Thank you for your question. I will first give a general overview of the function of the parietal association cortex and then go into some of the lesions which may make it easier to understand function. Also keep in mind that the anatomical basis of some of these lesions is not clearly set in stone and there are always reported cases of other regions producing similar disorders.
The term association cortex refers to regions of the cortex which have extensive connections with multiple sensory and motor areas. They combine information to give rise to complex parts of cognition. The parietal association cortex refers to the posterior parietal cortex, the part that does not include the primary somatosensory cortex. This region is anatomically divided into the superior and inferior parietal lobules by the intraparietal sulcus.
The posterior parietal cortex is connected to motor, somatosensory and visual areas of the cortex. It integrates this information to inform movement and give a sense of where the body is in space, and where objects are in relation to each other. These two functions are often interrelated. For example, when reaching for a book on your desk while looking at your computer screen, your brain must know where the book is within your visual field, in relation to the screen and to your hand. The dorsal part of the cortex is predominantly connected to the somatosensory cortex and is more involved in motor function, whereas the ventral parts of the cortex is mainly connected to the dorsal stream of the visual cortex and is more involved in spatial awareness (you can read more about the two-stream hypothesis here).
This is when the patient ignores events in the half of space that is contralateral to the lesion. They are usually caused by strokes occluding the middle cerebral artery, affecting the ventral aspect of the parietal lobe (referred to as the inferior parietal lobule). Patients can even neglect shaving one half of their face, and when asked to draw objects, it is only half complete.
Hemispatial neglect usually occurs from a lesion on the right side of the brain. It is thought that the right parietal lobe is dominant in processing spatial information from both sides of the body. So when there is a lesion on the right side, the left side is not able to compensate. Note that lesions in the frontal cortex and superior temporal gyrus have also been implicated in this condition.
A similar condition is asomatognosia where a patient ignores the existence of their arm or leg on the opposite side of the lesion. This impacts the sense of self, rather than the awareness of objects in space.
Damage to more dorsal parts of the posterior parietal cortex, usually on the left side can cause apraxia, which refers to an inability to perform actions even though the muscles required are perfectly normal. There are many types of apraxia depending on the exact region affected. Ideomotor apraxia is characterised by an inability to perform hand gestures such as waving goodbye and acting out movements such as pretending to brush their teeth.
Acalculia and agraphia
The posterior parietal cortex is also involved in abstract thinking. Acalculia refers to a difficulty performing calculations, especially with multi-digit numbers. Lesions in the left angular gyrus can cause agraphia and alexia, where patients cannot read, write or spell. These functions depend on the ability to remember the sequence of letters in a word, or the place value of digits which is a form of spatial awareness.
This deficit reflects the importance of the posterior parietal cortex in combining visual and motor information to guide movement. Patients with optic ataxia are unable to reach for an object in their visual field. They can however reach when visual information is not required, such as reaching their leg in the dark.
Essentially, the parietal association area serves multiple functions, with a predominant involvement in spatial awareness. I hope this answered your question and let us know if you require further clarification.
If the posterior parietal cortex is responsible for giving a sense of where the body is in space, then how is it that a patients with optic ataxia are able to reach for their leg in the dark? How would they know where their leg is?
There is great variation in the types of deficits seen in these patients but generally they have greater trouble reaching for objects in the peripheral parts of their visual field. The classical understanding is that this impairment causes difficulties in movements guided by vision. It is also usually part of a condition known as Bálint's syndrome which includes other deficits. You can get optic ataxia just by itself (even rarer than Bálint's syndrome) and interestingly these are caused by lesions in the superior parietal lobe or the intraparietal sulcus. There is a specific area in the macaque brain around that area called the parietal reach region which produces optic ataxia, but in humans they aren't entirely sure whether there is only one parietal reach region, so I would not worry too much about the localisation. This also is against what I said earlier about the more dorsal parts of the parietal cortex dealing with somatosensory information and ventral parts visual, but you must keep in mind that it is not as simple as drawing clear boundaries to dedicate to function and many exceptions exist, especially within these association areas.
Specifically in terms of your question, it really depends on the extent of damage. The function you are referring to is proprioception, which is essentially knowing where your body is in space. Now it was thought before that proprioception was not particularly impacted in optic ataxia since it is due to a deficit in combining vision with movement. Proprioception on the other hand relies more on the function of the cerebellum and cortical areas, rather than sight. This is why the patients can usually touch their knee when asked without relying on vision. Recently however, it was seen that it may in fact be much more complex than this and optic ataxia may impact proprioception, and indeed with more severe lesions they cannot do the pointing to their knee in the dark. I would not worry too much about the exact lesion and the variations but instead have this in mind to understand the function of the parietal cortex a bit better. There is very little known about optic ataxia since it is very rare so something like hemispatial neglect is much better to focus on since some more detail exists on it, though again it is still not that well understood.
I hope this was helpful and let us know if you have any more questions.
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