows one to "understand things that are implicitly signified in discourse—for example, the meanings of metaphors, or of questions like 'Do you have a light?' When right-handed people suffer damage to the right hemisphere of the brain, this pragmatic function is affected, and they tend to interpret such metaphors and questions literally. In fact, these people react exactly as if they were dealing with idioms in a foreign language: their grammar and phonology may be correct, but they do not understand the verbal humour or metaphors that native speakers of that language use every day. Thus, by contributing to the emotional and tonal components of language, the right hemisphere infuses verbal communication with additional meanings."
Damage to the right hemisphere illustrates its vital role in communication as well as the complex connectedness of the brain. The image to the right—taken while the subject was generating words—links to an advanced-level module of The Brain from Top to Bottom. This module explains that right-hemisphere damage can result in hemineglect, in which one pays no attention to stimuli coming in from the left side of the body.
In anosognosia, which Damasio discusses in Descartes' Error, an individual cannot recognize certain parts of their body as being their own. "Anosognosia, as the condition is known, is one of the most eccentric neuropsychological presentations one is likely to encounter. The word—which derives from the greek nosos, 'disease,' and gnosis, 'knowledge'–denotes the inability to acknowledge disease in oneself." Damasio writes: "No less dramatic than the oblivion that anosognosic patients have regarding their sick limbs is the lack of concern they show for their overall situation, the lack of emotion they exhibit, the lack of feeling they report when questioned about it. … Patients with the type of anosognosia described above have damage in the right hemisphere. Although drawing up a full characterization of the neuroanatomical correlates of anosognosia is an ongoing project, this much is apparent: There is damage to a select group of right cerebral cortices which are known as somatosensory (from the Greek root soma, for body; the somatosensory system is responsible for both the external senses of joint position, visceral state, and pain) and which include the cortices in the insula; the cytoarchitectonic areas 3, 1, 2 (in the parietal region); and area S2 (also parietal, in the depth of the sylvian fissure)."
Right-hemisphere damage also can affect an individual's use of prosody, which is the ability to use intonation and stress to convey the emotions one feels. Afflicted individuals therefore communicate in a way that seems flat and emotionless.
In Brainscapes, Restak illuminates what happens when the language-processing left hemisphere is taken out of the loop. "When J.W., a split-brain patient of neuropsychologist Michael Gazzaniga, is shown for less than a second a picture of a horse flashed only to his right hemisphere, he denies that he has seen anything. But when asked to draw 'what goes on it,' he picks up a pencil with his left hand and draws an English saddle, a rather primitive drawing difficult to interpret outside of the context of the experiment. The patient doesn't recognize what he has drawn. He is then asked to draw, rather than say, what picture has been flashed. With his left hand he then draws a horse and, after completing the picture, he grins and says of the first drawing: 'That must be a saddle.'"
In a module taken from The Brain from Top to Bottom, you can find the following excellent summary of Brodmann's Areas: "The cellular architecture differs sufficiently from one part of the neocortex to another to be used as a criterion for defining cortical areas that are functionally distinct. That is what the German anatomist Korbinian Brodmann did in the early 20th century, when he developed a map of the brain based on the differences in the cellular architecture of the various parts of the cortex. Brodmann assigned each part of the cortex that had the same cellular architecture a number from 1 to 52."
In the Wikipedia images above right, you can see how Brodmann labeled the cortical areas. These images link to Wikipedia's list of Brodmann's Areas for quick reference. Please remember to click your browser's BACK button to return to MyBrainNotes.com.
The Canadian internet resource referenced above went on to explain: "Brodmann's intuition, whose accuracy has been confirmed many times since, was that a particular anatomical structure corresponded to a particular function. For example, Brodmann's area 17, which receives information from a nucleus of the thalamus that is connected to the retina, turns out to correspond precisely to the primary visual cortex. And Brodmann's area 4, from which the axons of the large pyramidal cells project to the motor neurons of the spinal cord, corresponds broadly to the motor cortex."
In the Brodmann's Areas illustration to the left, the primary motor cortex corresponds to the area labeled 4. The neurosurgeon Wilder Graves Penfield (1891-1976) explored this region while treating patients with severe epilepsy in Montreal. Penfield's aim was to destroy nerve cells in the brain responsible for seizures. While the patient was under local anesthesia, Penfield stimulated the brain with electrical probes and observed the patient's responses. He did this to identify areas requiring surgery and to avoid vital areas that should not be destroyed. In doing this, he observed that stimulation to certain areas of the cortex triggered highly localized muscle contractions on the opposite side of the body.
