Where is the posterior parietal cortex?
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.
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.
Reference (in addition to linked text above):
Caspers S, Amunts K, Zilles K. Posterior Parietal Cortex: Multimodal Association Cortex. In: Mai JK and Paxinos G, eds. The Human Nervous System. 3rd ed. New York: Elsevier; 2012.