Pekcan Ungan

Differences between the N1 waves of the responses to interaural time and intensity disparities: scalp topography and dipole sources☆

OBJECTIVES Being the two complementary cues to directional hearing, interaural time and intensity disparities (ITD and IID, respectively), are known to be separately encoded in the brain stem. We address the question as to whether their codes are collapsed into a single lateralization code subcortically or they reach the cortex via separate channels and are processed there in different areas. METHODS Two continuous trains of 100/s clicks were dichotically presented. At 2 s intervals either an interaural time delay of 1ms or an interaural level difference of 20 dB (HL) was introduced for 50 ms, shifting the intracranial sound image laterally for this brief period of time. Long-latency responses to these directional stimuli, which had been tested to evoke no potentials under monotic or diotic conditions, as well as to sound pips of 50 ms duration were recorded from 124 scalp electrodes. Scalp potential and current density maps at N1 latency were obtained from thirteen normal subjects. A 4-sphere head model with bilaterally symmetrical dipoles was used for source analysis and a simplex algorithm preceded by a genetic algorithm was employed for solving the inverse problem. RESULTS Inter- and intra-subject comparisons showed that the N1 responses evoked by IID and ITD as well as by sound pip stimuli had significantly different scalp topographies and interhemispheric dominance patterns. Significant location and orientation differences between their estimated dipole sources were also noted. CONCLUSIONS We conclude that interaural time and intensity disparities (thus the lateral shifts of a sound image caused by these two cues) are processed in different ways and/or in different areas in auditory cortex.

Comparison of Wiener filtering and selective averaging of evoked potentials

The application of the Wiener filter to the estimation of evoked potentials is criticized, and this method is compared with an a posteriori selective averaging method. It is shown that Wiener filtering may cause information loss for certain types of evoked potentials, since the transient evoked response components of the brain are of damped oscillatory character and are not stationary signals. The selective averaging method is briefly described and suggested to obtain consistent, dependable and more descriptive averaged evoked potentials (AEPs) of a brain structure for well-defined waking and sleep stages. The stated arguments are supported by a comparative representation of AEPs obtained from the cat inferior colliculus by means of Wiener filtering and selective averaging.

Changes in the alpha and beta amplitudes of the central EEG during the onset, continuation, and offset of long-duration repetitive hand movements.

Electroencephalographic alpha and beta activities recorded from central electrodes are known to display movement-related suppression or enhancement. We investigated whether the suppression that is known to occur during the onset of a single movement would persist or otherwise habituate when the movement is continuously repeated for a long period of time. Fourteen subjects took part in the experiments. They performed repetitive simultaneous extension-flexions of the fingers II-V in one hand, continuously for a period of at least 30 s. They then stopped this self-paced movement and rested for at least 30 s. Bipolar recording was made from C3-Cz and C4-Cz. Patterns of amplitude changes in the alpha and beta bands were calculated against a resting baseline. Following a bilateral alpha and beta suppression at the movement onset, alpha amplitude gradually but not fully recovered towards the baseline during the 30 s post-onset. Habituation of afferences and transfer of the cortical function were discussed as the two alternative explanations for this gradual recovery. Beta amplitude, however, displayed no recovery as long as the movement continued. Considering the relatively rapid beta recovery reported for sustained movements, this finding demonstrated that the sustained and continuous movements are conducted through quite different processes. A transient contralateral beta rebound was observed only after the end of the long movement period, strengthening the viewpoint that links the beta rebound with the closure of the cortical processes running throughout a motor sequence. Modulation of the beta amplitude, rather than the changes in alpha amplitude, appeared to be more closely correlated with the execution of a continuous movement.

