Category: neuroanatomy

Posted on

Claustrum of the Brain

The claustrum is a thin, irregular sheet of neurons situated between the insular cortex and the striatum within each cerebral hemisphere of the brain. Despite its small size and somewhat elusive nature, the claustrum is believed to play a significant role in various cognitive processes. Here’s a detailed look at its structure, connections, and proposed functions:

Structure and Location

  • Location: The claustrum is located deep within the brain, nestled between the insula and the putamen of the basal ganglia. It is part of the telencephalon.
  • Anatomy: It is a slender, elongated structure composed of a dense network of neurons and fibers. The claustrum has two distinct parts: the dorsal and ventral claustrum.

Connections

  • Cortical Connections: The claustrum has extensive bidirectional connections with almost all areas of the cerebral cortex. These connections are topographically organized, meaning that different regions of the claustrum connect to specific cortical areas.
  • Subcortical Connections: Besides its cortical connections, the claustrum also interacts with subcortical structures, including parts of the thalamus and basal ganglia.

Proposed Functions

  1. Integration of Sensory Information:
    • Multisensory Processing: One of the primary hypotheses is that the claustrum acts as a hub for integrating multisensory information. It receives inputs from various sensory modalities and may help in creating a unified perceptual experience. This integration is thought to be critical for coherent perception and consciousness.
  2. Attention and Consciousness:
    • Attention Modulation: The claustrum is believed to play a role in controlling attention. It might help focus attention by selectively enhancing relevant sensory inputs and suppressing irrelevant ones. This function is crucial for maintaining a cohesive stream of consciousness.
    • Consciousness: Some researchers have proposed that the claustrum could be involved in generating or maintaining consciousness. This idea stems from its widespread cortical connections and its potential role in integrating information across different brain regions.
  3. Coordination of Cortical Activity:
    • Synchronization: The claustrum may help synchronize activity across different cortical areas, facilitating coordinated neural activity necessary for complex cognitive tasks. This synchronization could be essential for tasks that require the integration of information across different domains, such as language processing and spatial awareness.
  4. Cognitive and Behavioral Functions:
    • Learning and Memory: Although less understood, the claustrum might have a role in learning and memory processes. Its connections with the hippocampus and prefrontal cortex suggest potential involvement in these functions.
    • Executive Function: The claustrum’s interaction with the prefrontal cortex indicates it could be involved in higher-order executive functions, such as decision-making, planning, and inhibitory control.

Research and Clinical Implications

  • Lesion Studies: Studies involving lesions or dysfunction in the claustrum have provided insights into its potential roles. Damage to the claustrum has been associated with disruptions in attention, sensory processing, and consciousness, supporting its proposed functions.
  • Epilepsy: The claustrum has been implicated in certain types of epilepsy, where abnormal activity in this region might contribute to the spread of seizures across the cortex.
  • Neurological and Psychiatric Disorders: Dysfunctions in claustral activity or connectivity might be related to various neurological and psychiatric conditions, such as schizophrenia, autism, and Alzheimer’s disease. Understanding its role could lead to new therapeutic approaches.
Posted on

Blood-Brain Barrier

The blood-brain barrier (BBB) is a highly selective permeability barrier that separates the circulating blood from the brain and extracellular fluid in the central nervous system (CNS). It plays a critical role in maintaining the homeostasis of the CNS, protecting it from potentially harmful substances, and regulating the transport of essential molecules.

Structure of the Blood-Brain Barrier

  1. Endothelial Cells: The primary component of the BBB is the endothelial cells that line the capillaries in the brain. Unlike endothelial cells in other parts of the body, those in the brain are tightly joined together by complex structures called tight junctions. These tight junctions prevent most substances from passing between the cells, forcing materials to pass through the cells instead.
  2. Basement Membrane: Surrounding the endothelial cells is a thin, fibrous extracellular matrix called the basement membrane. This layer provides structural support and further regulates the movement of substances.
  3. Astrocytic End-feet: Astrocytes, a type of glial cell, extend their end-feet processes to cover the surface of the capillaries. These end-feet secrete factors that maintain the tight junctions and overall integrity of the BBB.
  4. Pericytes: These contractile cells are embedded in the basement membrane and play a role in regulating blood flow, maintaining the BBB, and participating in immune responses within the CNS.

