Event-related potential measures of the disruptive effects of trains of auditory stimuli during waking and sleeping states.

Acoustic backup alarms have been reported to particularly disrupt sleep. The present study simulated backup alarms by presenting trains of five consecutive 500 ms duration audible tones, with the time between the onset of each tone being 1 s and the time between trains (offset to onset) between 15 and 20 s. In different conditions, the tones were set at either 80 or 60 dB sound pressure level (SPL). Twelve young adults spent two consecutive nights in the laboratory. Stimuli were presented only on the second night. Measures of traditional sleep architecture (sleep stages) were not affected by the acoustic trains. Event-related potentials were also measured following presentation of the stimuli. In the waking state, the initial 80 dB stimulus elicited a large amplitude N1, peaking at about 100 ms, followed by a positive peak, P3, peaking at about 325 ms. N1 was attenuated following presentation of the 60 dB stimulus. The amplitude of N1 was much reduced following presentation of the subsequent second to fifth stimuli in the train. During non-rapid eye movement (NREM) sleep, the initial 80 dB stimulus elicited a large and later negativity (N350) that was reduced in amplitude for the 60 dB stimulus. A K-Complex (a composite N350 and a much larger N550) was elicited following 35% of the initial 80 dB tones and 12% of the initial 60 dB tones. The amplitude of N550 did not, however, significantly vary as a function of stimulus SPL. During REM sleep, N1 continued to be elicited by the initial louder stimulus, but the later positive wave was not apparent. A late negativity peaking at about 350 ms was, however, apparent. When queried the next morning, subjects rarely indicated that the stimulus presentations disturbed their sleep. This might be because of the absence of the late positivity. The presence of the long latency negativities (N350 and N550) might serve to protect sleep from obtrusive sound during sleep.

The intensity of masking noise affects the mismatch negativity to speech sounds in human subjects.

The present study investigated the effects of decreased audibility of the speech sounds 'ba' and 'da' on the mismatch negativity (MMN). Audibility was altered by varying the intensity of the masking noise to be either 0 (no noise), 65, 70 or 75 dB speech perception level. Cortical event-related potentials were recorded while normal listeners (n=7) were instructed to ignore the stimuli. In a separate condition, subjects were asked to signal detection of the deviant stimulus by button-press. As audibility of speech sounds was decreased, MMN peak amplitude decreased and MMN peak latency increased. Behavioral responses were in accordance with the MMN data. Hit rates decreased as audibility decreased. These results suggest that both pre-attentive and attentive speech discrimination deteriorate following a reduction in audibility.

Multiple P3 evidence of a two-stage process in word gender decision.

All French nouns must be assigned to one of two grammatical genders: masculine or feminine. Participants used either the superordinate labels masculin/féminin or the singular indefinite articles un/une to classify French target nouns. Reaction time to the labels masculin/féminin was about 200 ms longer than to the un/une labels. When the indefinite articles were used, a single P3 peak of the event-related potential was elicited. When superordinate labels were used, a double-peaked positivity was observed. The latency of the initial P3 in the masculin/féminin trials was not significantly different from that in the un/une trials. The second positive wave peaked approximately 300 ms following the first. An explanation consistent with these data is that subjects used a two-stage process to classify the nouns appearing with superordinate labels.

Mismatch negativity to inclusions of noise within complex spectrotemporal sound patterns.

Event-related potentials to complex spectrotemporal sound patterns were recorded during non-attend conditions. A high-intensity noise deviant elicited a large negative wave. This is probably a combined mismatch negativity (MMN) and N2b since it was followed by a large amplitude frontal positive wave (P3a). When the noise intensity was reduced, a smaller amplitude negative wave was elicited. This probably corresponds to a true MMN since it was not followed by the positive wave. These findings are consistent with previous research in which N2b is elicited in non-attend conditions when the deviant stimulus was particularly difficult to ignore. These findings demonstrate that complex spectrotemporal sound patterns are represented in auditory memory and that inclusions of noise within these complex stimuli can be passively discriminated.