Improving the reliability of the auditory middle latency response by monitoring EEG delta activity.

The auditory middle latency response (MLR), a useful tool in the assessment of low-frequency auditory sensitivity, can be consistently recorded in young children during wakefulness, stages 1 and 2, and REM sleep. Responses are often absent or questionable during stage 4. An on-line measure indicating favorable periods for recording MLR during sleep is important for interpretation of absent potentials. Here, for children 5 to 7 years old, the reliability and detectability of MLR was compared to sleep state and the dominance of delta activity (0-3 Hz) in the EEG frequency spectrum. Dominance of delta activity, a characteristic of stage 4, was expressed in a "delta ratio," a measure of relative EEG energy in the 0 to 3 Hz frequency spectrum. A fixed delta ratio (DR = 9) allowed the differentiation of periods favorable for MLR. MLR wave Pa amplitude and latency also varied with delta ratio. Results indicate that on-line monitoring of the delta ratio will allow reliable testing of MLR in clinical situations.