Function of the Blood-Brain Barrier

  1. Selective Permeability: The BBB selectively allows the passage of essential nutrients, such as glucose and amino acids, while restricting the entry of harmful substances, pathogens, and large molecules. Transport proteins facilitate the movement of these nutrients across the endothelial cells.
  2. Protection: By restricting the entry of potentially neurotoxic substances and pathogens, the BBB protects the brain from infections and toxins that could disrupt neural function.
  3. Homeostasis: The BBB helps maintain the ionic balance and extracellular environment of the CNS, which is crucial for proper neuronal function. It regulates the levels of ions, neurotransmitters, and other substances in the brain.
  4. Metabolic Barrier: Enzymes within the endothelial cells metabolize certain substances, providing an additional layer of protection by breaking down potentially harmful compounds before they can reach the brain tissue.
  5. Immune Surveillance: While the BBB limits the entry of immune cells, it is not completely impermeable to them. Microglia, the resident immune cells of the CNS, and pericytes play roles in immune responses, providing a controlled environment for immune surveillance and response.

Transport Mechanisms

  1. Passive Diffusion: Small, lipophilic (fat-soluble) molecules can diffuse passively across the BBB. Examples include oxygen, carbon dioxide, and certain lipid-soluble drugs.
  2. Facilitated Transport: Specific transport proteins and carriers in the endothelial cell membranes facilitate the movement of essential hydrophilic (water-soluble) substances like glucose and amino acids. An example is the GLUT1 transporter for glucose.
  3. Active Transport: Certain substances require active transport mechanisms, which use energy (ATP) to move molecules against their concentration gradient. This is seen with ions and other essential molecules.
  4. Receptor-Mediated Endocytosis: This mechanism involves the binding of specific molecules to receptors on the endothelial cell surface, triggering endocytosis and transport into the brain. Examples include insulin and transferrin.

Challenges and Clinical Implications

The BBB poses a significant challenge for drug delivery to the brain, necessitating the development of novel strategies to treat CNS disorders. Conditions such as multiple sclerosis, Alzheimer’s disease, and stroke can disrupt the BBB, leading to increased permeability and subsequent neural damage. Understanding the BBB’s function and structure is crucial for developing therapeutic interventions that can protect or restore its integrity in these diseases.

Posted on

Introduction to the Human Brain

The human brain is an incredibly complex and intricate organ, consisting of approximately 100 billion nerve cells (neurons) and trillions of supportive glial cells. It is the central control center for the body and is responsible for coordinating and integrating all bodily functions, from basic reflexes and movement to higher cognitive processes such as learning, memory, and decision making.

The brain is divided into three main divisions: the cerebrum, the cerebellum, and the brainstem. The cerebrum is the largest and most complex part of the brain and is responsible for most higher brain functions. It is divided into two hemispheres (left and right), which are connected by a bundle of nerve fibers called the corpus callosum. The cerebrum is further divided into four main lobes: the frontal lobe, parietal lobe, temporal lobe, and occipital lobe.

The frontal lobe is located at the front of the brain and is responsible for a variety of functions including voluntary movement, problem solving, planning, and decision making. The parietal lobe is located behind the frontal lobe and is responsible, among other functions, for processing sensory information from the body, such as touch and temperature. The temporal lobe is located on the sides of the brain and is responsible for processing auditory information and memory. The occipital lobe is located at the back of the brain and is responsible for processing visual information.

The cerebellum is located underneath the cerebrum and is responsible for coordinating voluntary movement and balance. It also connects to the frontal lobes and other brain regions and is involved in most functions. The brainstem is located between the cerebrum and the spinal cord and is responsible for controlling many of the body’s basic survival functions such as heart rate, blood pressure, and breathing.

The brain is surrounded and protected by the skull, which is made up of 22 bones that are fused together. The brain is also surrounded by three layers of protective membranes called meninges. The outermost layer is the dura mater, the middle layer is the arachnoid mater, and the innermost layer is the pia mater.

