Functional Connectivity in Mild Cognitive Impairment During a Memory Task: Implications for the Disconnection Hypothesis

Abstract

Mild cognitive impairment (MCI) has been considered an intermediate state between healthy aging and dementia. The early damage in anatomical connectivity and progressive loss of synapses that characterize early Alzheimer's disease suggest that MCI could also be a disconnection syndrome. Here, we compare the degree of synchronization of brain signals recorded with magnetoencephalography from patients (22) with MCI with that of healthy controls (19) during a memory task. Synchronization Likelihood, an index based on the theory of nonlinear dynamical systems, was used to measure functional connectivity. During the memory task patients showed higher interhemispheric synchronization than healthy controls between left and right -anterior temporo-frontal regions (in all studied frequency bands) and in posterior regions in the γ band. On the other hand, the connectivity pattern from healthy controls indicated two clusters of higher synchronization, one among left temporal sensors and another one among central channels. Both of them were found in all frequency bands. In the γ band, controls showed higher Synchronization Likelihood values than MCI patients between central-posterior and frontal-posterior channels and a high synchronization in posterior regions. The inter-hemispheric increased synchronization values could reflect a compensatory mechanism for the lack of efficiency of the memory networks in MCI patients. Therefore, these connectivity profiles support only partially the idea of MCI as a disconnection syndrome, as patients showed increased long distance inter-hemispheric connections but a decrease in antero-posterior functional connectivity.

Synchronization during an internally directed cognitive state in healthy aging and mild cognitive impairment: a MEG study

“…These findings are in agreement with many studies which employed cognitive tasks in the MCI population (Pijnenburg et al 2004;Jiang 2005;Jiang and Zheng 2006;Bajo et al 2010). In the alpha band, however, MCIs were desynchronized during the task.…”

“…However, other resting state studies have failed to find functional connectivity differences between MCI and healthy controls (Stam et al 2003;Jiang 2005;Zheng et al 2007;Tao and Tian 2005). By contrast, most task studies found an increased synchronization in MCIs when compared to healthy people during a cognitive task (i.e., memory task) (Pijnenburg et al 2004;Jiang 2005;Zheng et al 2007;Bajo et al 2010). For instance, Jiang and Zheng (2006) reported a higher inter-and intrahemispheric EEG coherence in MCI in a working memory task.…”

“…For instance, Jiang and Zheng (2006) reported a higher inter-and intrahemispheric EEG coherence in MCI in a working memory task. Bajo et al (2010) found higher interhemispheric synchronization likelihood in MCI between left and right temporo-frontal sensors in most frequency bands during a short-term memory task and higher connectivity for those that later on developed dementia (Bajo et al 2012). This increase in functional connectivity is usually interpreted as a compensatory mechanism and is associated with the risk for the progression to AD (Rossini et al 2006).…”

“…The concept of functional connectivity has emerged to evaluate functional relations between brain regions. It quantifies statistical interdependencies between different physiological signals, providing information about functional interactions between the corresponding brain regions (Friston 2001) and was proven to be also altered in MCI (Bajo et al 2010). Moreover, synchronization abnormalities seem to be related to the degree of dementia, being more accentuated in patients with a higher cognitive decline (Stam et al 2003;Jeong 2004).…”

“…Compared with fMRI which gives an indirect estimate of brain activity through hemodynamic responses, MEG provides a direct measure of electrophysiological activity, reflecting neuronal communication with great temporal resolution (Hämäläinen et al 1993) and, therefore, allowing the investigation of several brain rhythms (1-100 Hz). Indeed, MEG oscillatory activity has been proven useful in measuring both spontaneous and task-induced brain rhythms in normal and pathological aging (Fernández et al 2006;Osipova et al 2006;Stam 2010;Leirer et al 2011;Bajo et al 2010;Zamrini et al 2011).…”