θ power responses in mild Alzheimer's disease during an auditory oddball paradigm: lack of theta enhancement during stimulus processing.

There is evidence that theta responses reflect cognitive performance: good performances are associated with a decrease in tonic theta power as well as an increase in phasic theta power. In the present study, both tonic and phasic theta activity were analysed in 22 patients with mild Alzheimer's disease (AD), and 16 healthy elderly controls. Single-trial theta power responses were evaluated by an active auditory oddball paradigm along an early poststimulus window (0-250 ms) and a late time window (250-500 ms), and then compared to prestimulus theta power during both target tone and standard tone processing. The main findings were: (1) in AD patients, there was an increased prestimulus theta power, as well as no significant poststimulus theta power increase upon both target and non-target stimulus processing; (2) in healthy aged controls, only during target tone processing, an enhancement of both early and late theta responses relative to the prestimulus baseline was found. Moreover, healthy controls had a frontal dominance of theta power. The results might indicate that, during target processing, theta response is not functionally sensitive in AD and cannot be involved in processing demands as efficiently as in healthy controls. From a psychophysiological point of view, this might suggest an impairment of attentional allocation resources. The psychological implications might be related to selective attention/working-memory impairment from the early stage of the disease. Our data confirm that both tonic and phasic theta are relevant indicators of cognitive performance: the lack of a phasic theta and an increase in tonic theta are congruous findings in cognitive decline. Another factor worth noting is that in AD patients theta response is not dominant at the frontal site (as observed in healthy controls), indicating a weaker frontal lobe network reactivity during stimulus processing.