Onur Bayazit

Dichotic listening revisited: trial-by-trial ERP analyses reveal intra- and interhemispheric differences.

The dichotic listening (DL) paradigm is often used to assess brain asymmetries at the behavioral level. The aim of this study was to evaluate the dynamic temporal and topographical characteristics of event related potentials (ERPs) obtained with diotic and dichotic consonant-vowel (CV) stimuli from the same subjects. We used a novel approach in which we concurrently analyzed on a trial-by-trial basis ERP parameters during trials that resulted in a right ear advantage (REA) or left ear advantage (LEA) or that were presented under diotic (homonymous) conditions. CV syllables were used as auditory stimuli (/ba/, /da/, /ga/, /ka/, /pa/, /ta/). The EEG measurements were performed with 64 channels by mainly focusing on the N1P2, N2P3 and late negativity (LN) components. Overall, behavioral data revealed a clear REA. The central area showed higher amplitudes than the other locations for N1P2 responses. Additionally, responses were faster for the diotic, compared to the dichotic conditions. The LN had shorter latencies in trials resulting in a REA, compared with those producing a LEA. This result makes it likely that the overall REA is a time-bound effect, which can be explained by the structural theory of Kimura. Furthermore, the results demonstrated a specific spatiotemporal shift from central to frontal areas between N1P2 and LN that was pronounced in dichotic trials. This shift points towards the involvement of frontal areas in resolving conflicting input.

Brain function assessment in different conscious states.

Background The study of brain functioning is a major challenge in neuroscience fields as human brain has a dynamic and ever changing information processing. Case is worsened with conditions where brain undergoes major changes in so-called different conscious states. Even though the exact definition of consciousness is a hard one, there are certain conditions where the descriptions have reached a consensus. The sleep and the anesthesia are different conditions which are separable from each other and also from wakefulness. The aim of our group has been to tackle the issue of brain functioning with setting up similar research conditions for these three conscious states. Methods In order to achieve this goal we have designed an auditory stimulation battery with changing conditions to be recorded during a 40 channel EEG polygraph (Nuamps) session. The stimuli (modified mismatch, auditory evoked etc.) have been administered both in the operation room and the sleep lab via Embedded Interactive Stimulus Unit which was developed in our lab. The overall study has provided some results for three domains of consciousness. In order to be able to monitor the changes we have incorporated Bispectral Index Monitoring to both sleep and anesthesia conditions. Results The first stage results have provided a basic understanding in these altered states such that auditory stimuli have been successfully processed in both light and deep sleep stages. The anesthesia provides a sudden change in brain responsiveness; therefore a dosage dependent anesthetic administration has proved to be useful. The auditory processing was exemplified targeting N1 wave, with a thorough analysis from spectrogram to sLORETA. The frequency components were observed to be shifting throughout the stages. The propofol administration and the deeper sleep stages both resulted in the decreasing of N1 component. The sLORETA revealed similar activity at BA7 in sleep (BIS 70) and target propofol concentration of 1.2 µg/mL. Conclusions The current study utilized similar stimulation and recording system and incorporated BIS dependent values to validate a common approach to sleep and anesthesia. Accordingly the brain has a complex behavior pattern, dynamically changing its responsiveness in accordance with stimulations and states.

Brain asymmetry measurement using EMISU (embedded interactive stimulation unit) in applied brain biophysics

The basic motive to study the brain asymmetry using a dichotic paradigm, forced the authors to develop an interactive stimulus system, where not only the stimuli but also the responses could be interactively registered real time on to the EEG data. Therefore, an embedded interactive stimulation unit (EMISU) was constructed and applied in such a task, incorporating behavioral and evoked data from 20 volunteers (10 male, 10 female, 21.15 years). The results provided a significant factor of laterality in favor of right ear responses (p<0.001) and also proved the applicability of the proposed design.

Spectral Pattern Analysis of Propofol Induced Spindle Oscillations in the Presence of Auditory Stimulations

This study’s primary objective is to analyze human EEG spindle oscillations during propofol-induced anesthesia and to address possible activation sources. Such an analysis also has a secondary role of investigating the short- term spectral patterns and their functional role. Artifact-free epochs of spindle activations were selected from the electroencephalograms of patients undergoing propofol anesthesia. Power spectral analysis and source localization using standardized low-resolution-brain-electromagnetic-tomography (sLORETA) were performed. Additionally, spectrograms were obtained by means of using the Complex Morlet-based algorithm. In order to highlight the functional properties, auditory stimulations were conducted during the propofol administration. The loss of consciousness was reached at a level of 0.8-1.2 µg/mL, which also provided distinct spindle oscillations in the continuous EEG. The un-evoked (spontaneous) and evoked (auditory) conditions were examined across non-medicated and medicated conditions (propofol). The propofol administration resulted in appearance of 12-14 Hz spindle activity mostly localized in BA6, BA9, BA10, BA21, BA24 and BA37 areas. The presence of auditory stimulations slightly shifted these maximum activities to different locations. Between the medicated and non-medicated conditions, there was a significant reduction of spindle activity, which was pinpointed to BA7 (precuneus area). The findings indicate that spindle oscillations may have a dual nature. That is, spindle oscillations may be activity dependent and disruptive for large-scale information processing networks in the brain. Hence, the study of spindle oscillation may provide a basis for understanding the short-term spectral patterns of human EEG.

