Know your brain: Posterior parietal cortex

Where is the posterior parietal cortex?

posterior parietal cortex in blue.

posterior parietal cortex in blue.

The posterior parietal cortex comprises the region of the parietal cortex that is posterior to the primary somatosensory cortex and its adjacent sulcus, the postcentral sulcus. The posterior parietal cortex itself is divided into an upper and lower portion: the superior parietal lobule and inferior parietal lobule, respectively. These two lobules are separated from one another by a sulcus called the intraparietal sulcus.

What is the posterior parietal cortex and what does it do?

The posterior parietal cortex receives input from a collection of sensory areas as well as a variety of other regions of the brain, and is thought to integrate that input to facilitate the execution of functions that require diverse information. It has been associated with a number of these functions, which are sometimes called "higher-order" functions; it is probably best known, however, for its role in attention.

Through attempts to find the brain regions that facilitate attention, researchers have identified two attention-related networks that involve the posterior parietal cortex; these are termed the dorsal and ventral fronto-parietal systems. The dorsal system is found in both cerebral hemispheres and includes areas of the superior parietal lobule and intraparietal sulcus as well as a region of the frontal cortex that is involved in eye movements and visual perception known as the frontal eye field. The dorsal system is thought to be involved with what is known as "endogenous attention," which involves attention that is directed based on individual goals or desires. For example, if you are attempting to focus your attention to read this article, you are utilizing endogenous attention. 

The ventral system is found primarily in the right cerebral hemisphere and includes the area where the temporal and parietal lobes meet (the temporo-parietal junction), the intraparietal sulcus, and areas of the frontal cortex. The ventral system seems to be involved more in what is termed "exogenous attention," or attention that is directed towards external stimuli that are not being attended to by endogenous attentional processes. For example, if you were reading this article in a library and someone a few tables over shouted, breaking the complete silence of the room, you would suddenly and reflexively direct your attention to the person who shouted. This type of attention is not associated with your own goals or desires, and falls under the rubric of exogenous attention.

an example of what a clock drawn by a patient with hemispatial neglect might look like.

an example of what a clock drawn by a patient with hemispatial neglect might look like.

The importance of the posterior parietal cortex to attention is perhaps best exemplified by a condition that can occur after damage to the posterior parietal cortex known as hemispatial or contralateral neglect. Hemispatial neglect is most frequently associated with damage to the posterior parietal cortex in the right cerebral hemisphere (due to stroke, head trauma, etc.), after which the patient ceases to devote attention to the left side of their body and visual field. These patients can act as if they don't perceive anything in a certain part of their visual field; if asked to draw a picture, they will often not include a significant portion (up to half) of the item drawn, they may eat only about half of the food off of a plate, and shave or put makeup on only half of their face. Some patients may even deny that part of their body on the neglected side is theirs in an attempt to reject the idea that they are suffering from a neurological condition.

The posterior parietal cortex is also believed to be involved in some aspects of motor function, such as planning movements and integrating visual information with movement to facilitate actions like reaching and grasping. Additionally, regions of the posterior parietal cortex are thought to contain neurons called mirror neurons, which are activated not only when a particular action is performed but also when someone else is observed performing the same action. The true function of mirror neurons is yet to be determined. Some hypothesize that they are important to allowing us to learn by imitation, or even for understanding the actions of others; but there are also many who are critical of these hypotheses, arguing they are too speculative and lacking evidential support.

Additionally, the posterior parietal cortex is thought to be involved in language as well as the ability to understand numbers and arithmetic. Thus, its functions span a large spectrum ranging from attention to movement to number processing. Research is still being done to better understand the role of the posterior parietal cortex in these actions and others. What is known about the posterior parietal cortex already, however, makes it one of the more intriguing areas in the brain.

Caspers S, Amunts K, Zilles K. Posterior Parietal Cortex: Multimodal Association Cortex. JK Mai and G Paxinos (Eds.). 2012; Elsevier, New York.