The brain is supplied with blood by two main arteries: the carotid arteries and the vertebral artery. These arteries branch off into smaller arteries that supply the various regions of the brain with blood.

The brain receives a constant supply of oxygen and nutrients from the blood and removes waste products through a network of tiny blood vessels called capillaries. The brain also has its own system of waste removal called the glymphatic system, which helps to remove waste products such as amyloid beta, a protein that has been linked to the development of Alzheimer’s disease.

One of the most important cell types in the brain are neurons, which are responsible for transmitting information throughout the brain and body. Each neuron has a cell body, dendrites, and an axon. The cell body contains the cell’s nucleus and other organelles, and the dendrites receive signals from other neurons. The axon is a long, thin extension of the cell body that sends signals to other neurons or muscles.

Neurons communicate with each other through a process called neurotransmission. When a neuron receives a signal, it sends an electrical impulse down the axon to the terminal buttons, which release chemical neurotransmitters into the synapse (the small gap between neurons). These neurotransmitters bind to receptors on the dendrites of the receiving neuron, transmitting the signal across the synapse.

In addition to neurons, the brain also contains a variety of other cell types, including glial cells. Glial cells, also known as glia, are non-neuronal cells that provide support and insulation for neurons. There are several types of glial cells, including astrocytes (astroglia), microglia, and oligodendrocytes. There is a growing interest in the functions of glial cells, including their role in neuroinflammation, metabolism, and other functions.

In summary, the brain is complex. It allows us to have life as well as learn from and experience the world around us.

Posted on

One in five older adults experience brain network weakening following knee replacement surgery

Gainesville, FL – A new University of Florida study finds that 23 percent of adults age 60 and older who underwent a total knee replacement experienced a decline in activity in at least one region of the brain responsible for specific cognitive functions. Fifteen percent of patients declined across all brain networks the team evaluated.

“In essence, normally synchronized parts of the brain appeared more out of sync after surgery,” said Jared Tanner, Ph.D., the study’s co-lead author and a research assistant professor in the department of clinical and health psychology in the UF College of Public Health and Health Professions, part of UF Health.

Patients who were cognitively weaker before surgery – with worse working memory, slowed mental processing and evidence of brain atrophy as seen in imaging scans – demonstrated the biggest network declines after surgery.

Researchers say they do not yet know if or how patients perceive these network declines. They may contribute to brain “fuzziness” some patients experience right after surgery.

The study, which was published today online ahead of print in the Journal of Alzheimer’s Disease, was conducted to help scientists understand the causes of postsurgical cognitive impairment, which causes memory and thinking problems in about 15 to 30 percent of older adult patients, Tanner said. In most cases, these thinking and memory problems will resolve within six months to a year after surgery.

“Our study builds on 50 years of research into how the aging brain responds to anesthesia and surgery,” Tanner said. “We know older age and cognitive impairment before surgery are risk factors, but the specific causes are not known.”

For the UF study, the team conducted cognitive and brain imaging tests before and after surgery on 48 patients ages 60 and older undergoing a knee replacement. Results were compared with age-matched adults who have knee osteoarthritis, but did not have surgery.

The researchers used resting state functional MRI to look at patterns of blood flow in the brain while patients were lying still. Imaging data helped researchers understand how blood flow changes affected connections across brain networks that are responsible for functions such as memories of oneself and others, determining what outside stimuli deserve further attention, and working memory.

Participants who did not have surgery did not demonstrate any changes across the two brain scans, but 23 percent of participants who had knee replacement surgery showed large declines in connectivity in at least one brain network when tested 48 hours after surgery.

“It was surprising to observe such significant effects of orthopedic surgery on the human brain,” said Haiqing Huang, Ph.D., the study’s other lead author, a data manager at the University of Pittsburgh’s Brain Aging & Cognitive Health Lab and a graduate of the biomedical engineering program at the UF Herbert Wertheim College of Engineering.

The investigators say more research is needed to learn if the brain network changes persist.

