As people with Alzheimer’s disease (AD) progress from mild cognitive impairment to moderate and severe dementia, higher-order forms of awareness gradually decline. In contrast, more basic sensory awareness tends to remain comparatively preserved. As the disease advances, many conscious mental processes become increasingly disrupted, including attention, working memory, episodic memory, and executive functioning. In contrast, several unconscious or implicit processes—such as procedural or so-called “muscle” memory, operant conditioning (behaviour shaped by consequences), and priming, in which prior exposure to a stimulus influences the processing of a related one—are relatively spared. Nevertheless, as neurodegeneration spreads across multiple cortical regions in dementias such as AD, specific components of conscious awareness are progressively weakened and ultimately lost.
One way of quantifying brain complexity is through the perturbation complexity index–state transitions (PCI-ST), which is derived from electroencephalography (EEG) recordings taken after a pulse of transcranial magnetic stimulation. This metric has previously been used to distinguish between different levels of consciousness, such as coma and minimally conscious states. A recent study investigated whether this same measure could be applied to assess the integrity of conscious processing in individuals with Alzheimer’s disease.
Researchers from Boston University’s Chobanian & Avedisian School of Medicine report that it can. Their findings show that brain complexity in response to magnetic stimulation was significantly lower in people with AD compared with those undergoing typical ageing, indicating a measurable reduction in the brain’s capacity for integrated conscious activity.
As senior author Andrew Budson, MD, professor of neurology at the institution, explains, individuals with Alzheimer’s disease may retain the ability to rely on intact implicit forms of memory, such as procedural memory, to manage familiar daily activities at home, despite marked impairments in conscious memory. However, when removed from familiar environments, these preserved routines offer little support, and deficits in conscious memory can result in confusion, disorientation, and distress.
In the study, the researchers evaluated 28 participants diagnosed with Alzheimer’s disease alongside 27 healthy control participants. They assessed cognitive function and disease severity, while also measuring PCI-ST following stimulation of both motor and parietal cortical regions. Across both sites, individuals with AD exhibited lower PCI-ST values than controls, suggesting that this measure captures aspects of the impaired conscious cognition and reduced functional capacity characteristic of the disease.
According to Budson, these findings open new directions for research in cortical dementias. Future studies may use PCI-ST to explore further the links between conscious processes, global measures of consciousness, and their underlying neuroanatomical foundations. In doing so, this line of work may deepen understanding of dementia and contribute to the development of potential therapeutic approaches.
More information: Brenna Hagan et al, Evaluating Alzheimer’s disease with the TMS-EEG perturbation complexity index, Neuroscience of Consciousness. DOI: 10.1093/nc/niaf062
Journal information: Neuroscience of Consciousness Provided by Boston University School of Medicine
