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This study investigated the effect on hearing, sound attenuation, and sound source identification of a prototype neck and two prototype mandible guards attached to a combat helmet. Ten male subjects participated. Free-field hearing thresholds were measured from 250 Hz to 8,000 Hz with the head bare and fitted with the helmet alone and with the guards. Sound source identification was assessed using a horizontal array of eight loudspeakers surrounding the subject. The stimulus was a 75-dB SPL, 300-ms noise burst. Neither the helmet worn alone or with the guards affected hearing or provided significant sound attenuation. The helmet combinations resulted in a significant decrease in sound source identification, of 11.6%. This was due to diminished accuracy for loudspeakers close to the interaural axis of the head. The neck guard induced a frontal bias for these positions. This error pattern is not likely to interfere with localization during combat.

In order to test the functional significance of rapid eye movement (REM)-sleep and noradrenergic activity for cerebral cortex maturation, rat pups were daily injected with clonidine from 8 to 21 days of life. Previous studies have shown that this treatment reduces the amount of time spent in REM-sleep and the level of noradrenaline turnover in the brain. For long-term consequences of such treatment in adulthood, cortical neuron responses to micro-iontophoretically applied neurotransmitters were studied. No significant differences were found in the single cell responses to glutamate, GABA or noradrenaline in the cerebral cortex of clonidine treated rats as compared with age matched controls. However, the magnitude of GABAergic depression of glutamate induced neuronal responses was greater in the clonidine than in the control group.

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