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T Demiralp

Publications and source records attributed to T Demiralp.

33 records · Page 2Linked to original sources
PubMed1998

Slow cortical potential shifts modulate P300 amplitude and topography in humans.

Effects of the spontaneous slow cortical potential (SCP) shifts of the electroencephalogram (EEG) on the P300 response were investigated on ten healthy volunteers. P300 responses were recorded using an auditory oddball paradigm, where target stimuli were presented regularly after every four standard stimuli. Single event-related potential (ERP) sweeps exhibiting negative or positive SCP shifts were averaged separately. The P300 amplitude was significantly larger during negative SCP shifts. Furthermore, the topographies of P200 and P300 waves obtained during negative and positive SCP shifts showed significant differences. The results indicate that the SCP shifts in single ERP sweeps, which are considered to be correlated with the arousal or basic activity level of the cortex, explain at least part of the inter-trial variability of P300 response.

Adult↗
PubMed1998

Analysis of event-related potentials (ERP) by damped sinusoids.

Several researchers propose that event-related potentials (ERPs) can be explained by a superposition of transient oscillations at certain frequency bands in response to external or internal events. The transient nature of the ERP is more suitable to be modelled as a sum of damped sinusoids. These damped sinusoids can be completely characterized by four sets of parameters, namely the amplitude, the damping coefficient, the phase and the frequency. The Prony method is used to estimate these parameters. In this study, the long-latency auditory-evoked potentials (AEP) and the auditory oddball responses (P300) of 10 healthy subjects are analysed by this method. It is shown that the original waveforms can be reconstructed by summing a small number of damped sinusoids. This allows for a parsimonious representation of the ERPs. Furthermore, the method shows that the oddball target responses contain higher amplitude, slower delta and slower damped theta components than those of the AEPs. With this technique, we show that the differentiation of sensory and cognitive potentials are not inherent in their overall frequency content but in their frequency components at certain bands.

Evoked Potentials, Auditory↗
PubMed1998

Analysis of the electroencephalographic activity during the Necker cube reversals by means of the wavelet transform.

In previous studies, a perceptual switching related potential was obtained during the observation of a multistable dynamic reversal pattern, where the averaging of the single responses was triggered by subjects pressing a button. The present methodological study aims to increase the signal quality of perceptual switching related potentials considering the dependence of the measurement method on the reaction time of the subject, which may vary significantly during a session, leading to low-amplitude waveform in the averaged event-related-potential (ERP). To overcome this problem in measuring the electrophysiological correlate of an internal event, a pattern selection method based on the wavelet transform (WT) is proposed to choose a subset of single ERPs with more homogenous latencies. Nine subjects observed a Necker cube and were instructed to press the button immediately after perceptual switching. A slow, low-amplitude positive wave with frontocentral amplitude maxima was observed around 250 ms prior to the button press. After the application of a 5 octave WT on single sweeps, the time-frequency coefficients obtained in each octave were averaged across trials. The most dominant feature representing the averaged ERP was the delta (0.5-4 Hz) coefficient occurring between 250 and 125 ms before the button press. By averaging the subset of the single sweeps containing this property, a sharpening and significant amplitude increase of the response peak was observed.

Animals↗
PubMed1998

Spontaneous EEG theta activity controls frontal visual evoked potential amplitudes.

Frontal visual evoked potentials (VEPs) were studied in order to extend the application of a recently introduced algorithm for selective averaging of evoked potentials. This algorithm is based on the inverse relationship between amplitudes of alpha or theta components of the spontaneous EEG activity and evoked potential (EP) amplitudes. Stimuli were only applied if the root mean square (RMS) value of the ongoing EEG at the lead F4 was below an individual threshold level ('selective stimulation'). For this comparison, the EEG was filtered in one of the frequency ranges 'alpha', 'theta' and 'alpha and theta', respectively. 'Alpha' and 'alpha-and-theta-dependent' selective stimulation conditions resulted in significant amplitude increases (P < 0.05) at the input reference channel F4 and partly at ipsihemispherical temporal and parietal leads and at Cz. The largest increase of 35% at F4 (P < 0.01) was obtained with visual stimulation during low prestimulus theta activity. We conclude that spontaneous theta activity of the frontal cortex may be a factor influencing the amplitudes of frontal VEPs. A sophisticated analysis of frontal EPs, mainly in the framework of cognitive studies, should consider the theta activity prior to stimulation.

Adult↗
PubMed1997

The phase-locking of auditory gamma band responses in humans is sensitive to task processing.

