Short latency afferent inhibition is not impaired in mild cognitive impairment

To determine whether cortical cholinergic circuit impairment exists in the mild cognitive impairment (MCI) brain.

Fifteen healthy elderly controls (NC), 16 amnesic MCI subjects and 12 probable Alzheimer's disease (AD) subjects were recruited. Conditioning stimuli were delivered at the right wrist followed by test transcranial magnetic stimulation (TMS) of the left motor cortex. The center of the linear contiguous segment of the coil was placed over a point 5 cm lateral to the vertex on the interaural line. The interstimulus intervals (ISIs) between the conditioning stimuli and the test stimuli were set at 20, 40, 100, 200 and 600 ms. An inhibitory effect that occurred at ISIs as short as 20 ms was defined as short-latency afferent inhibition (SAI).

SAI was significantly reduced in subjects with AD compared with NC, but it was not reduced in subjects with MCI.

A difference in cortical excitability between subjects with AD and subjects with MCI could be captured by an in vivo neurophysiological method.

The state of the neurotransmitter systems, including the cortical cholinergic system, is thought by some compensatory mechanisms to be kept at the normal level in subjects with MCI.