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The N1 complex to gaps in noise: Effects of preceding noise duration and intensity AbstractObjective: To study the effects of duration and intensity of noise that precedes gaps in noise on the N-Complex (N 1a and N 1b ) of EventRelated Potentials (ERPs) to the gaps. Methods: ERPs were recorded from 13 normal subjects in response to 20 ms gaps in 2-4.5 s segments of binaural white noise. Within each segment, the gaps appeared after 500, 1500, 2500 or 4000 ms of noise. Noise intensity was either 75, 60 or 45 dBnHL. Analysis included waveform peak measurements and intracranial source current density estimations, as well as statistical assessment of the effects of pre-gap noise duration and intensity on N 1a and N 1b and their estimated intracranial source activity. Results: The N-Complex was detected at about 100 ms under all stimulus conditions. Latencies of N 1a (at 90 ms) and N 1b (at 150 ms) were significantly affected by duration of the preceding noise. Both their amplitudes and the latency of N 1b were affected by the preceding noise intensity. Source current density was most prominent, under all stimulus conditions, in the vicinity of the temporo-parietal junction, with the first peak (N 1a ) lateralized to the left hemisphere and the second peak (N 1b ) -to the right. Additional sources with lower current density were more anterior, with a single peak spanning the duration of the N-Complex. Conclusions: The N 1a and N 1b of the N-Complex of the ERPs to gaps in noise are affected by both duration and intensity of the pre-gap noise. The minimum noise duration required for the appearance of a double-peaked N-Complex is just under 500 ms, depending on noise intensity. N 1a and N 1b of the N-Complex are generated predominantly in opposite temporo-parietal brain areas: N 1a on the left and N 1b on the right. Significance: Duration and intensity interact to define the dual peaked N-Complex, signaling the cessation of an ongoing sound.

a b s t r a c tObjective: To define cortical brain responses to large and small frequency changes (increase and decrease) of high-and low-frequency tones. Methods: Event-Related Potentials (ERPs) were recorded in response to a 10% or a 50% frequency increase from 250 or 4000 Hz tones that were approximately 3 s in duration and presented at 500-ms intervals. Frequency increase was followed after 1 s by a decrease back to base frequency. Frequency changes occurred at least 1 s before or after tone onset or offset, respectively. Subjects were not attending to the stimuli. Latency, amplitude and source current density estimates of ERPs were compared across frequency changes. Results: All frequency changes evoked components P 50 , N 100 , and P 200 . N 100 and P 200 had double peaks at bilateral and right temporal sites, respectively. These components were followed by a slow negativity (SN). The constituents of N 100 were predominantly localized to temporo-parietal auditory areas. The potentials and their intracranial distributions were affected by both base frequency (larger potentials to low frequency) and direction of change (larger potentials to increase than decrease), as well as by change magnitude (larger potentials to larger change). The differences between frequency increase and decrease depended on base frequency (smaller difference to high frequency) and were localized to frontal areas. Conclusions: Brain activity varies according to frequency change direction and magnitude as well as base frequency. Significance: The effects of base frequency and direction of change may reflect brain networks involved in more complex processing such as speech that are differentially sensitive to frequency modulations of high (consonant discrimination) and low (vowels and prosody) frequencies.