“Our goals include investigating if patients who have this brain change after surgery continue to show this change later in their recovery, say at three months or one year after the surgery,” said Catherine Price, Ph.D., the study’s senior author and a UF associate professor of clinical and health psychology and anesthesiology.

People with concerns about their attention or memory should discuss them with their surgical team, Tanner said. At UF Health, neuropsychologists and anesthesiologists have established what is believed to be the first clinical service to identify older adults who may be at risk of developing cognitive problems after surgery so that health care providers can intervene to lessen the impact.

“We strongly believe clinicians need to consider preoperative memory and attention abilities in their patients,” said Price, also the co-director of the Perioperative Cognitive and Anesthesia Network, or PeCAN, service. “Across the nation, however, cognition is not routinely assessed prior to surgery.”

There are also actions patients can take on their own, based on previous studies of healthy aging.

“The brain is resilient and there are things we can do to help protect our brains before and after surgery,” Tanner said. “Exercise, following a Mediterranean-style diet (primarily vegetables, fruits and whole grains), remaining mentally and socially active and otherwise striving to stay as healthy as possible – all might help patients’ brains cope with surgery better,” Tanner said.

—

Mingzhou Ding, Ph.D., of the J. Crayton Pruitt Family department of biomedical engineering in the Herbert Wertheim College of Engineering, served as the study’s other senior author. The project is part of a larger investigation involving Thomas Mareci, Ph.D., of the department of biochemistry and molecular biology in the College of Medicine and the Evelyn F. and William L. McKnight Brain Institute; Hari Parvataneni, M.D., of the department of orthopaedics and rehabilitation in the College of Medicine; Ilona Schmalfuss, M.D., of the department of radiology in the College of Medicine; Mark Rice, M.D., and Cynthia Garvan, Ph.D., of the department of anesthesiology in the College of Medicine; and Ann Horgas, Ph.D., of the department of biobehavioral nursing science in the College of Nursing. The research was supported by funding from the National Institutes of Health.

Press release source.

Reference

Huang H, Tanner J, Parvataneni H, Rice M, Horgas A, Ding M, Price C (2018) Impact of Total Knee Arthroplasty with General Anesthesia on Brain Networks: Cognitive Efficiency and Ventricular Volume Predict Functional Connectivity Decline in Older Adults. J Alzheimers Dis 62, 319-333.

Posted on

Memory Problems in Some With Parkinson’s Disease

From a recent news release by Jill Pease at the University of Florida.

Using a combination of neuropsychological testing and brain imaging, University of Florida researchers have discovered that in a group of recently-diagnosed patients with Parkinson’s disease, about one quarter have significant memory problems.

Parkinson’s disease is commonly known as a movement disorder that leads to tremors and muscle rigidity, but there is growing recognition of cognitive problems associated with the disease. One of the most common is slower thinking speed that causes patients to have trouble quickly retrieving information. The UF study identifies a subset of patients who have a different kind of cognitive issue — memory problems, or difficulty learning and retaining new information.

The findings were published July 24 in the journal PLOS ONE.

“While a large proportion of people with Parkinson’s will experience slower thinking speed, which may make them less quick to speak or have difficulty doing two things at once, we now know that there are a subset of individuals with Parkinson’s disease who have memory problems,” said Catherine Price, Ph.D., the study’s senior author and an associate professor in the UF College of Public Health and Health Professions’ department of clinical and health psychology, part of UF Health. “It is important to recognize which people have issues with learning and memory so we can improve diagnostic accuracy and determine if they would benefit from certain pharmaceutical or behavioral interventions.”

For the UF study, 40 people in the early stages of Parkinson’s disease and 40 healthy older adults completed neuropsychological assessments and verbal memory tests.

About half the participants with Parkinson’s disease struggled with an aspect of memory, such as learning and retaining information, or recalling verbal information, said lead author Jared Tanner, Ph.D., an assistant research professor in the UF department of clinical and health psychology who conducted the study as part of his dissertation research for a UF doctoral degree in clinical psychology.

“And then half of those participants, or nearly one quarter of all participants with Parkinson’s, were really having a difficult time consistently with their memory, enough that it would be noticeable to other people,” said Tanner, adding that researchers were encouraged by the fact that most participants in the initial stages of Parkinson’s were not having significant memory problems.

