People in early stages of Alzheimer's disease have greater difficulty shifting attention back and forth between competing sources of information, a finding that offers new support for theories that contend breakdowns in attention play an important role in the onset of the disease.
"Our results provide evidence that breakdowns in attention produce a clear change in the early stages of Alzheimer's-related dementia," said study co-author David A. Balota, a professor of psychology in Arts & Sciences at Washington University in St. Louis.
Published in a recent issue of the journal Neuropsychology, the study suggests that subtle breakdowns in attention may offer a reliable clue that a patient is grappling with early symptoms of Alzheimer's-related dementia.
The findings are important because they offer clinicians and researchers another tool by which to better predict and understand dementia of the Alzheimer's type early in its history. Psychologists focus on early detection in part because current medications are useful only when given very early in the course of the disease.
While it's well known that memory skills deteriorate as Alzheimer's progresses, recent research by Balota and Duchek, among others, have championed the notion that breakdowns in attention may be at the heart of many cognitive problems linked to Alzheimer's. Although memory problems also show up in early stages of the disease, this study suggests that underlying declines in attention may be contributing to these memory mishaps and to other cognitive difficulties often associated with the disease.
"Because attention is prerequisite for memory, one might suspect that attention is one of the contributing culprits, at least early on in the disease," suggests study lead author Janet M. Duchek, an associate professor of psychology.
Participants for the study were drawn from volunteers at the Alzheimer's Disease Research Center at Washington University. Duchek and Balota studied 94 older participants, average age mid-70s, who were healthy control individuals or individuals diagnosed with very mild, or mild dementia of the Alzheimer's type.
In an effort to gauge each group's ability to effectively monitor and switch among competing channels of information, Duchek and Balota relied on a well-established psychological testing technique know as the dichotic listening task.
Developed in the 1950s, the dichotic listening test plays off the fact that humans are hardwired to process sensory information in a cross-lateral fashion - words heard in the left ear tend to be processed in the right hemisphere of the brain, and vice-versa. Since the left hemisphere of the brain is typically dominant for language processing, words presented in the right ear often have an advantage over words presented simultaneously in the left ear — the right ear-left hemisphere processing channel is said to be "pre-potent" in that it has a default processing advantage over the left ear-right hemisphere channel.
Using the dichotic listening task, Duchek and Balota presented participants with distinct streams of audio information via headphones. One stream of information — computer-generated speech naming three digits (such as 4, 3, 1) — went to the left ear; a different stream (such as 9, 2, 5) went to the right ear.
By asking participants to recall numbers in the order they were presented to either ear, the researchers were able to measure an individual's ability to switch back and forth between right-left processing channels, and more importantly, to monitor how well attention skills allowed them to overcome the "pre-potent" tendency to favor information presented to the default right ear-left hemisphere language channel.
As predicted, people with early dementia tended to rely more often on the default channel, reporting digits presented to the right ear far better than they reported digits presented to the left ear. When the researchers controlled for overall recall performance, the mild dementia group recalled 21.7% more information from their right ear vs. left ear, and even the very mildly affected group recalled 11.3% more from the right ear. The control participants only recalled 5.8% more from the right vs. left ear.
The right-ear advantage increased with dementia severity. People farther along in the disease relied even more on the dominant left-side channel; in other words, they found it even harder to override the usual path to process what went through the left ear to the right brain. Poor attentional control can leave people falling back on familiar, pre-programmed information pathways.
The study confirms that very early in the disease, people have problems with selective attention. This problem, although not as obvious as memory loss, may also explain why early-stage patients start to struggle with everyday tasks that call for processing a lot of information, such as driving. This speculation is supported by prior findings that performance on dichotic listening predicts accident rates in commercial bus drivers.
Findings from this study, the research team suggests, converge with accumulating evidence that individuals with early stage Alzheimer's Disease have breakdowns controlling prepotent pathways across a variety of experimental paradigms, which place minimal demands on memory systems.
"Our hope," Duchek said, "is that this work increases recognition that Alzheimer's Disease is not simply a disease of memory."
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