Chapter 2: Cognitive Neuroscience
The mind-body dichotomy has long concerned philosophers and scientists. Until very recently, there was little evidence and much speculation as to where the mind was located inside the body, and even though we now understand the workings of the nervous system, many of our metaphors for cognition still hearken to a time when far less was known. For example, the present-day debate over whether reason or emotions govern behavior is merely a refinement that clings to the primitive notion that some portion of the human mind existed in the brain and another portion of it exited in the heart.
It is now considered that anatomy and physiology, which consider the structures and functions of the parts of the body, have much to offer in terms of understanding the way people think. Specifically, we look to the brain as the organ that directly controls thoughts, emotions, and motivations as well as many functions of the body that occur and perpetuate without a conscious process of thought. Therefore considering the brain and, to a lesser degree, the central nervous system provides insight to cognitive psychology.
Observing the Brain
Presently, it's generally accepted that the nervous system receives and processes information from the external environment as well as controlling the actions of the body. The system consists of a network of neurons that carry information to and from the brain, which is the central organ of the network.
The brain itself is not a mechanical organ, but its activity is largely electrochemical, which is to say that its operation cannot be directly observed, but only inferred from secondary evidence. The author considers various methods of study, in terms of their nature and the kinds of information they have provided.
Postmortem Studies
Postmortem studies involve the dissection of the brain after the subject has died. This enables close inspection of the tissues of the brain, but as its function is electrochemical, little can be discovered about its operation by this method.
In some instances, researchers have attempted to correlate the structure to behavior by documenting the traits of the individual prior to death and seeking to correlate it to anomalies in the organ. This has been particularly enlightening in individuals who have suffered some trauma to the organ, and distinct correlations have been drawn between the damage to the organ and the atypical behavior that was observed. By this method, it has been inferred that certain parts of the brain are closely correlated to specific mental processes.
(EN: This conventional view was largely an derivative of phrenology, the theory that suggested observable forms such as the shape of the skull and the location of bumps on the head indicated personality traits - which was complete hogwash, yet was carried forward to suggest that certain parts of the brain have very specific functions. Postmortem studies have been skewed and misrepresented to support this view, but while some studies show a strong correlation, others have demonstrated the brain's ability to adapt and use other neural pathways in the brain to perform certain functions when the injury occurred early enough in life. For those injured later in life, this adaptation is not as effective.)
Animal Studies
It is generally regarded as unethical to perform an in vivo examination of a human subject's brain, but there is less reluctance to perform such experiments on animal subjects: to damage part of the brain intentionally in order to observe its effects on behavior, or to insert sensors to record electrochemical activity.
While some of these experiments have captured the attention of the scientific community, they are generally regarded with some suspicion as to whether the findings based on animal subjects are applicable to human ones, in regard to whether the intrusive methods of research may have corrupted the results (the insertion of a probe damages tissue, making the observed brain abnormal), and as a result of comprehensiveness (such methods study small, localized areas of the brain in certain regards, and do not consider other brain activity).
Electrical Recordings
Researchers and practitioners have recently adopted the use of the electroencephalography (EEG) as a method for observing the electrical activity in the brain. This device uses a set of eight to sixteen sensors that attach to the scalp to detect the level of electrical activity in regions of the brain, recording the way in which these levels change over time.
Subjects are wired to the device and exposed to stimuli, or are asked to perform a given task (physical or mental) so that researchers may observe the changes in the levels of activity, which is interpreted to mean activity in a given section of the brain. These readings are collected from large samples and analyzed for statistical correlation.
While this has generated much excitement, there are limitations:
- There is a lack of resolution. Given that there are over 100 billion neurons in the human brain, even a sixteen-sensor EEG makes generalizations about what occurs in groups of 6.25 billion neurons
- There is a lack of depth. EEG sensors can only measure the electrical activity in neurons that are relatively close to the surface of the cerebral cortex and cannot measure activity deeper in the brain.
- There is a lack of speed. The time interval of an EEG reading can be as much as 0.2 seconds, meaning it cannot detect electrical activity of a shorter duration
Static Imaging
Another recent tool is magnetic resonance imaging, which is capable of gathering more comprehensive data about the electrical activity of the brain, even the deep-brain tissues that the EEG misses, and on a more granular level.
However, the MRI provides a snapshot of the brain, and these snapshots cannot be taken at a fast enough interval to monitor brain activity in real time. The patient must also be immobilized in a bulky device in order to be scanned. This makes MRI impractical for many research applications.