All participants received brain scans, which used new imaging techniques that allowed the scientists to navigate the pathways of white matter fibers, the tissue through which messages travel across the brain. The methodology was developed by the research group ofThomas Mareci, Ph.D., a professor of biochemistry and molecular biology in the UF College of Medicine, and is described in a paper published July 14 in PLOS ONE.

Experts have theorized that cognitive problems in Parkinson’s are caused by a shortage of the brain chemical dopamine, which is responsible for patients’ motor issues. But with the help of imaging, the UF researchers were able to spot changes in the brain’s gray and white matter that appear unrelated to dopamine loss and are specific to those patients with Parkinson’s who have memory problems.

“Not only is gray matter important for memory, in Parkinson’s disease white matter connections between the temporal lobe and a region in the posterior portion of the brain called the retrosplenial cortex were particularly important in the recall of verbal information,” Tanner said. “People with Parkinson’s disease who had stronger connections between these areas of the brain did better at remembering information.”

Tanner’s study is part of a larger research project supported by a $2.1 million grant from the National Institute of Neurological Disorders and Stroke of the National Institutes of Health. Researchers led by Price are using imaging and cognitive testing to determine profiles for the cognitive problems that most commonly affect patients with Parkinson’s. The information gleaned from the research could help clinicians foreshadow the type of cognitive impairment a patient may experience over time, if any, and tailor treatment plans accordingly.

Posted on

Parkinson’s Disease and the Brain

The Michael J. Fox Foundation has a good, basic introduction to the neurobiology of Parkinson’s disease. The brief animate video provides an overview of affected parts of the brain as well as the role that dopamine, a neurotransmitter – a chemical in the brain that allows brain cells to communicate with each other – plays in Parkinson’s disease. Click on the link below and then click on the video link titled PARKINSON’S AND THE BRAIN to learn more about how Parkinson’s disease affects the brain.

Learn More

Posted on

Modems and White Matter

Yesterday my connection to the Internet decided to stop working. I tried restarting the cable modem, the wireless router, and other attached devices. That didn’t fix the problem. That’s usually a good first step though. I saw that the internet connectivity light was lit on the modem but the PC/Activity light was not lit. That told me that maybe the router was bad. I tried plugging my computer directly into the modem via ethernet and my computer did not recognize that a cable was plugged in. I had discovered what was wrong. While it hadn’t taken me long to figure out the problem, I did what many people do and look for solutions in the hardware first rather than in the connections. That’s not necessarily wrong, cables are more hardy than electronic components, but it did reveal my biases. So what was the problem?

The components were all okay – modem, router – but the connections were not. Wiring was the problem. Being interested in the brain, I immediately knew this would make  great brain analogy.

When someone’s cognitive functioning changes, one of the first things clinicians usually jump to is which part of the cortical or subcortical gray matter went bad, so to speak. While those components can and do go bad, we often overlook, just as I did at first, the connections. In my case, the ethernet cable had gone bad. There are many times when what’s affected in the brain are not the components but rather, the wiring – the axons. White matter might be just as important or even more important than the gray matter for cognition, even if its contribution might be more subtle. Much of my current research revolves around this idea.

So the moral of the story is that when things are not working correctly, the wiring might be the culprit.

How did my ethernet cable get damaged? Maybe it just stopped working spontaneously but it also had experienced a bit of acute stress earlier in the day (the modem fell off its stand). Something might have happened to the cable during this time. The white matter of our brain can similarly be affected by traumatic injury, nontraumatic injury (anoxia, hypoxia, etc.), stroke, or a long history of cerebrovascular problems. Just as we can take care of our electronic equipment (by not dropping it or knocking it off its home or stepping on it or whatever else we can do to our technology), we can take care of our white matter by avoiding similar injuries.

Exercise, weight control, managing diabetes, managing blood pressure, and managing cholesterol, can all help protect white matter from going bad and disconnecting different brain areas. We can’t connect to the Internet if our wiring is bad.

