Parietal Lobe Pathologies
Unilateral neglect: loss of awareness of half of the experiential field This condition has been associated with lesions in the supramarginal gyrus. If damage occurs in the right parietal lobe, a person may lose awareness of the left side of the world. If it occurs in the left parietal lobe, awareness of the right side of the world might be lost, but this is uncommon because of duo-lateral distribution of spatial processing. Normally, the right hemisphere handles our entire spatial world, while the left hemisphere handles only the right. As with prosopagnosia (inability to recognize faces), it is difficult to grasp the experience involved in unilateral neglect. The experience is not one of being blind to parts of the visual field. Someone with simply partial or occluded vision may have such awareness. The person with unilateral neglect is unaware of anything missing.
The experience often involves the body as well. A person with unilateral neglect caused by right parietal lobe damage might dress improperly on the left side, comb their hair only on the right side of their head, make the bed only on the right side, and leave food on the left side of the plate. If asked to draw the face of a clock, the person might draw the right side only. Some patients deny ownership of one or more limbs, such as is the case with alien hand syndrome. In one case, a person attempted to throw away his own leg (Sacks 1998, p. 55). Whatever the experience is like, neglect patients act as if parts of the world didn’t exist.
Phantom limbs: experiencing dismembered limbs or organs as alive and attached Phantom limb patients experience a disconnection between body image and body. A limb such as an arm is missing; yet the parietal lobe retains false awareness of it. It is unclear why. Work by neurologist Vilayanur Ramachandran (1951—) shows that phantom limb pains can be alleviated through therapy. He has worked on patients with cramps in phantom hands. Ramachandran puts the patient’s remaining hand inside a box with a vertical mirror (the stump is placed on the other side of the mirror). The person moves his remaining hand and imagines making symmetrical phantom movements. As the person sees a hand moving in the mirror, the brain comes to associate it with the phantom limb’s body image. The person feels as if his phantom limb is moving freely, and the cramp diminishes. Ramachandran describes his first mirror box experiment this way:
The first patient we tried this on, Jimmie, had an intact right arm, phantom left arm. His phantom jutted tike a mannequin's resin-cast forearm out of his stump. Far worse, it was also subject to painful cramping that his doctors could do nothing about. I showed him the mirror box and explained to him this might seem like a slightly off-the wall thing we were about to try, with no guarantee that it would have any effect, but he was cheerfully willing to give it a try. He held out his paralyzed phantom on the left side of the mirror, looked into the right side of the box and carefully positioned his right hand so that its image was congruent with (superimposed on) the felt position of the phantom. This immediately gave him the startling visual impression that the phantom had been resurrected. I then asked him to perform mirror-symmetric movements of both arms and hands while he continued looking into the mirror. He cried out, “It’s like it’s plugged back in!” Now he not only had a vivid impression that the phantom was obeying his commands, but to his amazement, it began to relieve his painful phantom spasms for the first time in years. It was as though the mirror visual feedback (MVF) had allowed his brain to “unlearn” the learned paralysis. (Ramachandran 2011, p. 33)
The cause of phantom pains has traditionally been thought to be inflamed nerve endings and/or scar tissue, but Ramachandran thinks the brain is in denial. It refuses to accept that the arm is gone. Ramachandran suggests that mirror box therapy can help the brain accept it:
When one of our patients, Ron, took the mirror box home and played around with it for three weeks in his spare time, his phantom Limb vanished completely, along with the pain.
All of us were shocked. A simple mirror box had exorcised a phantom. How? No one has proven the mechanism yet, but here is how I suspect it works. When faced with such a welter of conflicting sensory inputs—no joint or muscle feedback, impotent copies of motor-command signals, and now discrepant visual feedback thrown in via the mirror box—the brain just gives up and says, in effect, “To hell with it; there is no arm.” (Ramachandran 2011, p. 34)
With enough evidence, the brain accepts reality. One reason why the brain comes to live in postamputation denial in the first place may be a reconfiguration of neural pathways.
Apraxia: restricted ability to move The person afflicted with ideomotor apraxia is unable to move well, because of impairment of neural structures—often parietal— for timing, sequencing, and controlling motor action. The patient knows what to do but not how to do it. Conceptual apraxia is a reverse form of this condition. Conceptual apraxia patients can perform movements but with poor understanding of them. They might confuse different tools and usage patterns. If asked to brush their teeth, patients may pick up a comb and perform a shaving motion. There are also many other forms of apraxia.
Gerstmann’s syndrome: impaired writing and mathematical skills, difficulty in keeping track of left, right, and individual fingers This was discovered by Josef Gerstmann (1887-1969) as a body image disturbance caused by parietal cortex damage.
Balint’s syndrome: difficulty perceiving visual change, multiplicity, and depth This was first described by Hungarian neurologist Rezso Balint (1874-1929) in 1909 and is usually caused by multiple lesions to the parietal lobes. Lesions to the angular gyrus are strongly correlated with the syndrome. A Balint’s patient is unable to see and track more than one object at a time in space (simultagnosia) and cannot shift gaze easily (occulomotor apraxia). Balint’s patients lose much of their understanding of space. The visual world of a patient with Balint’s syndrome is chaotic, with flickering appearances of things, which cannot be easily grasped or located. It is a torturous world to interact with, and patients need assistance in their daily life. For clinical purposes, Balint’s syndrome patients are typically considered functionally blind, although they may have clear visual perception of isolated single objects.
Parietal Lobe Summary
The parietal lobes facilitate our abilities to position and orient ourselves in the world. They are crucial for our sense of being embodied and for normal perception. With lesions in the parietal lobes, people typically find themselves lost and disoriented in one way or another.