Amplitude and phase-shift effects on dichotic listening performance

Abstract Objective: Previous dichotic listening (DL) studies have shown that interaural amplitude or phase shifts have powerful effects on dichotic ear advantage. However, it is not known how these two factors interact. The present study aims to explore this interaction when amplitude (intensity) and phase (time) shifts occur simultaneously. Design: Participants listened to dichotically presented pairs of consonant-vowel syllables (216 trials) under nine different experimental conditions, systematically varying amplitude (+9 dB) and phase (+35 ms) shifts. Participants reported which syllable they perceived on each trial. Study sample: Thirty-two healthy adults. Results: The results showed an expected right ear advantage (REA) in the baseline (nosound manipulation) condition. While amplitude shifts favoring the right ear were found to have a greater effect on REA than phase shifts, phase shifts favoring the left ear were found to have a greater effect on left ear advantage (LEA) than amplitude shifts. Furthermore, phase shifts favoring the left ear had a greater effect on LEA than phase shifts favoring the right ear on REA. Conclusions: Present results may have consequences for the training of individuals with auditory and phonetic difficulties, e.g. auditory processing deficits or dyslexia, by aiding the design of optimal combinations of acoustic and phonetic training tools.

Neuroelectric responses of sportsmen and sedentaries under cognitive stress

Stress and anxiety are states which sportsmen are continuously exposed to. Our study aimed to evaluate neuroelectrical peripheral and central nervous system responses of sportsmen (SPR) and sedentary individuals (SED) during concentration grid test (CGT) employed under time pressure. Forty three SPR and 33 SED participated in the study. Neuroelectrical responses were simultaneously obtained during baseline and CGT. All responses were observed to increase under stress in both SED and SPR. The SPR’s stress related peripheral responses were lower than SED’s. When central values were evaluated a stress related increase according to baseline was observed in all frequency powers in all of the participants. Statistical comparison of increase rates revealed a significantly greater increase in beta in SED compared to SPR. Beta has been associated to alertness and cortical arousal. As SED exhibit greater beta increase under stress compared to SPR their state of cortical arousal and alertness may be interpreted to be higher than SPR. However the SPR’s weak increase in beta and their lower peripheral responses taken together may imply that they are better in stress management. In fact according to their performance scores the SPR’s higher level of performance under stress compared to SED shows that they are better at maintaining and focusing their attention under stress than SED.

Comparison of soprano and baritone dichotic listening test scores between genders: Preliminary results

Perception of voices of different genders by male and female individuals is in the research scope of many science areas. In this study, consonant-vowel syllables vocalized by baritone and soprano are presented to participants separately in the same session. In the dichotic application, homonymous or heteronymous syllable combinations are presented to each ear simultaneously. Participants are asked to indicate the syllable that they hear dominantly. For baritone and soprano voices, 36 syllable combinations are presented to 17 healthy subjects (8 females, age: 21.43) in three different conditions: when no attention is directed to any ear (non-forced/NF), when attention is directed to left ear (forced left ear attention/FL), and when attention is directed to right ear (forced right ear attention/FR). In NF condition, male and female subjects had high right ear advantage (REA) for both baritone and soprano voices. In FL condition, male and female subjects had high left ear advantage (LEA). In FR condition, REA was higher. Furthermore, REA score of males when soprano voice was presented was higher than the scores when baritone voice was presented, while females had higher REA scores when baritone voice was presented. To sum up, soprano and baritone voice presentation in dichotic listening test results in right ear preference in NF and FR conditions and left ear preference in FL condition. However, in FR condition REA is increased male and female subjects respond differently to baritone voices such that voice of opposite sex has higher advantage. Additionally, in FL and FR condition, that are indicators of attention, female subjects had better results than males. In further periods of the study, consistency of these results will be tested by increasing participant numbers.

EEG and sLORETA: The Dichotic Listening test preliminary results

Dichotic Listening (DL) paradigm which indicates the brain asymmetry behaviorally is a non-invasive neurocognitive method. Phonetic combinations are used in this method. During the DL paradigm, recently developed system is able to mark the time point on the continuous electroencephalography (EEG) at which the DL stimulus is applied to the person in real time. By applying this new method, DL paradigm, we can evaluate the event related potentials of each ear advantages. Also we can elucidate how auditory system processes the knowledge. In this preliminary study, the main aim was to apply the standardized “Low Resolution Electromagnetic Tomography“ (sLORETA) method to EEG data of DL paradigm. Secondly, sLORETA was applied to different ear advantage (heteronymous and homonymous) event related potentials (ERPs) as a new approach. The preliminary results indicate that for the heteronymous condition the activation found in the superior temporal gyrus in the early stages, moves to central frontal gyrus in the late stages. The differences between the conditions of heteronymous and homonymous arise in frontal activation in the late stages. These findings which indicate the spatial-temporal resolution of the brains dynamic processing can be beneficial for the clinical applications.