Researcher:
Ungan, Pekcan

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Pekcan

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Ungan

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Ungan, Pekcan

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2012 - 2019102020 - 20214

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    A diversity combination model incorporating an inward bias for interaural time-level difference cue integration in sound lateralization
    (MDPI, 2020) N/A; N/A; Department of Computer Engineering; N/A; Mojtahedi, Sina; Erzin, Engin; Ungan, Pekcan; PhD Student; Faculty Member; Faculty Member; Department of Computer Engineering; Graduate School of Sciences and Engineering; College of Engineering; School of Medicine; N/A; 34503; N/A
    A sound source with non-zero azimuth leads to interaural time level differences (ITD and ILD). Studies on hearing system imply that these cues are encoded in different parts of the brain, but combined to produce a single lateralization percept as evidenced by experiments indicating trading between them. According to the duplex theory of sound lateralization, ITD and ILD play a more significant role in low-frequency and high-frequency stimulations, respectively. In this study, ITD and ILD, which were extracted from a generic head-related transfer functions, were imposed on a complex sound consisting of two low- and seven high-frequency tones. Two-alternative forced-choice behavioral tests were employed to assess the accuracy in identifying a change in lateralization. Based on a diversity combination model and using the error rate data obtained from the tests, the weights of the ITD and ILD cues in their integration were determined by incorporating a bias observed for inward shifts. The weights of the two cues were found to change with the azimuth of the sound source. While the ILD appears to be the optimal cue for the azimuths near the midline, the ITD and ILD weights turn to be balanced for the azimuths far from the midline.
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    Facial muscle activity contaminates EEG signal at rest: evidence from frontalis and temporalis motor units
    (Iop Publishing Ltd, 2019) N/A; N/A; N/A; N/A; N/A; Yılmaz, Gizem; Budan, Abdullah Salih; Ungan, Pekcan; Topkara, Betilay; Türker, Kemal Sıtkı; PhD Student; Undergraduate Student; Faculty Member; PhD Student; Faculty Member; Graduate School of Health Sciences; School of Medicine; School of Medicine; Graduate School of Health Sciences; School of Medicine; N/A; N/A; N/A; 353320; 6741
    Objective: In order to reach electroencephalography (EEG) electrodes on the scalp, synchronized activity of neurons needs to pass thorough several tissue layers, including the skull and muscles covering the scalp. the contamination of EEG signal by temporalis and frontalis muscles has been well documented for voluntary muscle contraction even at low contraction levels. the extent of myogenic contamination during postural and/or rest activity of the temporalis and frontalis remains an impediment for EEG research. Approach: In this study, we first aimed to observe involuntary, continuous motor unit activity of the frontalis muscle at rest and evaluate motor unit level frontalis interference on the EEG electrodes. Second, we compared motor unit interference from the frontalis before and after artefact pruning via an independent component analysis (ICa) algorithm. Main Results: We demonstrated that motor unit activity of the frontalis muscle produces interference potentials on the frontal electrodes at rest and the interference was significantly reduced after ICa on the frontal electrodes, but not completely eliminated. Likewise, the temporalis interference at rest was significantly smaller after ICa on the fronto-temporal electrodes, but not completely removed. Significance: We documented the existence of resting involuntary activity of the temporalis and frontalis muscles underneath EEG electrodes and the removal of the EEG signal from their contiguous interference is not possible even after the use of ICa technology. We recommend that EEG researchers readdress the definition of 'rest' for EEG recordings and the ICa experts should extend their electromyography removal strategies to motor unit level interference.
