Arousal from sleep: the differential effect of frequencies equated for loudness

The present research required individual subjects to personally equate the loudness of three different auditory frequencies. When these same auditory stimuli were presented to the subject during sleep they produced equal levels of arousal (increases in cortical desynchronization) during sleep characterized by nonREM fast-wave EEG activity but different levels of arousal during sleep characterized by slow-wave EEG activity. During this latter type of sleep, the individual's arousal appeared more related to physical sound pressure level than subjective loudness. These results are discussed in relation to the possible occurrence of cortical inhibition during sleep characterized by slow-wave electrocortical activity.