The MRI is therefore generally regarded as an enhanced version of an X-ray: it shows the structure of the brain, enabling researchers to consider its shape and the location of things such as scar tissue, lesions, voids, and tumors, but it is not useful in making observations over time.
Metabolic Imaging
Rather than measuring electrical impulses, metabolic imaging uses methods such as positron emission tomography (PET scans) to investigate chemicals within the brain - specifically the levels of glucose and oxygen. The theory is that parts of the brain that are active consume these chemicals at a higher rate than areas of the brain that are not active.
This carries with it the same limitations as electric imaging, plus the problem of migration of chemicals in a manner that is not related to thought processes. That is, chemical levels are replenished, so the reduction of oxygen and glucose from an inactive part of the brain can result from the nee to replenish levels in an active part of the brain, which would give a false positive for activity in an inactive sector.
The latest technique is functional magnetic resonance imaging (FMRI), which combines the measurement of electrical activity from the MRI with metabolic data to record the flow of oxygenated blood in the brain, under the same premise (oxygen is consumed by mental activity) and with the same limitation (blood moves about for reasons other than mental activity). It is less invasive and has higher resolution than the PET.
Cognition in the Brain
The author considers the structure of the brain as an organ, the science of which has evolved over time. Historical perspectives on the structure of the brain seem laughable today, but consider that our most sophisticated models will seem laughable in the future, as more about the organ is discovered.
Gross Anatomy: Forebrain, Midbrain, and Hindbrain
In the most general sense, the brain is divided into three major sections from front to back, with a few fundamental parts that are believed to have specific functions.
The forebrain includes:
- Cerebral Cortex - Considered to be responsible for receiving sensory information and the majority of cognition
- Basal Ganglia - Control center for motor systems
- Limbic System - Important to emotion, motivation, memory, and learning. It is believed that humans have far more developed limbic systems, enabling them to create and adjust behavior rather than relying on instinct.
- Thalamus - Relays incoming sensory information to different areas of the brain, helps control sleep and waking, has soe relation to memory and linguistic functions.
- Hypothalamus - Regulates behavior related to survival, particularly fighting, feeding, fleeing, and mating. Regulates emotions and reactions to stress.
- Amygdala - Plays an important role in emption, especially anger, aggression, and fear
- Hippocampus - Plays an essential role in memory formation
The midbrain includes:
- Superior Colliculi - Receives and processes visual stimuli
- Inferior Colliculi - Receives and processes auditory stimuli
- Reticular Activating Systems - Important in sleep, attention, and movement
- Ventral Region - Important to controlling movement
The Hindbrain Includes:
- Cerebellum - Essential to balance, coordination, and muscle control
- Pons - Involved in sleep and arousal, facial nerves, and coordination of the other parts of the brain
- Medulla Oblongata - Routing signals between body and brain, also involved in cardiorespiratory function, digestion, and swallowing.
The assumptions about the function of each part of the brain are drawn by comparative anatomy - specifically, the comparison between the human brain and those of animals, as well as the comparison between the normal brain and one that is damaged or underdeveloped.
Cerebral Cortex and Localization of Function
The cerebral cortex consists of a thin layer, 1-3mm, that wraps the surface of the brain, much like bark wraps a tree, and has a wrinkled appearance (sulci are the small wrinkles, fissures are large ones, and gyri are the bulges between them). If peeled off and flattened out, it would be about two square feet of tissue.
The cortical area is attributed the most complex brain functions: in human beings, this is believed to be the majority of thought processes, perception, language, and other of the higher mental faculties. A large cortical area is considered to be a distinguishing feature of the human brain.
The cerebral cortex is dense in "gray matter" (the bodies of neurons, where information is processed) whereas other parts of the brain have more "white matter" (the tendrils extending from the bodies of neurons, which transmit information).
There is some mention of the hemispheres of the brain, as the brain appears has a distinctively deep cleft down the center. It is believed that the left and right hemispheres have distinctly different functions, though this is a generalization of the functions of specific parts of each hemisphere. In terms of the body, it's generally accepted that the left hemisphere controls the right side of the body and vice-versa.
