Biophysics

This study is carried out on single (not averaged) recordings combining the spontaneous activity preceding the stimulus onset and the El' recorded upon acoustical stimulation. These recordings, which we call EEG-El,ograms, 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 El,s, 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 ~ and /~ frequency ranges; (2) there exist significant coupling and synchtony 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.

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. D

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.

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. D

Amplitude enhancement in the N1 component of auditory eventrelated potentials (ERPs) to alternately presented sounds has been referred as a typical example for the e¡ect of release from neural refractoriness. We tested this hypothesis to see whether some other e¡ects 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' di¡erence 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. NeuroReport19:49^53

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.

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 cat's 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.

Amplitude enhancement in the N1 component of auditory eventrelated potentials (ERPs) to alternately presented sounds has been referred as a typical example for the e¡ect of release from neural refractoriness. We tested this hypothesis to see whether some other e¡ects 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' di¡erence 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. NeuroReport19:49^53

Spontaneous tension changes of isolated strips of guinea-pig portal vein were analysed using the time series analysis (evaluation of autocorrelation function and power spectra). The results indicated three typical power spectral maxima with frequencies of 0.01--0.03 Hz, 0.05--0.08 Hz and 0.09--0.2 Hz. These vascular smooth muscle oscillation components were compared with the vascular autooscillations of the isolated rat kidney and with the mechanical oscillations of the taenia coli which had been previously determined.

We used electrophysiological measures to investigate the effects of obstructive sleep apnea on attention, learning, and memory. Thirty subjects (OSA group, n = 15, control group n = 15) participated in n-back tests, accompanied by P300 recordings, to investigate working memory and attention. The mirror-drawing test was used to study procedural memory, and the trail-making test (TMT) was used to evaluate divided attention and executive function. No significant group difference in reaction time was found in the 0-back and 1-back tests. In the 2-back test, reaction times of patients were longer than those of the control group. No P300 wave was obtained in the OSA group in any (0-, 1-, or 2-back) n-back test. In contrast, in the control group, significant P300 waves were recorded except for the 2-back test. The mirror-drawing scores were unaffected by sleep apnea. There was no difference between groups in the TMT-A test on any of the trials. Although no group difference was found in the first or second trials of the TMT-B test, OSA patients were less successful in learning on the third trial. According to our study results, OSA affects attention and executive function adversely however, we could not detect a significant effect on working or procedural memory.

Interaural time and intensity disparities (ITD and IID) are the two cues to sound lateralization. ''Time-only'' hypothesis claims that an IID is first converted to an interaural afferent delay (Dt), and is then processed by the central ITD mechanism, rendering a separate IID processor unnecessary. We tested this hypothesis by assessing the contribution of the cochlear latency effect to the psychophysical ITD/IID trading ratio. Auditory brainstem responses (ABRs) were used to measure the interaural afferent delays (Dts) that developed with a 20/sec dichotic click train used in the trading experiment. Except for small IIDs at low loudness levels, the physiological Dt delay produced by an IID was significantly smaller than the ITD psychophysically traded for the same IID. We concluded that the cochlear latency effect alone cannot explain the psychophysical ITD/IID trading ratios and a separate IID mechanism must be involved.

It is a common psychophysical experience that a train of clicks faster than ca. 30/s is heard as one steady sound, whereas temporal patterns occurring on a slower time scale are perceptually resolved as individual auditory events. This phenomenon suggests the existence of two different neural mechanisms for processing of auditory sequences with fast and slow repetition rates. To test this hypothesis we used Weber's law, which is known to be valid for perception of time intervals. Discrimination thresholds and Weber fractions (WFs) for 12 base inter-click intervals (ICIs) between 5 and 300 ms were measured from 10 normal hearing subjects by using an "up-down staircase" algorithm. The mean WF, which is supposed to be constant for any perceptual mechanism according to Weber's law, displayed significant variation with click rate. WFs decreased sharply from an average value of around 5% at repetition rates below 20 Hz to about 0.5% at rates above 67 Hz. Parallel to this...

