After a patient is diagnosed with dementia, it is important to determine the kind of dementia that is present. Dementias are often classified by the region of the brain that is affected. One of the main classifications divides dementias into two main groups: cortical and sub-cortical based on the area of the brain where degeneration occurs. The cortical region consists of the cerebral cortex while the sub-cortical region is comprised of the other structures of the brain including the thalamus, hypothalamus, cerebellum and brain stem (See Figure 1). To learn more about the brain click herefor the HOPES Brain Tutorial. Whether cortical and subcortical dementias should be considered separately is still controversial among researchers and physicians. In general, studies have shown that some differences do exist, but there is disagreement on the degree to which the two dementias differ.
Figure 1: The brain can be divided into the cortical and subcortical regions. The cortical region consists of the cerebral cortex while the sub-cortical region consists of the thalamus, hypothalamus, cerebellum and brain stem.
To more clearly define the two types of dementia, researchers have studied whether their effects on memory differ. Alzheimer's patients are often used as a model for cortical dementia because patients with this disease have large amounts of degeneration in the cerebral cortex. Clinical studies of Alzheimer's patients have shown that cortical dementias have difficulty performing tasks that require semantic memory. Semantic memory is what we use to store facts without respect to the setting where we learned the facts (See Figure 1). To evaluate this type of memory, patients are asked to perform tasks such as matching pictures and generating definitions of words. To a lesser degree, cortical dementia can also affect episodic memory, which is used to remember experiences and the setting in which facts are learned. For example, after a boy bumps his head in a bike accident, semantic memory would enable him to remember that wearing a helmet is important when riding bikes, while episodic memory would enable him to recollect the specific time when the accident occurred.
Sub-cortical dementias have a slightly different effect on memory than cortical dementias in that they have a smaller effect on semantic memory. HD and Parkinson's disease are considered sub-cortical dementias. In HD, patients instead find it challenging to accomplish cognitive tasks that require retrieval and synthesis of known facts, such as forming abstractions. Unlike patients with Alzheimer's, however, those with HD can accomplish tasks that require semantic memory under the right conditions. For example, one study tested patients on a category fluency task in which patients were asked name as many items as possible from certain categories (e.g. foods, animals, plants) within an allotted time period. Under what scientists call "cued" conditions in which patients were given hints or clues that help with the task, HD and Parkinson's patients improved their scores. Alzheimer's patients, however, did not perform better under "cued" conditions. This suggests that patients with HD or other types of sub-cortical dementia have not experienced degradation of semantic memory per se, but instead have difficulty retrieving facts from their memory.
Sub-cortical dementias like HD do not affect memory following a time-dependent gradient. Memories and knowledge obtained recently are not more susceptible to degeneration than those from the distant past, as is the case in Alzheimer's Disease. To learn more about the effects of HD on memory click here.
Figure 2: Our brains use different types of memory. Cortical dementia is thought to affect semantic memory to a greater degree than episodic memory. Sub-cortical dementia, as in HD, is thought to affect semantic memory to a lesser degree than cortical dementias.
Although memory is one of the leading areas of interest in the study of cortical and subcortical dementias, other differences between the two dementias exist. Sub-cortical dementias almost always result in motor disorders. Chorea in HD patients and tremors in Parkinson's tremors are examples of motor impairment that accompany sub-cortical dementias. In terms of other cognitive effects, differences between dementias are still being studied.
Last Modified: 05/22/2009
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