The hemispheres of the cerebellum are not independent - the organ, being efficient, tends to localize some processes (related functions are closely grouped to minimize the distance trough which signals mist pasts) to one or the other hemisphere so that transmission is ipsilateral (occurs on the same site of the brain), but there is also considerable contralateral activity that shifts information from one side to the other, across a fibrous area called the corpus callosum, which separates the hemispheres. Damage to this region prevents the two sides of the brain from communicating to one another.
The observation of hemispheric activity arises from ease of observation. While it is not possible to pinpoint damage in a live subject, it is relatively easy to generalize that an injury has occurred on one side of the head or the other. As early as the nineteenth century, physicians were able to correlate certain dysfunctions to injury to one or the other side of the head, but hadn't the technology to be much more specific.
Roger Sperry (1964) performed some of the earliest experiments to investigate the function of the hemispheres and the corpus callosum by means of experimenting on cats and monkeys, as well as observation of "split-brain" patients whose corpus has been damaged.
It has been estimated that approximately 90% of the adult population have language functions that are predominately localized to the left hemisphere. And a similar parentage demonstrate spatial processing in the right hemisphere. In cognitive terms, the right hemisphere deals more with interpreting information received from the environment, whereas the left is more involved with externalizing by means of action and speech.
To add specificity, each of the hemispheres is divided into four basic lobes, as demarked by the larger fissures in the surface of the brain:
- Frontal Lobe (roughly the front 50% of the brain, to the central fissure) - Associated to motor functioning, abstract thinking, higher reasoning, and conscious thought
- Parietal Lobe (another 30% between central and longitudinal fissures) - Associated to receiving and processing sense data as well as somatic signals from within
- Occipital Lobe (last 20% between the longitudinal fissure and the back) - Associated to visual processing, including color, shape, form, and location
- Temporal Lobe (a distinctive on each side, extending from the occipital to temporal lobes) - Associated with processing audio input
Along the central fissure are two significant cortexes: the motor cortex at the back of the frontal lobes controls movement, and the somatosensory cortex at the front of the parietal lobe receives and processes input.
The term "association area" is used to indicate a portion of the brain that is related to a given behavior or function. It is here that neurology most approaches phrenology, in the assumption that a specific "part" of the brain is the primary or sole demesne of a given function. There is evidence supporting and contradicting this theory.
As such, our present scientific knowledge generally accepts that it has answered the question of where the mind exists in the body, and is now focused on determining where specific thoughts occur within the brain.
Brain Disorders
In their studies of the human brain, there have been identified three categories in which a given condition has been associated to a specific part of the organ.
Stroke is a vascular disorder that results from a disruption of the flow of blood to the brain, which damages brain tissue by depriving it of oxygen. An ischemic stroke is caused by a blockage of blood vessels by fatty tissue and a hemorrhagic stroke occurs when the vessel itself breaks. The worst effects of a stroke tend to be immediate, wit some functionality restored over time as the damage is repaired, but some damage is irreparable.
Brian tumors are tee second category of condition, in which an abnormal growth of tissue consumes or compresses brain cells and causes trauma to the brain, the effects of which are slow to progress until considerable damage has occurred. Unless the growth of the tumor is arrested, naturally or by treatment, the damage will progress. Even if a tumor's growth is arrested, or if it atrophies, some of the damage done will remain permanent.
Head injuries at the third category, whether the injury itself damages brain tissue, or the damage is a secondary consequences of swelling due to an injury compresses and displaces the tissue. Such injuries are still surprisingly common, with an estimated 700,000 North Americans suffering them each year, and 10% 10 12.5% result in permanent disability. Any trauma that causes loss of consciousness likely involves damage to the brain itself.
(EN: my sense is that the author overlooks a number of causes that will either damage the brain or prevent it from developing normally: certain diseases, use of drugs, and birth defects come immediately to mind, and there are likely others.)
The author stresses that a medical doctor, rather than a psychologist, should be consulted for treatment of damage to the organ. A rehabilitation psychologist may be useful in enabling the patient to cope with the consequences, but cannot hope to deal with the non-behavioral causes.
Key Themes
One significant emphasis is that the biological and psychological investigation of the function of the brain are separate, but not entirely mutually exclusive. Psychologists and neurologists work together to understand the causes of human behavior.
A second pertinent theme is the nature-nurture dichotomy, which seems to surface even though it is generally acknowledge that both contribute to cognition.
A third theme is a return to basic or applied research: basic research discovers fundamental truths that are meaningless without having a practical application, and application is random without the information discovered by basic research.