Posted on

Video Introduction to the Cingulum

I posted this on my neuroimaging blog and thought I should post it here too. This is a video I put together about the cingulum, a prominent white matter fiber track in the brain that is involved in emotion, attention, memory, among many other functions. All images except one from Gray’s Anatomy (the anatomy book, not the T.V. show) were created by me using some fairly advanced imaging techniques. If you are interested about some of the techniques, read my neuroimaging blog.

Posted on

The Relationship Between Executive Function and Processing Speed

Understanding the relationship between brain (specifically subcortical structures) and cognitive processes is a field still in its infancy. The rise of structural and functional neuroimaging that started in the 1970s and really began to mature in the 1990s (with even greater changes and advancements being made today), led to the ability to measure the structure and function of various brain regions in vivo. This was and is important for neuropsychologists because it allowed them to more accurately assess the relationship between the brain and cognitive and behavioral functions.

Processing speed is a basic cognitive or brain processes that subserves many other higher-order cognitive domains. Among those higher domains is executive functioning, a somewhat broad construct that involves the organization of behaviors and behavior responses, selective attention of pertinent information and suppression of unnecessary information, and maintenance and shifting of cognitive sets. Thus, executive functioning is dependent on processing speed but processing speed is not dependent on executive functioning. If executive functioning is a car, processing speed is the engine. Having a faster or more powerful engine means that the car can go faster. More efficient engines allow the car to function at a higher level of efficiency. Thus, while processing speed and executive functions are distinct, they are related with processing speed as one of the basic cognitive processes driving executive functions.

As an example of the interaction between executive functions and processing speed in clinical applications we can look at the Stroop Color-Word task. A person who is not only able to read the words or name the colors quickly but also able to inhibit the undesired but automatic process (namely, word reading on the incongruent color-word task) will receive a higher score on the Stroop task. This would, in combination with other executive function tests, be evidence for intact or even good executive functioning.

Even on non-speeded executive tasks those with fast processing speed can benefit because they can sort through information more quickly and hopefully, efficiently – speed and efficiency are related but not exactly the same. However, not all tests of executive function rely on processing speed. A person, for example, could be slow on the Wisconsin Card Sort Test, yet not exhibit any “executive dysfunction” in that they could complete all the categories and not have an abnormal number of perseverative errors. This simply demonstrates that “executive functions” are diverse and varied.

As a basic process that is dependent on basic neuronal function and glial support, any sort of focal or diffuse injury to the brain can affect processing speed. An example of this is traumatic brain injury, which frequently results in diffuse axonal injury; this diffuse injury negatively affects cognitive processing speed. Any time the white matter is focally or grossly disrupted, processing speed is in danger of disruption itself. This disruption of white matter could be anything from axonal damage, loss of oligodendroglia (which form the myelin), or even low levels of neurotransmitters.

White matter disruption also occurs in multiple sclerosis where diffuse lesions are apparent in the white matter. This disruption also occurs more often in people with heightened vascular risk factors, such as hypertension, diabetes, and cardiovascular disease. People who have these vascular risk factors and subsequent damage to their white matter (this damage or disruption is frequently termed leukoaraiosis) have reduced processing speed and attention (Viana-Baptista et al., 2008). Lesions to subcortical structures, such as the caudate, also result in reduced processing speed (Benke et al., 2003) in addition to executive dysfunction.

In subcortical disease processes such as Huntington’s disease, which usually starts with atrophy of the caudate nuclei, or Parkinson’s disease, which starts with a loss of the majority of dopaminergic cells in the substantia nigra, processing speed is consistently affected. Some common symptoms of Parkinson’s disease are freezing and a shuffling gait; even though these symptoms are motoric, they can be indicative of the global cognitive slowing that also occurs. However, it seems that processing speed is heavily dependent on the integrity of white matter.

Because of the diffusivity of processing speed, I am not aware of any areas of the brain shown to be necessary for processing speed, outside of global white matter. As I mentioned above, damage to the caudate has been shown to affect processing speed but damage to almost any area of the brain, especially if the white matter is disrupted results in slowed processing speed. Neuropsychologists often talk about a patient who has executive dysfunction, slowed speed of processing, as well as some other cognitive deficits as exhibiting signs of a frontal-subcortical disruption – a frontal-subcortical profile. So far, no one has localized processing speed to a single area – many brain structures or areas affect it.

