Neuromagnetic evidence of abnormal automatic inhibitory function in subjective memory complaint.

Subjective memory complaint (SMC), a self-perceived worsening in memory capacity concurrent with normal performance on standardized cognitive assessments, is considered a risk factor for the development of Alzheimer's disease (AD). Deficient sensory gating (SG), referring to the lack of automatic inhibition of neural responses to the second identical stimulus, has been documented in prodromal and incident AD patients. However, it remains unknown whether the cognitively normal elderly with SMC demonstrate alterations of SG function compared with those without SMC. A total of 19 healthy controls (HC) and 16 SMC subjects were included in the present study. Neural responses to the auditory paired-stimulus paradigm were recorded by the magnetoencephalography and analyzed by the distributed source imaging method of minimum norm estimate. The SG of M50 and M100 components were measured using the amplitude ratio of the second response over the first response at the cortical level. Compared to HC, subjects with SMC showed significantly increased M50 SG ratios in the inferior parietal lobule (IPL). Furthermore, M50 SG ratios in the right IPL yielded an acceptable discriminative ability to distinguish SMC from HC. However, we did not find a significant association between SG ratios and cognitive function requiring inhibitory control either in the HC or SMC group. In conclusion, although SMC subjects have intact cognitive functioning revealed by objective neuropsychological tests, their deficits in automatic inhibitory function could be detected through neurophysiological recordings. Our results suggest that altered brain function occurs in SMC prior to the obvious decline of cognitive performance.

Differential motor cortex excitability during observation of normal and abnormal goal-directed movement patterns.

Beta (∼20-Hz) activity induced by median nerve stimulation has been reported to be located in the primary motor cortex (M1) and associated with various movement-related tasks. It has been evident that the ∼20-Hz activity was suppressed more in the observation of goal-directed than non-goal-directed tool use. However, it remains unclear whether this rhythmic activity is further modulated by the observation of normal and abnormal goal-directed hand movements. We recorded neuromagnetic responses in 19 right-handed healthy adults while they kept relaxed (Rest), manipulated a small cube (Act), observed videotapes with normal (Normal) or abnormal (Abnormal) hand actions. The M1 ∼20-Hz rebound powers were quantified through minimum norm estimate and time-frequency analysis at the source level. The results showed that compared to the Rest condition, the M1 ∼20-Hz activities were significantly decreased when subjects observed normal and abnormal hand actions. The suppression was more profound in the Normal than Abnormal condition, indicating a stronger M1 activation during observing graceful, undistorted movement patterns. Our data provided a neurophysiological basis to differentiate the motor cortical activities in the observation of normal and abnormal hand actions. Further research is warranted to verify its clinical benefits in patients with stroke.