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    EEG-like signals can be synthesized from surface representations of single motor units of facial muscles
    (Springer, 2018) N/A; Yılmaz, Gizem; Ungan, Pekcan; Türker, Kemal Sıtkı; PhD Student; Faculty Member; Faculty Member; Graduate School of Health Sciences; School of Medicine; School of Medicine; N/A; N/A; 6741
    Electrodes for recording electroencephalogram (EEG) are placed on or around cranial muscles; hence, their electrical activity may contaminate the EEG signal even at rest conditions. Due to its role in maintaining mandibular posture, tonic activity of temporalis muscle interferes with the EEG signal particularly at fronto-temporal locations at single motor unit (SMU) level. By obtaining surface representation of a motor unit, we can evaluate its interference in EEG and if we could sum surface representations of several tonically active motor units, we could estimate the overall myogenic contamination in EEG. Therefore, in this study, we followed re-composition (RC) approach and generated EEG-like artefact model using surface representations of single motor units (RC). Furthermore, we compared signal characteristics of RC signals with simultaneously recorded EEG signal at different locations in terms of power spectral density and coherence. First, we found that RC signal represented the power spectral distribution of an EMG signal. Second, RC signal reflected the discharge rate of a SMU giving the greatest surface representation amplitude and strongest interference appeared as distinguishable frequency peak on RC power spectra. Moreover, for strong interferences, RC also contaminated the EEG at F7 and other EEG electrodes. These findings are important to illustrate the susceptibility of EEG signal to myogenic artefacts even at rest and the research using EEG coherence comparisons should consider muscle activity while drawing conclusions about neuronal interactions and oscillations.
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    Event-related potentials to single-cycle binaural beats and diotic amplitude modulation of a tone
    (Springer, 2019) Yağcıoğlu, Suha; N/A; N/A; Ungan, Pekcan; Ayık, Ece; Faculty Member; Master Student; School of Medicine; Graduate School of Sciences and Engineering; N/A; N/A
    When two tones with slightly different frequencies are dichotically presented, binaural beats (BBs) are experienced. BBs resulting from the cycling change in interaural phase difference elicit electroencephalographic responses. Because they repeat at short periods, allowing poor recovery of the cortical responses, these steady-state responses have small amplitudes, and their various wave components intermingle and might mask each other. Using single-cycle BBs separated by relatively long inter-onset intervals would be a solution, but introducing a transient interaural frequency shift requires response subtraction which may not be acceptable for non-additive brain responses. The proposed stimulation method employs transient and monaurally subthreshold frequency shifts in opposite directions in the two ears to produce single-cycle BBs of a 250Hz tone. These shifts are perceived as distinct BBs when presented dichotically, but remain subthreshold when presented monotically. Therefore, no frequency-shift response is elicited, and the specific BB response is obtained with no need for waveform subtraction. We recorded from 19 normal hearing participants the event-related potentials (ERPs) to single-cycle BBs and also to temporary diotic amplitude modulation (AM) with matched perceptual salience. The ERPs to single-cycle BBs presented at 2s inter-onset intervals had N1-P2 responses with up to seven times larger amplitudes than the conventional steady-state BB responses in the literature. Significant differences were found between the scalp potential distributions of the N1 responses to BB and AM stimuli, suggesting that the cortical sites, where envelope-based level processing and temporal fine structure-based spatial processing of the stimulus take place, are not totally overlapped.
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    Event-related potentials to single-cycle binaural beats of a pure tone, a click train, and a noise
    (Springer, 2019) Yağcıoğlu, Suha; N/A; N/A; Ungan, Pekcan; Ayık, Ece; Faculty Member; Master Student; School of Medicine; Graduate School of Sciences and Engineering; N/A; N/A
    There are only few electrophysiological studies on a phenomenon called "binaural beats" (BBs), which is experienced when two tones with frequencies close to each other are dichotically presented to the ears. And, there is no study in which the electrical responses of the brain to BBs of complex sounds are recorded and analyzed. Owing to a recent method based on single-cycle BB stimulation with sub-threshold temporary monaural frequency shifts, we could record the event-related potentials (ERPs) to BBs of a 250-Hz tone as well as those to the BBs of a 250/s click train and to the BBs of a recurrent 4-ms Gaussian noise. Although fundamental components of the click train and noise stimuli were lower in intensity than the tonal stimuli in our experiments, the N1 responses to the BBs of the former two wide-spectrum sounds were recorded with significantly larger amplitudes and shorter latencies than those to the BBs of a tone, suggesting an across-frequency integration of directional information. During a BB cycle of a complex sound, the interaural time differences (ITDs) of the spectral components are all equal to each other at any time; whereas their interaural phase differences (IPDs) are all different. The ITD rather than the IPD should, therefore, be the cue that is relied upon by the binaural mechanism coding the perceived lateral shifts of the sound caused by BBs. This is in line with across-frequency models of human auditory lateralization based on a common ITD, fulfilling a straightness criterion.
