Damage one brain lobe and half the world can simply vanish
After an injury to the right parietal lobe, some people stop noticing everything on their left: food on that side of the plate, the left half of a drawing, even their own left arm. This region knits touch, vision and body position into a working map of space, and losing it shows how much we take for granted.
The parietal lobe is one of four big lobes of the cerebral cortex, sitting behind the frontal lobe and above the temporal lobe. Its name comes from the parietal bone, from the Latin word for wall. Its front strip, the postcentral gyrus, is the main receiving area for touch. Signals for touch, heat and pain from the skin pass through the thalamus to reach it, and each hemisphere handles the opposite side of the body. Drawn to scale by how much cortex each part gets, the body becomes the lopsided cortical homunculus, or little man.
Further back, the posterior parietal cortex blends sight and touch to steer the arms, hands and eyes. Vision researchers call it the dorsal stream, nicknamed both the where pathway and the how pathway. Studies of macaques in the 1990s found that separate patches along the intraparietal groove each track different parts of space. One maps which spots deserve attention and helps aim eye movements; another codes reach targets relative to the eyes; a third responds to the shape, size and angle of objects about to be grasped. Human brain scans have since found comparable regions, organised around where the eyes are pointing.
Which side is damaged changes the result. Right-sided injury tends to wipe out spatial imagery and produce that neglect of the left side of space and body. Left-sided injury more often disrupts arithmetic, writing, sustained reading and handling symbols. Gerstmann syndrome bundles four problems together: trouble with calculation and writing, inability to tell fingers apart, and confusion between left and right. Other signs include failing to recognise a three-dimensional object by touch, or to read a letter traced on the palm with eyes closed.
The lobe also seems to shape remembering. When the inferior parietal region is damaged on both sides, people do not become amnesic, yet their memories grow weaker, complex events lose detail, and they feel little confidence in what they recall. Researchers still debate whether this reflects inward attention, the feeling of remembering, or the way evidence builds up toward a decision.
Source: Parietal lobe