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).

In this study, we analyze the important relation between the spontaneous and evoked activities of the substructures of the cat brain, such as the reticular formation, hippocampus, inferior colliculus, medial geniculate nucleus and acoustical cortex, with an ensemble of systems theory method s consisting of the following steps: (1) single auditory and/or visual evoked potentials (EPs) and the spontaneous activities (EEG) just preceding the stimuli are recorded from the brain center under study; (2) selectively averaged evoked potentials (SAEPs) are obtained from the recorded EPs; (3) amplitude frequency characteristics are computed from the SAEPs by means of Fourier transform; (4) the single EEG-EP sweeps are theoretically pass-band filtered with adequate band limits determined according to the selectivities revealed by the amplitude characteristics; (5) the EEG and EP components obtained in this way are compared with regard to the amplification in the population response upon the application of the stimulus. The results of this analysis support quantitatively our prediction of various types of resonance phenomena in a number of nuclei in the cat brain and in a large scale of frequencies from 1 Hz to 1000 Hz and show that the amplification factor related to resonance phenomena has probabilistic nature. Therefore, the analogy which we have recently drawn between the behaviors of a neural population and a random-phase probabilistic harmonic oscillator is extended by assigning also the amplitude and the frequency of the oscillations as random variables. A working hypothesis for the dynamics of neuronal populations is elaborated accordingly.

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.

An acoustic stimulus elicits an electroencephalographic response called auditory event-related potential (ERP). When some members of a stream of standard auditory stimuli are replaced randomly by a deviant stimulus and this stream is presented to a subject who ignores the stimuli, two different ERPs to deviant and standard stimuli are recorded. If the ERP to standard stimuli is subtracted from the ERP to deviant stimuli, the difference potential (DP) waveform typically exhibits a series of negative-positive-negative deflections called mismatch negativity (MMN), P3a, and reorienting negativity (RON), which are associated with pre-attentive change detection, involuntary attention switching, and reorienting of attention, respectively. The aim of the present study was to investigate how these pre-attentive processes are affected if the change occurs earlier than its usual timing implied by isochronous standard stimuli. In the MMN paradigm employed, 15% of the standards were randomly replaced by deviant stimuli which differed either in their pitch, their earlier onset time, or in both. Event-related responses to these three deviants [timely pitch change (R TP), earlier onset (R EO), earlier pitch change (R EP)] and to standards (R S) were recorded from 10 reading subjects. To maintain identical stimulation histories for the responses subtracted from each other, "deviant-standard" difference potentials (DP) for "timely" and "early" pitch deviances were derived as follows: DP TP = R TP − R S and DP EP = R EP − R EO. Interestingly, the MMN components of the DPs to timely and early pitch deviances had similar amplitudes, indicating that regularity of stimulus timing does not provide any benefit for the pre-attentive auditory change detection mechanism. However, different scalp current density (SCD) dynamics of the MMN/P3a complexes, elicited by timely and early pitch deviances, suggested that an auditory change in a stimulus occurring earlier-than-usual initiates a faster and more effective call-for-attention and causes stronger attention switching than a timely change. SCD results also indicated Abbreviations: DP, Difference Potential; DP EP , Difference Potential to early pitch deviance; DP TP , Difference Potential to timely pitch deviance; ERP, Event-related Potential; IOI, Inter-onset Interval; MMN, Mismatch Negativity; MMN EP , Mismatch Negativity to early pitch deviance; MMN TP , Mismatch Negativity to timely pitch deviance; P3a, The positive component around 250 ms of the ERP to deviant stimulus; R EP , ERP to early pitch deviant stimulus; R EO , ERP to standard stimulus with early onset; R TP , ERP to timely pitch deviant stimulus; R S , ERP to timely standard stimulus; RON, Reorienting negativity; SCD, Scalp Current Density.