Parietal lobe · Grey Matter

The parietal lobe is the upper back part of each hemisphere that builds the brain's model of where things are: where the body is, where objects are around it, where attention should go and, by extension, how large a quantity is.

Parietal lobe. The parietal lobe is the upper back part of each hemisphere that builds the brain's model of where things are: where the body is, where objects are around it, where attention should go and, by extension, how large a quantity is.

It lies behind the central sulcus, above the lateral fissure and in front of the occipital lobe. Its front strip, the postcentral gyrus, is the primary somatosensory cortex, which receives touch, pressure, temperature, pain and vibration. Behind it the posterior parietal cortex combines touch with vision and the sense of limb position to guide reaching and grasping. The intraparietal sulcus divides it into a superior and an inferior lobule; the inferior lobule holds the supramarginal gyrus (Brodmann area 40) and the angular gyrus (area 39), which border the temporal lobe at the temporoparietal junction.

Number lives here more than anywhere else. The horizontal segment of the intraparietal sulcus is active whenever quantities are compared or estimated, and the left angular gyrus supports arithmetic facts learned by rote (multiplication tables).

The right side attends to both sides of space, the left side mostly to the right. That asymmetry is why damage to the right inferior parietal lobe leaves hemispatial neglect of the left side, while the mirror lesion rarely produces a lasting deficit.

In the left hemisphere the angular and supramarginal gyri also belong to the language network (reading, writing, the sound of words).

Questions: Which areas work together when you search for the keys you put down somewhere? The hippocampus tries to recover the episode of putting them down, with its place and time, using the map built by its place cells and the grid cells of the entorhinal cortex. The posterior parietal cortex turns the remembered location into a position relative to your body and guides the eyes and steps toward it. The dorsolateral prefrontal cortex organises the search (where was I last, which rooms are already checked) and keeps the goal in mind while attention moves from place to place. Is there a mathematics area, or which areas solve a sum in your head? There is no single mathematics area; several networks share the work. The horizontal segment of the intraparietal sulcus represents quantity (it is active whenever two numbers are compared), the left angular gyrus retrieves facts learned verbally such as multiplication tables, and the posterior superior parietal lobe moves attention along the mental number line. The dorsolateral prefrontal cortex holds intermediate results in working memory and plans the steps, and the anterior cingulate cortex catches errors and conflicts along the way. Why does damage to the right parietal lobe make people ignore the left side, while left damage rarely does the reverse? Attention to space is lateralised: the right parietal and frontal network attends to both sides of space, while the left attends mainly to the right side. After a left lesion the intact right hemisphere still covers both sides, so neglect of the right is rare and mild. After a right lesion, especially of the inferior parietal lobe and temporoparietal junction, only the left hemisphere is left, and it has little interest in the left side, so the patient eats half the plate and draws half the clock.