Know your brain: Fornix

Where is the fornix?

fornix.

The term fornix comes from Latin and means "arch." It is used to refer to various arch-like structures in the body, but when used in reference to the brain it indicates a bundle of white matter fibers that arches around the thalamus. The fornix originates in the hippocampus, where it emerges from a collection of fibers called the fimbria. It then stretches up and around the thalamus toward the front of the brain. When it reaches a tract called the anterior commissure, it branches downward. Some fibers then split off and terminate mainly in the septal nuclei, preoptic nuclei, and ventral striatum, while others enter the hypothalamus and form connections with the mammillary bodies.

What is the fornix and what does it do?

In 1937 the neuroanatomist James Papez described what came to be known as the Papez circuit. The Papez circuit consisted of a group of structures---including the hippocampus, mammillary bodies, anterior nucleus of the thalamus, cingulate gyrus, and parahippocampal gyrus---that Papez hypothesized were the "anatomic basis of emotions." The fornix was a critical component of the Papez circuit, acting as a primary connection among several structures within the circuit. The Papez circuit would later be expanded upon and termed the limbic system. 

The diverse group of structures known as the limbic system is now thought to be involved in much more than emotion, and the fornix is still considered an important part of the limbic system. The fornix acts as the primary outgoing pathway from the hippocampus, and thus its most recognized function is its involvement in memory. The hippocampal projections that travel in the fornix are thought to be important for memory consolidation, and damage to the fornix has been associated with anterograde amnesia, which involves the inability to create new memories. Fornix damage is primarily linked to deficits in declarative memories, or memories for factual information---and especially episodic memories, which are a type of declarative memory that deals with autobiographical information.

Neurodegeneration in the fornix has also been associated with the cognitive impairment seen in Alzheimer's disease. The integrity of the fornix may be compromised in the early stages of Alzheimer's disease, and thus may be an early indicator of the disease that can predict the progression of Alzheimer's disease from preclinical (i.e. asymptomatic) to clinical (i.e. symptomatic) stages. Degeneration of the fornix in Alzheimer's disease seems to precede degeneration of the hippocampus, an area that is known to be severely affected by the disease.

Although the functions of the fornix are still relatively poorly understood, its role in memory processes seems to be one that is relatively well supported. Due to its diverse connections, the fornix likely is involved in a list of other brain activities, but more research will be needed to further elucidate these roles.

Know your brain: Septum

Where is the septum?

the septum, with separate indicators for the septum pellucidum and septal nuclei.

The term septum, when used in reference to the brain (it is a common anatomical term used to refer to a partition), indicates a subcortical structure in the forebrain that is found near the midline of the brain. The septum in humans can be separated into two structures: the septum pellucidum and septum verum. Each of these is sometimes referred to on is own as the "septum," which can make references to the structure a bit confusing.

The septum pellucidum, which is Latin for “translucent wall,” is a thin, almost transparent membrane that runs down the middle of the brain from the corpus callosum to the area of a large fiber bundle called the fornix. The septum pellucidum acts as a partition between a portion of the lateral ventricles, forming part of the walls of the anterior region of the lateral ventricles. It is made up of a thin two-layered structure that consists of white matter, some neurons, fiber bundles, and blood vessels. The septum pellucidum is surrounded by neurons that make up the septum verum, which consists of assorted nuclei commonly referred to as the septal nuclei. The septal nuclei themselves are often categorized based on location and are split up into lateral and medial (and sometimes additional caudal and ventral) divisions.  

What is the septum and what does it do?

Little is known about the functional role of the septum pellucidum, and it is often treated as simply an anatomical barrier in many discussions of the septum. However, its connections with the hippocampus and hypothalamus suggest a role at least as a relay station between these structures. Although abnormalities of the septum pellucidum are associated with several neurological conditions, it is at this point unclear what role, if any, the septum pellucidum plays in directly generating the symptoms of such disorders.