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    Effects of aging on event-related potentials to single-cycle binaural beats and diotic amplitude modulation of a tone
    (Elsevier, 2020) Yagcioglu, Suha; N/A; N/A; Ungan, Pekcan; Ayık, Ece; Faculty Member; Master Student; School of Medicine; Graduate School of Sciences and Engineering; Koç University Hospital; N/A; N/A
    Aim of the study is to determine whether the auditory processing of temporal fine structure (TFS) is affected with normal aging, even in the presence of normal audiometric hearing and fine cognitive state; and, if it is, to see whether a comparable effect is also observed in the processing of a diotic change in sound envelope. The event-related potentials (ERPs) to binaural beats (BBs), which are the responses of the binaural mechanisms processing TFS of a sound, and the ERPs to diotic amplitude modulation (AM) stimuli, which are the responses of the monaural mechanisms processing the changes in its envelope, were recorded from thirteen young university students and ten senior but active university professors, all with normal hearing in low frequencies. To obtain directly the specific BB responses without confounding monaural frequency change-evoked responses, we used single-cycle BB stimuli with temporary sub-threshold frequency shifts. BBs of a 250-Hz tone and diotic AM of the same tone with similar perceptual salience were presented with 2-second stimulus onset asynchrony. The N1 components of the ERPs to both stimuli displayed notable age-dependent changes in their scalp topography and significant amplitude reduction and latency prolongation in the elderly. These amplitude and latency changes were at similar rates for the two stimulus types, implying that the auditory TFS and envelope processing mechanisms are proportionally affected by physiological aging. These results may serve as control data in future studies investigating the effect of aging-associated cognitive pathologies on auditory TFS processing.
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    PublicationOpen Access
    A diversity combination model incorporating an inward bias for interaural time-level difference cue integration in sound lateralization
    (Multidisciplinary Digital Publishing Institute (MDPI), 2020) N/A; Department of Computer Engineering; Mojtahedi, Sina; Erzin, Engin; Ungan, Pekcan; Faculty Member; Faculty Member; Department of Computer Engineering; Graduate School of Sciences and Engineering; College of Engineering; School of Medicine; N/A; 34503; N/A
    A sound source with non-zero azimuth leads to interaural time level differences (ITD and ILD). Studies on hearing system imply that these cues are encoded in different parts of the brain, but combined to produce a single lateralization percept as evidenced by experiments indicating trading between them. According to the duplex theory of sound lateralization, ITD and ILD play a more significant role in low-frequency and high-frequency stimulations, respectively. In this study, ITD and ILD, which were extracted from a generic head-related transfer functions, were imposed on a complex sound consisting of two low- and seven high-frequency tones. Two-alternative forced-choice behavioral tests were employed to assess the accuracy in identifying a change in lateralization. Based on a diversity combination model and using the error rate data obtained from the tests, the weights of the ITD and ILD cues in their integration were determined by incorporating a bias observed for inward shifts. The weights of the two cues were found to change with the azimuth of the sound source. While the ILD appears to be the optimal cue for the azimuths near the midline, the ITD and ILD weights turn to be balanced for the azimuths far from the midline.
