The 40/sec middle-latency auditory response in Alzheimer's disease, Parkinson's disease, and age-matched normals

The 40/sec middle-latency auditory response (MLR) was studied in order to determine its relationship to the 40-Hz EEG activity shown previously to be altered in patients with Alzheimer's disease (AD). As controls, patients with Parkinson's disease (PD), a neurodegenerative disorder involving different brain pathways from those in AD, were studied as were age-matched normals. No significant group differences were found in response waveform, latency, or amplitude, suggesting that this response has a different neural substrate from that of the AD-sensitive 40-Hz EEG. Additional studies in normals involving the use of short trains of 10/, 20/, and 40/sec clicks rather than the continuous stimulation typically employed suggests a new technique for the elucidation of the neural substrate underlying the 40/sec MLR. It is speculated that the 40/sec MLR arises from interactions between structures within the primary auditory pathways and others within the nonspecific thalamic system.