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.
Objective: The auditory Event-Related Potentials (ERP) of component P 50 to sound onset and offset have been reported to be similar, but their magnetic homologue has been reported absent to sound offset. We compared the spatio-temporal distribution of cortical activity during P 50 to sound onset and offset, without confounds of spectral change. Methods: ERPs were recorded in response to onsets and offsets of silent intervals of 0.5 s (gaps) appearing randomly in otherwise continuous white noise and compared to ERPs to randomly distributed click pairs with half second separation presented in silence. Subjects were awake and distracted from the stimuli by reading a complicated text. Measures of P 50 included peak latency and amplitude, as well as source current density estimates to the clicks and sound onsets and offsets. Results: P 50 occurred in response to noise onsets and to clicks, while to noise offset it was absent. Latency of P 50 was similar to noise onset (56 ms) and to clicks (53 ms). Sources of P 50 to noise onsets and clicks included bilateral superior parietal areas. In contrast, noise offsets activated left inferior temporal and occipital areas at the time of P 50 . Source current density was significantly higher to noise onset than offset in the vicinity of the temporo-parietal junction. Conclusions: P 50 to sound offset is absent compared to the distinct P 50 to sound onset and to clicks, at different intracranial sources. P 50 to stimulus onset and to clicks appears to reflect preattentive arousal by a new sound in the scene. Sound offset does not involve a new sound and hence the absent P 50 . Significance: Stimulus onset activates distinct early cortical processes that are absent to offset.
IntroductionSpatio‐temporal distributions of cortical activity to audio‐visual presentations of meaningless vowel‐consonant‐vowels and the effects of audio‐visual congruence/incongruence, with emphasis on the McGurk effect, were studied. The McGurk effect occurs when a clearly audible syllable with one consonant, is presented simultaneously with a visual presentation of a face articulating a syllable with a different consonant and the resulting percept is a syllable with a consonant other than the auditorily presented one.MethodsTwenty subjects listened to pairs of audio‐visually congruent or incongruent utterances and indicated whether pair members were the same or not. Source current densities of event‐related potentials to the first utterance in the pair were estimated and effects of stimulus–response combinations, brain area, hemisphere, and clarity of visual articulation were assessed.ResultsAuditory cortex, superior parietal cortex, and middle temporal cortex were the most consistently involved areas across experimental conditions. Early (<200 msec) processing of the consonant was overall prominent in the left hemisphere, except right hemisphere prominence in superior parietal cortex and secondary visual cortex. Clarity of visual articulation impacted activity in secondary visual cortex and Wernicke's area. McGurk perception was associated with decreased activity in primary and secondary auditory cortices and Wernicke's area before 100 msec, increased activity around 100 msec which decreased again around 180 msec. Activity in Broca's area was unaffected by McGurk perception and was only increased to congruent audio‐visual stimuli 30–70 msec following consonant onset.ConclusionsThe results suggest left hemisphere prominence in the effects of stimulus and response conditions on eight brain areas involved in dynamically distributed parallel processing of audio‐visual integration. Initially (30–70 msec) subcortical contributions to auditory cortex, superior parietal cortex, and middle temporal cortex occur. During 100–140 msec, peristriate visual influences and Wernicke's area join in the processing. Resolution of incongruent audio‐visual inputs is then attempted, and if successful, McGurk perception occurs and cortical activity in left hemisphere further increases between 170 and 260 msec.
The results suggest that the main contribution to the 40 Hz BI is from rate resistant thalamo-cortical neurons. The results also suggest that the binaural cortical neurons contributing to the 40 Hz BI are less affected by increased rate than monaural neurons.
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