At this point, processing speed and executive functions cannot be “mapped” to separate basal ganglia structures or loops. Of the three classically recognized cortico-striato-thalamo-cortical loops involved in cognitive and emotional processes rather than basic motor processes, which were first introduced by Alexander, Delong, and Strick (1986), the dorsolateral prefrontal cortex circuit appears to be most correlated with processing speed (Mega & Cummings, 1994). This is also the circuit most strongly linked with executive functioning. It appears that rather than utilizing different circuits processing speed and executive functions utilize the same circuits; however, processing speed is much more globalized.

References

Alexander, G. E., DeLong, M. R., & Strick, P. L. (1986). Parallel organization of functionally segregated circuits linking basal ganglia and cortex. Annual Review of Neuroscience, 9, 357-381.

Benke, T., Delazer, M., Bartha, L., Auer, A. (2003). Basal ganglia lesions and the theory of fronto-subcortical loops: Neuropsychological findings in two patients with left caudate lesions. Neurocase, 9, 70-85.

Mega, M. S., & Cummings, J. L. (1994). Frontal-subcortical circuits and neuropsychiatric disorders. The Journal of Neuropsychiatry and Clinical Neurosciences, 6, 358-370.

Viana-Baptista M, Bugalho P, Jordão C, Ferreira N, Ferreira A, Forjaz Secca M, Esperança-Pina JA, Ferro JM. (2008). Cognitive function correlates with frontal white matter apparent diffusion coefficients in patients with leukoaraiosis. Journal of Neurology, 255, 360-366.

Posted on

What is Executive Function?

Executive function is a term that describes a wide range of cognitive behaviors and processes. It is broad enough of a term that some people simply describe it as, “what the frontal lobes do.” When asked what exactly the frontal lobes do do, some revert to the circular definition of “executive functions.” However, executive functions are distinct from – but related to – what the frontal lobes do. The frontal lobes are involved in motor functions (e.g., pre-motor and primary motor areas), eye movement (e.g., frontal eye fields), memory (e.g., acetylcholine-producing portions of the basal forebrain), and language (BA 44,45 or Broca’s area). In addition, some executive functions incorporate areas of the brain outside the frontal lobes – the parietal lobes or basal ganglia, for example. Like many cognitive domains, executive functions are part of a distributed network of brain structures and regions.

Most neuropsychologists however, would define (or at least accept the following definition of) executive function similar to this: Executive function is the ability to selectively attend to, work with, and plan for specific information. This means that executive function is deciding what information, cognitions, or stimuli are relevant, holding and working with that information, and then planning what to do with it. As such, executive function is largely the roles of planning and organization. It is also the ability to recognize and learn patterns (i.e., cognitive sets) but also have the cognitive flexibility to respond to set changes and make a shift in set. Executive function also involves being able to select the appropriate response or behavior while at the same time inhibiting inappropriate responses or behaviors.

Executive functions have been compared to the conductor of an orchestra who, in order to make sense of the disparate instruments, sounds, and parts, must coordinate the members and lead the efforts of all the components of the orchestra. Executive functions also have been compared to chief executive officers of companies. These comparisons demonstrate that executive functions are arguably the most complex and highest of all cognitive functions. However, just like most other cognitive functions, executive functions are comprised of relatively simple processes (e.g., attention and processing speed) – it is just the unique combination of these more basic processes that makes executive functions so powerful.

One potential problem with executive function as a cognitive domain is that it is large and loose. Many tests have been developed, or at least used, to assess executive function (e.g., Wisconsin Cart Sort Test, Stroop Color-Word Task, clock drawing, and so forth). Even though all such tests are used as measures of executive functioning, scores on them do not always correlate highly with each other. They do not always cluster together when subjected to principal components analysis or even structural equations modeling. This means that even though neuropsychologists have many purported tests of executive function, they all seem to measure different aspects of executive function. This might partially result from executive functioning tests being differentially affected by basic cognitive processes such as processing speed.

