People who go on to develop Alzheimer's disease start from losing the ability to process semantic or knowledge-based information, says a new study.
According to the study authors, previous research on mild cognitive impairment (MCI), an early sign of Alzheimer's diseases, focused mainly on short-term memory loss. However, people might begin losing the ability to recognize size and shapes - which requires some degree of knowledge - before memory related problems kick in.
"The semantic system is organized in networks that reflect different types of relatedness or association. Semantic items and knowledge have been acquired remotely, often over many repetitions, and do not reflect recent learning," researchers wrote in the American Journal of Psychiatry.
The study involved 25 patients with MCI, 27 patients with Alzheimer's and 70 people who weren't diagnosed with any mental disease.
The researchers decided to test the participants' ability to process semantic information by asking them to judge between two competing sets of data. "If you ask someone what is bigger, a key or an ant, they would be slower in their response than if you asked them what is bigger, a key or a house," explained Terry Goldberg, PhD, one of the study authors.
When the difference between objects is greater, any person can see the difference.
Researchers found that the reaction time of people with Alzheimer's disease was slower than healthy people. "This finding suggested that semantic processing was corrupted. MCI and AD (Alzheimer's disease) patients are really affected when they are asked to respond to a task with small size differences," said Dr. Goldberg.
Researchers then changed the test and showed participants pictures of small ant and a big house or big ant and a small house. People with MCI and AD didn't have a difficulty in recognizing size in the first part of the test - they could choose the house over the ant when asked what was bigger. However, for the second image, they took more time to arrive at a conclusion. "When the decision was harder, their reaction time was slower," Goldberg said.
David P. Salmon, PhD, of the Department of Neurosciences at the University of California in San Diego wrote in an accompanying editorial that the "semantic memory deficit demonstrated by this study adds confidence to the growing perception that subtle decline in this cognitive domain occurs in patients with amnestic mild cognitive impairment. Because the task places minimal demands on the effortful retrieval process, overt word retrieval, or language production, it also suggests that this deficit reflects an early and gradual loss of integrity of semantic knowledge."