Cerebral Cortex Parietal

The parietal lobe sits between the visual cortex at the back of the brain and the somatosensory cortex just behind the central sulcus. Textbooks have described it this way for decades. Most neurology students learn the basics in their second year and move on.

Edoardo Bisiach and Claudio Luzzatti did not move on.

In 1978, these two researchers at the Centro di Neuropsicologia dell'Università di Milano published a short paper in Cortex. The paper described two patients. One was an 86-year-old woman, a retired manager. The other was a 72-year-old male lawyer. Both had suffered strokes affecting the right temporoparietal region. Both had a condition called hemispatial neglect.

Patients with hemispatial neglect behave as if one side of the world does not exist. They eat food from only one side of their plate. They shave only one side of their face. They draw clocks with all the numbers crowded onto the right half. The left side of space simply disappears from their awareness.

Bisiach and Luzzatti wanted to know something specific. When these patients closed their eyes and imagined a familiar place, would the left side of that imagined place also disappear?

Then the researchers asked the patients to imagine standing at the opposite end of the square, facing the cathedral. The patients now described the previously neglected buildings. Those buildings were now on their right.

The patients knew the entire square. The information was in their memory. They just could not access all of it at once. Whatever side would have been on their left from their imagined viewpoint was simply gone.

This finding changed how researchers thought about the posterior parietal cortex. The region was not just processing incoming sensory information. It was involved in constructing internal representations of space. Damage to the right posterior parietal cortex disrupted something fundamental about how the brain organizes spatial awareness.

The posterior parietal cortex consists of Brodmann areas 5, 7, 39, and 40 in humans. In macaque monkeys, it consists of areas 5 and 7. In rodents, just area 7. The intraparietal sulcus divides it into a superior parietal lobule and an inferior parietal lobule. The inferior parietal lobule contains the supramarginal gyrus, the temporoparietal junction, and the angular gyrus.

These anatomical divisions matter because different parts do different things.

Giacomo Rizzolatti's team at the University of Parma discovered something unexpected about the parietal cortex in the 1990s. They were recording from neurons in the ventral premotor cortex of macaque monkeys, studying neurons that fired when the monkey grasped objects. A graduate student reached for a peanut. A neuron fired. The monkey had not moved.

The same neurons that fired when the monkey performed an action also fired when the monkey watched someone else perform that action. Rizzolatti's team found these mirror neurons in the inferior frontal gyrus and in the inferior parietal lobule. About 10% of neurons in these regions have mirror properties.

A neuron would fire differently when a monkey watched someone grasp food to eat it versus grasping food to place it in a container. The motor act was identical. The goal was different. The parietal cortex was reading intentions.

Stroke patients with right parietal damage lose this integration. The deficit shows up in ways that seem bizarre until you understand what the parietal cortex actually does.

A patient might deny that their left arm belongs to them. Somatoparaphrenia, this is called. The arm is there. They can see it. But the sense that it is their arm has been disrupted.

A patient might be unable to disengage attention from the right side of space. Michael Posner and colleagues showed this in 1984 using a simple cueing task. A cue appears on one side. A target appears on the other side. Parietal patients cannot shift attention away from the cued location to detect the target on the opposite side.

Recovery from hemispatial neglect varies. Most patients in the acute phase after stroke show some improvement as swelling decreases and reperfusion occurs. At twelve weeks, neglect persists in about 17% of patients with right hemisphere lesions. These patients have longer hospital stays and higher rates of falls. Many cannot return to work.

O'Shea's group tried something different. Instead of suppressing the left parietal cortex, they stimulated the left sensorimotor cortex while patients underwent prism adaptation. The idea was to strengthen memory for the adaptation.

The results exceeded expectations. Patients with chronic neglect, more than a year after stroke, showed improvements that lasted far longer than expected. Plasticity that had seemed dormant could be reawakened.

Modern imaging has revealed more subdivisions within the posterior parietal cortex than Brodmann's original maps suggested. The Human Connectome Project atlas identifies over twenty distinct regions. Area 7 alone contains multiple subdivisions: 7AL, 7Am, 7PC, 7PL, 7Pm. The intraparietal sulcus contains AIP, LIPd, LIPv, MIP, VIP, IP0, IP1, IP2.

Each region has distinct connectivity patterns. Each appears to serve somewhat different functions. The lateral intraparietal area in humans is implicated in the fine eye movement control required for reading. The ventral intraparietal area responds to stimuli approaching the face. The anterior intraparietal area is involved in grasping.

The complexity keeps increasing as techniques improve.

Hemispatial neglect affects about 80% of stroke patients with right hemisphere damage in the acute phase. Most recover. Some do not. For those with lasting neglect, daily life becomes difficult in ways that are hard to appreciate unless you see it.

A man walks down a hallway and bumps into the left wall repeatedly. He is not blind on that side. His eyes work fine. Something upstream from vision has broken. The left side of the world no longer registers as important.

A woman reads a newspaper and cannot find the beginning of each line. She starts reading in the middle. The left margin does not exist for her attention.

The posterior parietal cortex constructs the frame through which we experience space. Damage that frame and spatial experience itself becomes incomplete.

The region covers about 150 square centimeters in humans. It evolved extensively after the emergence of primates. No rodent model captures its full complexity. No current imaging technique resolves all its subdivisions.

What researchers know keeps changing as methods improve. Brodmann's maps from 1909 were a starting point. The Human Connectome Project atlas is more detailed. Future parcellations will be more detailed still.