Interaural delay-dependent changes in the binaural difference potential in cat auditory brainstem response: implications about the origin of the binaural interaction component

Auditory brainstem responses (ABRs) evoked by dichotic clicks with 12 different interaural delays (ITDs) between 0 and 1500 microsecond(s) were recorded from the vertices of 10 cats under ketamine anesthesia. The so-called binaural difference potential (BDP), considered to be an indicator of binaural interaction (BI), was computed by subtracting the sum of the two monaural responses from the binaural one. The earliest and most prominent component of BDP was a negative deflection (DN1) at a latency between 4 and 4.8 ms. Like all the other components of BDP, DNI was also due to binaural reduction rather than enhancement of the corresponding ABR wave, P4 in this case. Furthermore, the way its latency increased as a function of ITD was also not compatible with what would be predicted by the delay-line coincidence detector models based on the excitatory-excitatory units in the medial superior olive (MSO). We therefore proposed an alternative hypothesis for the origin of this BI component based on the inhibitory-excitatory (IE) units in the lateral superior olive (LSO). The computational model designed closely simulated the ITD-dependent attenuation and latency shifts observed in DN1. It was therefore concluded that the origin of this BI component in the cats vertex-ABR could be the lateral lemniscal output of the LSO, although the delay lines which have been shown to exist also in the mammalian brain may play an important role in encoding ITDs.

Origin of the binaural interaction component in wave P4 of the short-latency auditory evoked potentials in the cat: evaluation of serial depth recordings from the brainstem

There is no general agreement on the origin of the binaural interaction (BI) component in auditory brainstem responses (ABRs). To study this issue the ABRs to monaural and binaural clicks with various interaural time differences (ITDs) were simultaneously recorded from the vertex and from a recording electrode aiming at the superior olive (SO) in cats. Electrode path was along the fibers of the lateral lemniscus (LL). Binaural difference potentials (BDPs), which were computed by subtracting the sum of the two monaural responses from the binaural response, were obtained at systematic depths and across a range of ITD values. It was observed that only a specific BDP deflection recorded at the level at which lemniscal fibers terminate in the nuclei of LL coincided in time with the most prominent BDP in the cats vertex-recorded ABRs, the BDP in their wave P4. As ITD was increased, the latency shifts and amplitude decrements of the scalp-recorded far-field BDP wave exactly followed those recorded at this lemniscal near-field BDP locus. The data support our hypothesis that the BI component in wave P4 results from a binaural reduction in dischargings of axons ascending in the LL, with this reduction due to contralateral inhibition of the discharge activity of the inhibitory-excitatory units in the lateral nucleus of the SO. Furthermore, at the level of the SO, the BDP in the responses to contra-leading binaural clicks always had larger magnitudes than those evoked by ipsi-leading ones. This bilateral asymmetry is consistent with the view that the BDP in scalp-recorded ABRs is related to the function of sound lateralization.

Interaural delay-dependent changes in the binaural interaction component of the guinea pig brainstem responses ☆

Auditory brainstem responses to monaural and binaural clicks with 23 different interaural time differences (ITDs) were recorded from ten guinea pigs without anesthesia. Binaural interaction component was obtained by subtracting the sum of the appropriately time-shifted left and right monaural responses from the binaural one. With increasing ITD, the most prominent peak of the binaural difference potential so obtained shifted to longer latencies and its amplitude gradually decreased. The way these changes depended on binaural delay was basically similar to that previously observed in a cat study [P. Ungan, S. Yagcioglu, B. Ozmen. Interaural delay-dependent changes in the binaural difference potential in cat auditory brainstem response: implications about the origin of the binaural interaction component. Hear. Res. 106 (1997) 66-82]. The data were successfully simulated by the model suggested in that report. We therefore concluded that the same model, which was based on the difference between the mean onset latencies of the ipsilateral excitation and contralateral inhibition in a typical neuron in the lateral superior olive, their standard deviations, and the duration of the contralateral inhibition, should also be valid for the binaural interaction in the guinea pig brainstem. The results, which were discussed in connection with sound lateralization models, supported a model based on population coding, where the lateral position of a sound source is coded by the ratio of the discharge intensity in the left and right lateral superior olives, rather than the models based on coincidence detection.

