Semantic Dementia

Semantic dementia is a result of the lateral temporal lobes in the brain degenerating causing the person to slowly lose previous knowledge about the world. A semantic dementia patient may not recognize a teapot or be able to explain its function. The patient has word-finding difficulties. They have lost the meaning of certain words - “What is a teapot?” People who are afflicted with semantic dementia exhibit reduced skills when using semantic memory (Graham, et al, 2000). They have complications naming items, putting items into a category, matching a picture and the correct word, saying the correct color for a common item (yellow for a banana), categorizing items such as plants and animals, and knowing names of people or even themselves. A person with the onset of semantic dementia will normally speak fluently and articulate well, but use only a small number of content words. Research into semantic dementia indicates that new recollections were recovered easier compared to those from long-ago, leading to the belief that new learning is possible during the beginning stages of the disorder.

Our semantic memory is what we use to define objects, meanings of words, facts, concepts, and people. Associated to semantic memory is episodic memory which
is retrieval of personal experiences from the past. If a person has a deficit in semantic memory it is normally referred to as semantic dementia. When a person has a deficit in episodic memory they are referred to as having amnesia.

Tulving (Graham et al., 2000) originally proposed that episodic and semantic memory were separate in long – term memory based on how the brain acquires, processes, and stores information. After tests were completed on patients with amnesia showing a simple separation between episodic and semantic memory, Tulving revised his model. The current theory called Serial encoding, Parallel storage, and Independent retrieval (SPI) gives an opinion that episodic memory is reliant on semantic knowledge and it is a subsystem of semantic memory. In Tulving’s SPI model there are four key categories of cognitive memory organization: perceptual depiction; semantic; working; and episodic memory. He includes three fundamental building blocks in the model. First, information is encoded into systems consecutively, with encoding in one system dependent upon production from the prior phase. Second, information can be stored in different systems at the same time. Third, information in different systems can be retrieved independently without any effects on retrieval of information from other systems. This analysis clarifies how a person with amnesia can recover semantic information that was learned previously. Tulving said, “…a double dissociation between semantic and episodic memory is not possible and only single dissociations (impaired episodic memory and preserved semantic memory) can occur” (p.314). According to the SPI model, information in episodic memory cannot be formulated without information from semantic memory, and the semantic memory must have input from perceptual systems.

Graham and his coauthors disagree with Tulving’s theory that people with semantic dementia cannot learn new information. There are two experiments completed in the research to disprove his theory. The authors’ hypothesis states that new learning is usually a blend of sensory/perceptual information experienced during the learning episode and semantic information about the substance of the event. They predicted that patients with semantic dementia would demonstrate stability within recognition memory in the perceptually identical (PI) form but a reduced aptitude to choose a perceptually different (PD) item.

For the first experiment with semantic and episodic memory there were eight patients with semantic dementia, eight patients with Alzheimer’s, and eighteen people in the control group. In the recognition memory trial, to show a PI item they would use the same colored picture twice, the PD item was still the same item as shown previously, but from a different angle or side view. Initially the patients were asked to identify by name the items in the pictures. Later they were asked to select the pictures they had seen previously by using PI or PD. The results showed the patients with semantic dementia performed better on PI information than on PD, comparable to the control group. There were large gaps between the three groups on items shown PD. For the authors to evaluate their hypothesis of the impact on semantic knowledge associated with new learning they examined performance on the picture naming task with PD items in the episodic assessment. The results showed patients with semantic dementia had impaired semantic knowledge, but the episodic memory was maintained. These results oppose Tulving’s SPI model. Graham and his coauthors understanding of the outcome is that the manipulation in the PD form had two effects. First when the item was presented in the initial naming task then again PD the perceptual information previously learned was not helpful. Second the episodic decision relied on the abstract information of the original image.

In the second experiment, one person with semantic dementia previously tested in experiment one was reevaluated. Performance was evaluated in recognition memory. The patients were asked if items shown before in the first experiment were known items or unknown items. The results showed that the PI items were all known, but the PD items were unknown.

After reviewing the two experiments the authors proved that semantic memory is damaged and episodic memory is not when using PI items with patients who have semantic dementia. Graham and his coauthors were able to prove Tulving’s hypothesis false. A person with semantic dementia can learn new information. New learning was not impaired in patients with semantic dementia unless the patient was tested perceptually differently than they had previously learned.

New episodic learning relies on perceptual information and conceptual knowledge in normal brain functions. Perceptual areas in the brain help to identify items that are not familiar. The perceptual information processed goes straight to the episodic memory and the semantic system, to create new learning. When a person has semantic dementia to learn new episodic information the items have to be PI. When the items are PD the patient can not use their perceptual information, so they have to use their semantic knowledge which is impaired and they are not able to identify the object.

Participation from different neocortical (the roof of the cerebral cortex that forms the part of the mammalian brain that has evolved most recently and makes possible higher brain functions such as learning) (Encarta Dictionary, online) areas in the brain, which promote perceptual examination and semantic memory, work in harmony to support new learning. Neuroscientists believe that the hippocampus and the infero-lateral temporal lobes are damaged in patients who have semantic dementia. There must be a link from the visual structure contained by the perceptual system that is devoted to the breakdown of auditory and tactile stimuli.

People with impaired semantic memory maintained performance on tests of recognition memory if the stimulus was perceptually identical between learning and testing. Using the same perceptual stimulus is noted to dramatically increase semantic memory if introduced in the episodic task. The authors proved that sensory or perceptual knowledge and semantic memory work together to assist new learning. This type of semantic knowledge is incorporated in my classroom daily. Children work on tasks such as picture word associations and assembling puzzles with items named. I agree that new learning is improved by using sensory/perceptual processes with semantic memory.

Reference:
Graham, K.S., Simons, J.S., Pratt, K.H., Patterson, K., & Hodges, J.R. (2000). Insights from semantic dementia on the relationship between episodic and semantic memory. Neuropsychologia. 38, 313-324.