A bit more is known about the actions of the septal nuclei, which seem to be involved in a variety of functions, although their exact role in many of these functions is still relatively poorly understood. Additionally, most of what we do know about the septal nuclei comes from animal studies, as there is little research available on the functions of the septal nuclei in humans.

The septal nuclei are considered part of the limbic system, a group of subcortical structures that are often linked to emotion but are really involved in a long list of functions in the human brain. The septal nuclei receive afferent (i.e. incoming) connections from other limbic structures like the hippocampus, amygdala, and hypothalamus, as well as the dopamine-rich ventral tegmental area. The septal nuclei also send projections to the hippocampus, habenula, thalamus, ventral tegmental area, and hypothalamus.

One of the first functional roles to be linked to the septal nuclei was an involvement in processing rewarding experiences. In a now-famous group of experiments, researchers James Olds and Peter Milner found that electrical stimulation of the septal nuclei and several other areas of the brain seemed to be rewarding to rats. Rats, in fact, responded more strongly to stimulation in the septal region than any other part of the brain studied, leading Olds and Milner to hypothesize the septal region was perhaps the locus of the reward system.

Although our understanding of the reward system has since expanded and less importance has been placed on the septal nuclei in favor of other structures like the ventral tegmental area and nucleus accumbens, the septal nuclei are still thought to potentially play a role in reward processing. Neurons in the septal nuclei give rise to axons that travel in the medial forebrain bundle, a collection of fibers that connects the nuclei with the hypothalamus and ventral tegmental area. The medial forebrain bundle is an important part of the reward system, thought to stimulate dopamine neurons in the ventral tegmental area in response to rewarding stimuli.

The septal nuclei also are densely interconnected with the hippocampus, and through these connections may play a role in learning and memory. The septal nuclei and hippocampus are sometimes referred to as the septo-hippocampal complex, and projections to the hippocampus (which travel in a fiber bundle called the fornix and are often called septohippocampal fibers) are some of largest projections from the septal region. Although the precise role of the septal nuclei in memory functions is not yet clear, the hippocampus receives the majority of its acetylcholine projections from the septal region. These neurons are activated during learning and degenerate in conditions like Alzheimer's disease that are characterized by disruptions in memory processes.

The septal nuclei have been implicated in a number of other roles such as social behavior and the expression of fear, and abnormalities in septal functioning have been linked to a variety of disorders ranging from depression to schizophrenia. The septal area, however, including both the septum pellucidum and septum verum/septal nuclei, is still relatively poorly understood; it will take more research to fully elucidate its functions and influence on behavior.

Sheehan, T., Chambers, R., & Russell, D. (2004). Regulation of affect by the lateral septum: implications for neuropsychiatry Brain Research Reviews, 46 (1), 71-117 DOI: 10.1016/j.brainresrev.2004.04.009

2-Minute Neuroscience: Primary Somatosensory Cortex

In this video, I discuss the primary somatosensory cortex. The primary somatosensory cortex is responsible for processing somatic sensations, or sensations from the body that include touch, proprioception (i.e. the position of the body in space), nociception (i.e. pain), and temperature. The primary somatosensory cortex is generally divided into 4 areas: area 3a, 3b, 1, and 2. In the video, I discuss the relative functions of each of these areas. 

Read more - Know your brain: Primary somatosensory cortex

2-Minute Neuroscience: Suprachiasmatic Nucleus

The suprachiasmatic nuclei (SCN) are thought to be involved with maintaining circadian rhythms, or biological patterns that follow a 24-hour cycle. To accomplish this, the cells of the SCN contain biological clocks. In this video, I discuss the molecular mechanism driving the biological clocks in the cells of the mammalian SCN, and how a cycle of gene expression allows the activity of these cells to follow a 24-hour pattern. 

You can read more about the suprachiasmatic nucleus in this Know Your Brain article.