The present study assessed the effects of stimulus task-relevance and certainty on early and late 40 Hz (gamma) band responses (GBRs) in humans. Auditory GBRs of nine young adults were recorded in passive listening, simple reaction task, and choice-reaction task (target probability = 0.5) conditions and evaluated in three consecutive post-stimulus periods (0-120, 120-250, 250-400 ms) corresponding to the serial occurrence of gamma oscillation bursts. Amplitude and phase-locking of GBRs within these bursts were analysed separately at the level of single sweeps by applying a method that allows the independent quantification of between-sweep synchronization. Major results showed that the effects of stimulus certainty and task relevance on single response amplitude were specific and different from the effects on the phase-locking. The functional involvement of the early and late auditory gamma responses was distinct: early auditory gamma band responses appear primarily associated with focused attention, while the late gamma responses vary with motor-task relevance. It is concluded that along with power measures, the stability of phase-locking of gamma band responses should be regarded as a functionally meaningful parameter that varies with processing demands and recording site.

Acoustic Stimulation↗
PubMed1997

The effects of subthreshold visual stimulation on P300 response.

The effects of the subthreshold visual stimuli on the electrical activity of the brain were investigated by an oddball-like paradigm and a single-stimulus paradigm. In the oddball-like paradigm, suprathreshold and subthreshold stimuli were presented randomly where the suprathreshold stimulus probability was 0.24. The single-stimulus paradigm contained only the suprathreshold stimuli and was identical to the oddball-like paradigm apart from the absence of subthreshold stimuli. Suprathreshold P3b amplitude was larger for the oddball-like than for the single-stimulus condition particularly in the centro-parietal region. This result suggests that at least one component of the P300 wave could be assigned to an automatic comparison and memory updating process which runs unconsciously.

Adult↗
PubMed1997

Time-frequency analysis reveals multiple functional components during oddball P300.

A time-frequency decomposition was applied to rare target and frequent non-target event-related potentials (ERPs) elicited in an oddball condition to assess whether multiple functional components occur in the P300 latency range. The wavelet transform (WT) was used because it allows capture of simultaneous or partly overlapping components in ERPs without loosing their temporal relationships. The application of a four-octave quadratic B-spline wavelet transform at the level of single-sweep data allowed us to obtain new information and revealed the presence of separate events during P300 development. Several delta, theta, and alpha frequency components in the P300 latency range differed between target and non-target processing. These findings indicate that P300 is composed of multiple functional components and that the WT method is of use for the study of P300 functional correlates more precisely.

Acoustic Stimulation↗
PubMed1997

Effects of task variables on the amplitude and phase-locking of auditory gamma band responses in human.

The present study was designed to assess the effects of stimulus certainty and motor task-relevance on auditory transient 40-Hz or gamma band responses. To study the effects of these factors in a balanced design, auditory event-related potentials (ERPs) of 9 young adults were recorded in a passive listening, simple reaction task, and choice-reaction task (target tone probability = 0.5) conditions. Amplitude and phase-locking of event-related gamma activity were analyzed separately at the level of single sweeps by applying a method that allows the independent quantification of phase synchronization between consecutive single responses. Major results demonstrated that (1) During auditory stimulus processing discernible gamma oscillation bursts were observed in three time windows of the poststimulus epoch: early (0-120) ms, middle (120-250 ms) and late (250-400 ms). (2) Early gamma response amplitudes were significantly largest for highly expected motor-task stimuli, whereas the phase-locking did not depend on either of the two variables. (3) The phase-locking of late gamma responses, however, was significantly stronger to targets than to nontargets. These results indicate that auditory gamma responses are functionally involved in the processing of task variables such as stimulus certainty and motor-task relevance. It is also demonstrated that single gamma response amplitude and phase-locking have independent functional significance as being affected in a different manner by different task conditions.

Adult↗
PubMed1996

Distributed gamma band responses in the brain studied in cortex, reticular formation, hippocampus and cerebellum.

The transient evoked responses of auditory cortex, reticular formation, hippocampus and cerebellar cortex to auditory stimulation have been analysed in the gamma frequency band on cats with chronically implanted electrodes. We found gamma band transient responses consisting of wave packets with 3-4 oscillations in all of the studied brain structures in the first 100 ms of the poststimulus period. The responses were strongly time-locked to the stimulation time point. The observation that the gamma band responses exist simultaneously in various brain structures supports the tentative proposal of our group on the "diffuse gamma response system" of the brain, which seems to be an important, universal operator in brain function. Furthermore, it shows that in search for generalized approaches to brain phenomena it is important to analyse the simultaneous behavior of different brain structures.

Animals↗
PubMed1996

Dynamic properties of the responses to fast omitted stimulus potential paradigm in human.

