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Biomedical subjects

P Ungan

Publications and source records attributed to P Ungan.

At least 19 recordsLinked to original sources
PubMed2001

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.

Adolescent↗
PubMed1997

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 cat's 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.

Acoustic Stimulation↗
PubMed1996

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.

Acoustic Stimulation↗
PubMed1994

Alteration of tension response to stretch with spontaneous contraction frequency in the guinea pig taenia coli and stiffness-work characteristics.

Previously we developed a data acquisition procedure to measure the tension responses of smooth muscles during their spontaneous contraction. This study was undertaken to determine whether the time course of the tension responses to stretches was altered with the ongoing spontaneous contractions. In addition, the stiffness, phase and work characteristics were evaluated from the tension responses in order to see how these characteristics were affected by the changes in the tension responses. The tension responses of the spontaneously contracting taenia coli to stretches had four phases. However, the latencies of the phases changed with the type of the spontaneous contractions during which the measurement was carried out. In the responses recorded during the slow spontaneous contractions (40-70 s), the second phase reached the minimum at 2.26 +/- 1.12 s and the third phase had the peak tension at 9.56 +/- 3.08 s. These latencies were 0.89 +/- 0.30 s and 3.91 +/- 1.10 s, respectively, when the experiments were undertaken during high frequency contractions (10-30 s). Besides, the maxima and minima in the stiffness, phase and work characteristics were also shifted to higher frequencies as the spontaneous contraction became faster. Work had a positive peak around 0.03 Hz for the slowly contracting taenia coli. The peak shifted to 0.1 Hz when the taenia coli contracted at higher frequency. The results show that the time course of the tension responses is determined by the contractile state of the taenia coli.

Animals↗
PubMed1992

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.

Acoustic Stimulation↗
PubMed1989

Human laterality reversal auditory evoked potentials: stimulation by reversing the interaural delay of dichotically presented continuous click trains.

Long-latency responses to bidirectional reversals in the laterality of a sound image were recorded from vertex with left and right earlobe references simultaneously. Laterality reversal stimuli with regular ISIs of 3 sec were obtained by alternating between +1 msec and -1 msec the interaural delay (ID) of dichotically presented 70 c/sec continuous click trains. Monaural presentation of this temporal modulation did not evoke any potential or any sensation whereas, when dichotically presented, the same ID modulation was perceived as laterality reversals. Evoked potentials from 9 right-handed volunteers with normal hearing were evaluated. The recorded laterality reversal auditory evoked potentials (LRAEPs) consisted, characteristically, of a negative and a positive wave with average peak latencies of 160 msec and 260 msec, respectively. Statistical analysis indicated significant dependence of the response parameters on the direction of the sound image shifts, but the laterality of the earlobe reference did not prove to be important. The appropriateness of the stimulus parameters employed was discussed from the viewpoint of dynamic systems analysis. By contrasting with the monaural auditory off-responses recorded from the same subjects, it was demonstrated that the laterality reversal responses we obtained did not seem to be non-specific 'rest-to-motion' responses to any change in the acoustic background. Similarities and differences between these laterality reversal responses and various ERP components in the same latency range were also discussed. It is concluded that the LRAEPs introduced in this communication can be used in assessing the functional integrity and studying the dynamics of the neurophysiological system that accomplishes sound lateralization using cues of interaural time disparity (ITD).

Acoustic Stimulation↗
PubMed1989

Specificity of auditory evoked potentials from rat hypothalamus: differential recording by lateral and ventromedial electrodes.

Evoked potentials recorded from ventromedial nucleus (VMH) and lateral area (LHA) of the hypothalamus were studied in freely moving rats with respect to specificity of the components to recorded areas and to peripheral stimuli utilized. This study attempts to circumvent some methodological problems and contamination in data with respect to origin of recorded components by comparing monopolar and differential recordings made in VMH and LHA, areas accepted as being involved in food-intake behavior. Of the stable components, the complex with 15-20 ms latency was evaluated as of extra-hypothalamic origin. Meanwhile, those with 25-ms (positive) and 40-ms (negative) average peak latencies were considered as specific to recorded areas. Hypothalamic responses appeared not to be specific to stimulus modality, indicating convergence of different sensory modalities on studied areas. Hunger and satiety appeared to influence the amplitude of the specified components similarly, the study not providing evidence for the reciprocal relation stated in the dual center hypothesis between VMH and LHA.