According to a module in
The Brain from Top to Bottom, the areas of cortex assigned to various body parts "are proportional not to their size, but rather to the complexity of the movements that they can perform. Hence, the areas for the hand and face are especially large compared with those for the rest of the body. This is no surprise, because the speed and dexterity of human hand and mouth movements are precisely what give us two of our most distinctly human faculties: the ability to use tools and the ability to speak."
A homunculus is defined as any representation of a human being. The homunculus to the right (image links to source) displays the proportion of cortex dedicated to controlling different parts of the body. Note the large size of the tongue relative to the size of the foot.
In Evolving Brains, Allman cites the work of Hughlings Jackson in determining how the brain controls muscle movement. In observing that epileptic patients sometimes had seizures confined to a particular location in the body, Jackson "concluded that the muscles were 'represented' in the brain in a particular location, which he deduced to be somewhere in the cerebral cortex or in a nearby structure called the corpus striatum. This theory was a radical departure from the prevalent clinical view of the time, which was that epileptic seizures were caused by a disturbance in the lowest level of the brain stem."
Allman writes: "In 1870, Hughlings Jackson's topographic prediction was confirmed by the German physicians Gustav Fritsch and Eduard Hitzig, who discovered the motor cortex by stimulating the surface of the brain in dogs with weak electrical currents and observing discrete movements of the body."
Allman explains that Jackson's observations "relate to three fundamental properties of the neocortex. The first is that the neocortex contains topographic maps, the second is that the parts of these maps which are used the most have the largest representations, and the third is that the neocortex has a key role in the genesis of epilepsy." Allman continues, saying: "The cortical circuitry is highly plastic in that it can change its functional organization in response to experience, and it is crucial for memory formation and storage."
In the illustration to the left (image links to source), areas labeled 3, 1, and 2 represent the somatosensory cortex, another kind of topographical map. If your dog licks the bottom of your foot, tickling it, a specific area of the cortex will be activated. This mapping allows you to know that it is your foot being licked, not the back of your neck. In Evolving Brains, Allman tells us how such topographical maps were discovered. "With the development of electronic amplifiers and oscilloscopes in the 1930s it became possible to record the electrical activity of the cortex. Edgar Douglas Adrian, Clinton Woolsey, and their colleagues found that the region adjacent to the motor cortex was electrically activated by mechanical stimulation of the surface of the body and named it the somatosensory cortex, from the Greek soma, 'body.' When they recorded from a particular site in the somatosensory cortex, they were able to map out a receptive field on the body surface which activated that site. By systematically moving the recording electrode from point to point on the cortical surface they were able to determine the representation of the body surface in the somatosensory cortex; they also found a second map of the body surface nearby."
"In the 1970s," Allman explains, "by using microelectrode recordings, Michael Merzenich, Jon Kaas, and their collaborators were able to establish that there are at least four maps of the body surface in the somatosensory cortex of monkeys. Like the motor cortex maps, the somatosensory cortex maps in primates show a strong emphasis on the hand and face, indicating that the exquisitely sensitive surfaces of the hand, lips, and tongue are connected to much larger areas of cortex than are the less sensitive parts of the body. As with the motor cortex, the somatosensory cortical maps are plastic and the cortical representation expands for the parts of the body that are heavily used. The distinction between somatosensory cortex and motor cortex is not absolute. The motor cortex has some sensory functions and vice versa."
Regarding the role of experience in determining cortical representations, Allman writes: "Functional imaging experiments done in human subjects have also demonstrated that the hand representation expands as a result of performing complex finger movements. The expansion of the hand representation can be observed following short-term training, but it is most notable in Braille readers and in musicians who play stringed instruments. These findings demonstrating the role of experience build upon Hughlings Jackson's original observation: the finer the degree of control and use of a muscle, the larger its representation in the cortex."
Although Allman eloquently covers visual processing in Evolving Brains, at this point in my research, the material is a bit over my head, to say the least. If this is your area of interest, however, then I urge you to check out Evolving Brains. Meanwhile, I will provide here some basic information about cortical maps involved in visual processing.
