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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.

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.

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Motor evoked potentials and central motor conduction time (CMCT) were examined from both upper and lower limbs in patients with normal pressure hydrocephalus to find a predictor for the success of shunting procedures. The hypotheses that walking disturbances are due to pyramidal tract compression as weli as the possibility that the upper limbs are affected subclinically in these patients were also studied. The study suggests that the walking disturbances are not the result of a major pyramidal tract dysfunction but probably involve the sensorimotor integration leading to normal gait. Furthermore, CMCT measured with electromagnetic motor stimulation can help in selecting the patients that will benefit from shunting. The study does not provide electrophysiological evidence of upper limb involvement in normal pressure hydrocephalus. In this study we challenged the hypothesis that gait disturbances in normal pressure hydrocephalus are the result of pressure on the internal capsule. We also tested whether conduction to the upper limbs is affected, even without clinical manifestation. As electromagnetic stimulation of the motor cortex56 and measurement of the central motor conduction time (CMCT) allows direct evaluation of central motor function, we applied this method to consider these questions. In addition, we examined the possible utility of motor evoked potentials in prediction of outcome in patients with the clinical and radiological picture of normal pressure hydrocephalus. We report our findings and their implications on the nature of walking difficulties as well as on the criteria for patient selection. Methods CONTROLS AND PATIENTSThis study was approved by the hospital ethics committee. Informned consent was obtained from all the volunteers and from the patients' guardians. The study was prospective and patient selection for surgery was not influenced by the results of these electrophysiological tests.Sixteen healthy volunteers served as controls. Their age ranged from 30 to 67. Four were women. Their height was 157 cm to 180 cm. In the absence of evidence for significant age or sex effects on CMCT, we preferred a neurologically intact control group to perfect matching of age and sex.Twenty four consecutive unselected patients with suspected normal pressure hydrocephalus were examined preoperatively. All the patients were referred to the outpatient clinic by physicians other than the operating neurosurgeons. All patients fulfilled the clinical, radiological, and known standard laboratory criteria for the diagnosis of normal pressure hydrocephalus7 including: (a) clinical presentation: either appreciable gait difficulty or the full triad of dementia, ataxia, and incontinence; (b) CT or MRI with periventricular low density or small sulci along with expansion of the ventricular system. All patients underwent lumbar puncture and isotopic cisternography with technetium (TcDTPA) as well. Patients with negative lumbar puncture or isotope cisternography with a classic clinical presentation and positive CT o...