Sensory evoked and event related oscillations in Alzheimer's disease: a short review

Abstract

Diagnosis and treatment of Alzheimer's disease (AD) depend on clinical evaluation and there is a strong need for an objective tool as a biomarker. Our group has investigated brain oscillatory responses in a small group of AD subjects. We found that the de novo (untreated) AD group differs from both the cholinergically-treated AD group and aged-matched healthy controls in theta and delta responses over left frontal-central areas after cognitive stimulation. On the contrary, the difference observed in AD groups upon a sensory visual stimulation includes response increase over primary or secondary visual sensorial areas compared to controls. These findings imply at least two different neural networks, depending on type of stimulation (i.e. cognitive or sensory). The default mode defined as activity in resting state in AD seems to be affected electrophysiologically. Coherences are also very valuable in observing the group differences, especially when a cognitive stimulus is applied. In healthy controls, higher coherence values are elicited after a cognitive stimulus than after a sensory task. Our findings support the notion of disconnectivity of cortico-cortical connections in AD. The differences in comparison of oscillatory responses upon sensory and cognitive stimulations and their role as a biomarker in AD await further investigation in series with a greater number of subjects.

Keywords: Alzheimer; Dementia; Event related; Oscillations; P300.

Fig. 1

Grand-averages of theta oscillatory response healthy elderly controls and two groups of Alzheimer subjects; treated and non-treated

Fig. 2

The average delta oscillatory responses (thick lines) as superimposed single trials (thinner lines) in representative subjects from each group; i.e. a healthy elderly subject, untreated AD subject, and treated AD subject

Fig. 3

Grand averages of delta oscillatory responses elicited upon visual target stimuli from each group; i.e. healthy elderly controls (n = 20), untreated AD group (n = 11), and treated AD group (n = 11)

Fig. 4

Comparison of topology or changes of amplitudes in subject groups after sensory evoked and event related oscillatory responses indicate differential circuitry, depending on the type of stimuli, i.e. sensory or cognitive. Asterisks indicate significance at the level of 0.01

Fig. 5

Fronto-parietal connectivity (coherence) is decreased in Alzheimer groups relative to control group (n = 19) in delta and theta frequency ranges, whereas the Alzheimer subjects (n = 11) on cholinergic treatment display greater coherence values compared to the untreated Alzheimer group (n = 11). These responses are elicited upon the target stimulation to classical visual oddball paradigm

Fig. 6

Coherences of brain oscillations upon a cognitive task (i.e. target stimulus in classical visual oddball paradigm) reach higher values compared to those elicited upon simple sensory visual stimuli (i.e. basic light stimulation). Coherence reflecting functional connectivity between fronto-parietal regions shows higher values in controls in relation to Alzheimer (AD) subjects. The coherence values in alpha ranges are greater in the cholinergically treated Alzheimer group than those with no treatment, possibly indicating better functional connectivity after cholinergic remedy