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    PublicationOpen Access
    Accuracy in archery shooting is linked to the amplitude of the ERP N1 to the snap of clicker
    (Montenegrin Sports Academy and University of Montenegro, 2021) Ertan, Hayri; Yağcıoğlu, Süha; Yılmaz, Alpaslan; Korkusuz, Feza; Ungan, Pekcan; Faculty Member; School of Medicine
    An archer requires a well-balanced and highly reproducible release of the bowstring to attain high scores in competition. Recurve archers use a mechanical device called the clicker to check the draw length. The fall of the clicker that generates an auditory stimulus should evoke a response in the brain. The purpose of this study is to evaluate the event-related potentials during archery shooting as a response to the fall of the clicker. Fifteen high-level archers participated. An electro cap was placed on the archers' scalps, and continuous EEG activity was recorded (digitized at 1000 Hz) and stored for off-line analysis. The EEG data were epoched beginning 200 ms before and lasting 800 ms after stimulus marker signals. An operational definition has been developed for classifying hits corresponding to hit and/or miss areas. The hit area enlarged gradually starting from the centre of the target (yellow: 10) to blue (6 score) by creating ten hit area indexes. It is found that the snap of the clicker during archery shooting evokes N1-P2 components of long-latency evoked brain potentials. N1 amplitudes are significantly higher in hit area than that of miss areas for the 2nd and 4th indexes with 95% confidence intervals and 90% confidence intervals for the 1st and 3rd indexes with 90% confidence intervals. We conclude that the fall of the clicker in archery shooting elicits an N1 response with higher amplitude. Although evoked potential amplitudes were higher in successful shots, their latencies were not significantly different from the unsuccessful ones.
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    PublicationOpen Access
    Interference of tonic muscle activity on the EEG: A single motor unit study
    (Frontiers, 2014) Ugingius, Paulius; N/A; Ungan, Pekcan; Sebik, Oğuz; Türker, Kemal Sıtkı; Yılmaz, Gizem; Faculty Member; Researcher; Faculty Member; PhD Student; School of Medicine; N/A; N/A; 6741; N/A
    The electrical activity of muscles can interfere with the electroencephalogram (EEG) signal considering the anatomical locations of facial or masticatory muscles surrounding the skull. In this study, we evaluated the possible interference of the resting activity of the temporalis muscle on the EEG under conventional EEG recording conditions. In 9 healthy adults EEG activity from 19 scalp locations and single motor unit (SMU) activity from anterior temporalis muscle were recorded in three relaxed conditions; eyes open, eyes closed, jaw dropped. The EEG signal was spike triggered averaged (STA) using the action potentials of SMUs as triggers to evaluate their reflections at various EEG recording sites. Resting temporalis SMU activity generated prominent reflections with different amplitudes, reaching maxima in the proximity of the recorded SMU. Interference was also notable at the scalp sites that are relatively far from the recorded SMU and even at the contralateral locations. Considering the great number of SMUs in the head and neck muscles, prominent contamination from the activity of only a single MU should indicate the susceptibility of EEG to muscle activity artifacts even under the rest conditions. This study emphasizes the need for efficient artifact evaluation methods which can handle muscle interferences.
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    PublicationOpen Access
    Significant variations in Weber fraction for changes in inter-onset interval of a click train over the range of intervals between 5 ms and 300 ms
    (Frontiers, 2014) Yağcıoğlu, Süha; (TBD); Ungan, Pekcan; Faculty Member; (TBD); School of Medicine
    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 WE 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 steep transition, subjects reported that at rates below 20 Hz they perceived periodicity as a fast tapping rhythm, whereas at rates above 50 Hz the perceived quality was a pitch. Such a dramatic change in WE indicated the existence of two separate mechanisms for processing the click rate for long and short ICIs, based on temporal and spectral features, respectively. A range of rates between 20 and 33 Hz, in which the rate discrimination threshold was maximum, appears to be a region where both of the presumed time and pitch mechanisms are relatively insensitive to rate alterations. Based on this finding, we speculate that the interval-based perception mechanism ceases to function at around 20 Hz and the spectrum based mechanism takes over at around 33 Hz; leaving a transitional gap in between, where neither of the two mechanisms is as sensitive. Another notable finding was a significant drop in WE for ICI = 100 ms, suggesting a connection of time perception to the electroencephalography alpha rhythm.