Action Potentials↗
PubMed1980

Comparative frequency analysis of single EEG-evoked potential records.

The aim of this study is to expand our previous results on the important relation between the spontaneous EEG and EPs (Basar et al. 1976 a,b). These previous results were obtained from single recordings of combined EEG-EP epochs by comparing the rms-magnitude of the spontaneous EEG prior to stimulus with the maximal amplitude of the response. Both were pass-band-filtered by means of the response adaptive filter, determined according to the band limits of the studied selectivity channel. In this earlier analysis, although the relation between the amplitudes of EEG and EP components in various frequency channels could be determined in the time domain, no exact statement could be made (1) on possible relations between the frequency distributions of spontaneous and evoked activities of a given brain centre, nor (2) on the phase relationships between the electrical activities of various brain structures. These gaps are filled in our present report with the further analysis of the EEG-EPograms simultaneously recorded from 5 different brain structures.

Acoustic Stimulation↗
PubMed1979

Combined dynamics of EEG and evoked potentials. I. Studies of simultaneously recorded EEG-EPograms in the auditory pathway, reticular formation, and hippocampus of the cat brain during the waking stage.

This study is carried out on single (not averaged) recordings combining the spontaneous activity preceding the stimulus onset and the EP recorded upon acoustical stimulation. These recordings, which we call EEG-EPograms, are measured simultaneously from different subdural brain structures, such as the auditory cortex, medial geniculate nucleus, inferior colliculus, reticular formation and the hippocampus of awake cats. Using a combined analysis procedure (C.A.P.), the relevant frequency components of spontaneous EEG and EPs, recorded simultaneously from these brain nuclei, are analyzed according to the consistent selectivity bands depicted by the determined amplitude-frequency characteristics. These analyses provide us the following information: (1) there is an important congruency in the time courses of simultaneous response components in common frequency bands, especially in the alpha and beta frequency ranges; (2) there exist significant coupling and synchrony between the evoked amplitude enhancements in the simultaneously recorded single response components; (3) the inter-nuclei coherency in the brain's electrical activity is enormously increased upon stimulation; (4) the evoked response magnitude can be predicted, with reasonable accuracy, from the spontaneous activity preceding the stimulation. The strong dependence of the response magnitude on the stimulus-preceding EEG is explained by means of a model network consisting of a population of relaxation oscillators, which can be brought to different states of synchrony and asynchrony. Some suggestions and comments are also made for investigators working toward theories of signal transmission in the brain.

Animals↗
PubMed1979

Combined dynamics of EEG and evoked potentials. II. Studies of simultaneously recorded EEG-EPograms in the auditory pathway, reticular formation, and hippocampus of the cat brain during sleep.

This study is carried out on single (not averaged) recordings combining the spontaneous activity preceding the stimulus onset and the EP recorded upon acoustical stimulation. These recordings, which we call EEG-EPograms, are measured simultaneously from different subdural structures, such as the auditory cortex, medial geniculate nucleus, inferior colliculus, reticular formation and the hippocampus of the cat brain during the slow wave sleep stage. Using a combined analysis procedure (C.A.P.), the relevant frequency components of spontaneous EEG and EPs, recorded simultaneously from these brain nuclei, are analyzed according to the consistent selectivity bands depicted by the determined amplitude-frequency characteristics for the SWS-stage. In parallel with the results which we obtained for the waking stage, these analyses provide also the following information: (1) there is an important congruency in the time courses of simultaneous response components in common frequency bands, especially in the alpha and beta frequency ranges; (2) there exist significant coupling and synchrony between the evoked amplitude enhancements in the simultaneously recorded single response components; (3) the inter-nuclei coherency in the brain's electrical activity is enormously increased upon stimulation;(4) the evoked response magnitude can be predicted, with reasonable accuracy, from the spontaneous activity preceding the stimulus. All these findings are discussed with reference to those obtained for the waking stage.

Animals↗
PubMed1976

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.

Action Potentials↗