Human long-latency responses to brief interaural disparities of intensity

The electroencephalographic responses to abrupt changes in interaural differences of time (ITD) and intensity (IID) should provide important information on the dynamic characteristics and integrity of the binaural mechanisms detecting the azimuthal shifts of a sound image. However, a change in either or both of these cues to sound lateralization would stimulate not only the binaural mechanisms but also the monaural ones. There are several reports evidencing that in the case of ITD changes this problem can be overcome by using time-shifted noise or repetitive clicks. Any change in IID, however, will inevitably have a stimulating effect also on purely monaural mechanisms. Therefore, the stimulation techniques described in the literature so far for recording the long-latency responses related to IID mechanism cannot be regarded as being specific for binaural mechanisms. We used dichotically presented 100/s click trains which were amplitude modulated with a random sequence of 50 or 100 ms square wave-intervals, so that the sound intensities at the two ears simultaneously alternated between 60 dB and 80 dB levels except during brief periods of time (50 ms) in which the interaural intensity balance was impaired, leading to an IID of 20 dB every 2 s. Owing to the fact that the cortical mechanisms remain unresponsive to repetitive stimuli presented with intervals shorter than a certain recovery period, this stimulus did not evoke any significant potential when it was presented monotically or diotically, yet it could produce lateral sound image shifts and therefore evoke pronounced long-latency responses when presented dichotically. The main components N1 and P2 of these shift responses and those of the pip responses, also recorded from the same subjects, were compared with respect to their midline distributions and hemispheric or bilateral asymmetries. The significant differences found between the shift and pip responses indicated that those evoked by the IID stimulation we designed should not be considered simply as a non-specific vertex potential.

Refractoriness, habituation, and mismatch effects in the auditory event-related potential to alternating stimuli.

Amplitude enhancement in the N1 component of auditory event-related potentials (ERPs) to alternately presented sounds has been referred as a typical example for the effect of release from neural refractoriness. We tested this hypothesis to see whether some other effects also contribute to this phenomenon. Two tones of different frequencies were presented singly or in pairs, and ERPs were recorded using monotonous (mnt) and alternating (alt) sequences of these stimuli. Comparison of the ‘alt–mnt’ difference waveforms recorded with single and paired stimuli supported the refractoriness hypothesis. A mismatch negativity-like wave, however, was also observed, questioning the constraint of ‘at least two consecutive standards before deviant’ presumed in most mismatch negativity studies. This paradigm made it possible to delineate the ERP components related to refractoriness and mismatch detection processes.

Human long-latency potentials evoked by monaural interruptions of a binaural click train: connection to sound lateralization based on interaural intensity differences.

In 9 subjects with normal hearing, monaural offset (MO) responses in the long-latency range were recorded with and without an ongoing sound (click train at a rate of 250/s) at the opposite ear. In the latter case MOs were perceived simply as termination of a sound. In the former case, however, the abrupt transition from binaural to monaural (BM) stimulation was perceived as a shift of the fused image from the center to either side. Therefore, the fairly large difference potential obtained by subtracting the MO response from the BM response was evaluated as the cortical response to stimulation of the sound lateralization mechanism based on interaural intensity differences. These center-to-side responses, which could be characterized by an N1-P2 wave sequence at latencies of 120 and 220 ms, respectively, were compared with the auditory onset responses also recorded from the same subjects by means of a sequential stimulation paradigm. The scalp topography of the N1 components in all these responses recorded simultaneously from frontocentral, parietocentral and two superior temporal electrodes with a neck reference is discussed.

Dynamics of the contralateral white noise-induced enhancement in the guinea pig's middle latency response.

The peak-to-peak amplitude of temporal middle latency response (MLR) of the guinea pig, evoked by a click in the contralateral ear, according to the recording side, is increased with the presence of continuous white noise (CWN) in the ipsilateral ear and this specialty is defined as the white noise enhancement (WNE). This phenomenon is evaluated as an interesting electrophysiological finding from the viewpoint of binaural interaction and in this study, its dynamic specifications were investigated. After the beginning of ipsilateral CWN, significant WNE was observed at 275th ms and it reached to a maximum, with an increase more than 40%, at 350th ms. After a habituation occurred, WNE reached to 20% on the 4th second by gradually decreasing and came to a steady state. In the time window between 2 and 5 ms after CWN started, a surprising amplitude decrease is observed. Therefore, CWN causes an effect, like a click, in the short-term and this on-response type effect originates from low level binaural centers, which decreases the MLR amplitude. However, the same CWN increases the MLR amplitude (WNE) by the effects over the high level binaural centers in the succeeding period, by its continuous characteristic.