I found a concise explanation of the visual cortex in the Wikipedia entry for "Human brain." It reads: "In visual areas, the maps are retinotopic—that is, they reflect the topography of the retina, the layer of light-activated neurons lining the back of the eye." The entry explains that "the fovea—the area at the center of the visual field—is greatly overrepresented compared to the periphery. The visual circuitry in the human cerebral cortex contains several dozen distinct retinotopic maps, each devoted to analyzing the visual input stream in a particular way. The primary visual cortex (Brodmann area 17), which is the main recipient of direct input from the visual part of the thalamus, contains many neurons that are most easily activated by edges with a particular orientation moving across a particular point in the visual field. Visual areas farther downstream extract features such as color, motion, and shape."
Much of the brain's visual processing takes place in the occipital lobe. For orientation as to where these visual areas area, please reference the illustration to the left. Brodmann's area 17 is the primary visual cortex (V1). Brodmann's area 18 represents the secondary visual cortex (V2) and Brodmann's area 19 represents the associative visual cortex (V3). I have drawn a larger red circle around these three areas. Each one of these areas is a retinotopic map. As mentioned earlier when discussing the temporal lobe, Brodmann's areas 20 and 21 in the temporal lobe (also circled in red) represent the inferotemporal visual cortex, which includes neurons sensitive to the images of faces and sends visual information to the amygdala.
Allman, in Evolving Brains, details how the primary visual cortex, designated V1, was initially mapped:
In the Russo-Japanese War of 1905, many Japanese soldiers sustained bullet wounds that penetrated through the posterior part of their brains. Because of the higher muzzle velocity and the smaller bullet size of rifles developed in the late nineteenth century, these weapons tended to produce more localized brain injuries than were inflicted in earlier wars, and improved care of the wounded also resulted in higher rates of survival. Many of the wounded soldiers were partially blinded by these injuries, and Tatsuji Inouye, an ophthalmologist, was asked by the Japanese government to evaluate the extent of their blindness as a means to determine their pension benefits. Inouye found that the part of the visual field in which these soldiers were blind corresponded to the locations of their brain injuries as determined by the sites of the bullet's entry and exit through the head. By combining the visual field deficits from different soldiers he was able to deduce the topographic organization of the primary visual cortex. Inouye's map revealed that much more cortex was devoted to the representation of the central part of the retina than to the periphery. This is the portion of the retina with the highest acuity, and it is our most important means for probing our environment for information, and the part you are using to read this book. Inouye's map of the primary visual cortex has been confirmed by modern brain-imaging techniques.
In The Modular Brain: How New Discoveries in Neuroscience are Answering Age-Old Questions about Memory, Free Will, Consciousness, and Personal Identity (1994), Richard M. Restak provides an example of how damage to one area of the brain, in only one hemisphere, can dramatically change how we perceive the world. Restak discusses how Michael Gazzaniga conducted an experiment that involved a subject who had lost sight off to his left because of brain damage to the visual area on the right side of his brain. The subject "was asked to imagine himself looking toward California from New York and naming the states in between. He named only ten states, all located to the right of his imaginary vantage point. He omitted the states to the left, corresponding to the visual field mediated by his right brain lesion. What he could not see in the real world as a result of his brain damage, he could neither picture in imaginal space nor speak about."
I found a readable description of the various visual cortical areas in the human brain in an
intermediate-level module titled "The Eye" in The Brain from Top to Bottom.
advanced-level module in this same resource provides another good description of the various visual cortexes. The image to the right is taken from the advanced-level module (image links to source).
As a last note on the vastly complex visual cortical areas, in Evolving Brains, Allman notes the differences in the visual cortex that distinguish earlier mammals from primates and humans. He writes that "opossums and hedgehogs, which in many respects resemble the early mammals that lived more than 60 million years ago, have rather limited visual capacities and a small number of the visual cortical areas. In these mammals, the cortical maps of the retina are relatively uniform in that the amount of cortical space devoted to the more central part of the visual field in front of the animal is not much greater than the cortex devoted to the more peripheral parts of the visual field. By contrast, primates have extremely well developed visual capacities and have a large number of cortical maps devoted to visual perception and memory. Within most of these maps there is a strong emphasis of the representation of the central part of the visual field and a much smaller representation of the peripheral parts of the visual field."
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