Even though, as previously mentioned, I do not believe executive functions and frontal lobe functions are synonymous terms, are we able to localize executive functions to the frontal lobes? Largely we can. The most evidence from neuroimaging studies and neurological injuries demonstrate that the prefrontal cortex – the area of the brain that is phylogenetically youngest and most advanced and as such, proportionately larger in humans than any animal – is necessary (but not necessarily sufficient) for executive functioning. When this area is disrupted in humans, they exhibit poor decision-making skills, including poor planning and poor maintenance or self-regulation of behavior. One area of the prefrontal cortex particularly involved in executive functions is the dorsolateral prefrontal cortex (area 46) – although both the orbitofrontal and anterior cingulate are involved in aspects of executive functions.

In 1986 Alexander, Delong, and Strick published their seminal work on five parallel and closed cortico-striato-thalamo-cortical loops. These frontal-subcortical circuits were hypothesized to be involved in a range of behaviors and cognitions based on the varying cortical connections of the loops. Previously, many researchers did not well-understand the role that the basal ganglia played in any sort of “higher” function; in fact, most viewed the basal ganglia as involved mainly in motor behaviors. Alexander, Delong, and Strick’s article set off a flurry of research into the functions of these frontal-subcortical circuits, which have been verified as existent in humans (Middleton & Strick, 2000). Over time different theories have modified these circuits, including that they are composed of direct, indirect, and hyperdirect pathways, which all function at different speeds or timings to allow the basal ganglia to regulate behavior. Mink (1996) proposed that actions (e.g., producing a specific word) are regulated by the direct and indirect pathways, which serve as large components of our ability to select and inhibit correct and incorrect responses, respectively. It is as if each individual fronto-cortical loop allows us to properly attend to the correct behavior or response and inhibit all other behaviors or responses, much like the DLPFC and orbitofrontal cortex and their associated loops are involved in the selection and inhibition of behavior, both major aspects of executive function.

Just as damage to the dorsolateral prefrontal cortex (DLPFC) produces deficits in executive function, damage to any part of the DLPFC loop also results in executive dysfunction. Benke, Delazer, Bartha, and Auer (2003) presented two clinical cases of patients with left caudate lesions (the lesions also affected part of the anterior limb of the internal capsule as well as portions of the putamen and pallidum; however, the infarcts affected the caudate the most). Among other deficits, both patients had executive function impairments, including problem-solving deficits, many perseverative errors, and set-shifting problems. Even though the patients had no direct DLPFC damage, they exhibited similar deficits to patients with DLPFC lesions. These executive deficits persisted over time.

As a cognitive domain, and even as broad as it might be, executive functioning has ecological validity. Price and colleagues (2008) found that executive dysfunction was related to greater difficulty performing IADLs. Specifically, patients with executive dysfunction had more difficulty performing IADLs than patients with memory deficits did. Thus, how quickly, flexibly, and accurately people can organize, solve, plan, or attend to specific neuropsychological tasks seems to correlate with their accomplishment of everyday tasks of life, such as finances, driving, and shopping.

References

Alexander, G. E., DeLong, M. R., & Strick, P. L. (1986). Parallel organization of functionally segregated circuits linking basal ganglia and cortex. Annual Review of Neuroscience, 9, 357-381.

Benke, T., Delazer, M., Bartha, L., Auer, A. (2003). Basal ganglia lesions and the theory of fronto-subcortical loops: Neuropsychological findings in two patients with left caudate lesions. Neurocase, 9, 70-85.

Middleton FA, & Strick PL. (2001). Basal ganglia output and cognition: evidence from anatomical, behavioral, and clinical studies. Brain Cogn., 42, 183-200.

Mink, J. W. (1996). The basal ganglia: Focused selection and inhibition of competing motor programs. Prog Neurobiol, 50, 381-425.

Price, C.C., Garvan, C., and Monk, T. (2008). Type and severity of cognitive impairment in older adults after non-cardiac surgery. Anesthesiology, 108, 8-17.

Proudly powered by WordPress