Visual evoked potentials (VEPs) and omitted stimulus potentials (OSPs) were analyzed on scalp recordings of 10 healthy subjects. OSP responses, obtained after the due-time of the first omitted stimulus at the end of a conditioning stimulus train have been studied mostly in animals. Two types of visual OSP responses were observed in the previous studies: slow (< or = 2 Hz) and fast (> or = 5 Hz). In this study, some dynamic properties of the responses to high frequency flash stimulation were investigated. Very short conditioning trains and even only two stimuli were found to be sufficient for the generation of an OSP response. With longer conditioning periods, of ca. 250 ms the OSP response develops and reaches its final size and shape, similar to the results obtained from animal studies. In the topographical analysis, the steady state response (SSR) amplitudes were found to be larger in the occipital and parietal regions while the OSP responses spread homogeneously in all subjects.

Adult↗
PubMed1994

Event-related theta rhythms in cat hippocampus and prefrontal cortex during an omitted stimulus paradigm.

In a recent study on human subjects which performed a time prediction task in an omitted stimulus paradigm we found an increase in the amplitudes of evoked potentials (EPs) for the stimulus preceding the omitted stimuli, probably due to expectancy and focused attention. The amplitude increases were dominant over frontal and parietal association areas and were mainly reflected in enhancements of the theta (3-6 Hz) components. In the present study we analyse the responses of the auditory cortex (GEA, gyrus ectosylvianus anterior), different parts of hippocampus (CA1, CA3), reticular formation (RF), and motor cortex (MC) of the cat brain using a similar paradigm. Similar theta component increases of the EPs were obtained in various parts of the hippocampus, which were dominant in pyramidal cell layers (CA3, CA4), and in motor cortex. The results are interpreted as signs of a diffuse theta response system in the brain including mainly the hippocampus and frontal and parietal association areas. The diffuse theta system is probably involved in general attention and expectancy processes.

Acoustic Stimulation↗
PubMed1992

Theta rhythmicities following expected visual and auditory targets.

Evoked (EPs) as well as event-related potentials (ERPs) were recorded from two groups of 10 healthy, voluntary subjects in auditory and visual modalities. For ERP recordings 'the omitted stimulus paradigm' was employed, in which the subjects were expected to mark mentally the onset time (time prediction task) of the omitted stimulus (target). The standard auditory (AEP) and visual (VEP) evoked potentials and auditory and visual ERPs to the preceding stimuli of the omitted ones were analyzed in time and frequency domains. In the time domain the time prediction task induced increases of the amplitudes of waves existing in standard EPs; however, an additional wave or component could not be detected. Analysis of amplitude frequency characteristics (AFCs) revealed, however, selective, significant increases of the theta (3-6 Hz) frequency components of the responses concerned. These theta increases were especially evident in the frontal and parietal recording sites. Our findings suggest an association between the theta frequency components of transient evoked responses, the association areas of the brain and cognitive performance. The neurophysiological basis of scalp recorded ERPs are discussed in relation to the findings of animal studies with EEG and single unit recordings from cortical and subcortical structures.

Adult↗
PubMed1992

P300-response: possible psychophysiological correlates in delta and theta frequency channels. A review.

The present paper combines a review of event-related potentials (ERPs) with empirical data concerning the question: what are the differences between auditory evoked potentials (EPs) and two types of ERPs with respect to their frequency components? In this study auditory EPs were elicited by 1500 Hz tones. The first type of ERPs was responses to 3rd attended tones in an omitted stimulus paradigm where every 4th stimulus was omitted. The second type of ERPs was responses to rare 1600 Hz tones in an oddball paradigm. The amplitudes of delta and theta components of EPs and ERPs showed significant differences: in responses to 3rd attended tones there was a significant increase in the theta frequency band (frontal and parietal locations; 0-250 ms). In the delta frequency band there was no significant change. In contrast a diffuse delta increase occurred in oddball responses and an additional prolongation of theta oscillations was observed (late theta response: 250-500 ms). These results are discussed in the context of ERPs as induced rhythmicities. The intracranial sources of ERPs, their psychological correlates and the role of theta rhythms in the cortico-hippocampal interaction are reviewed. From these results and from the literature a working hypothesis is derived assuming that delta responses are mainly involved in signal matching, decision making and surprise, whereas theta responses are more related to focused attention and signal detection.

Adult↗
PubMed1990

Movement-related cortical potentials: their relationship to the laterality, complexity and learning of a movement.

Movement-related cortical potentials (MRCPs) to self-paced unilateral movements of different laterality, complexity and practice level were recorded from 14 healthy subjects using the EMG onset as the trigger. The amplitudes at certain time points and slopes of linear regression lines fitted to 3 main shifts have been evaluated. In the early phase of MRCP the potential and slope values were symmetrically distributed around the midline maximum, which indicates that this part of MRCP can not originate from the primary motor area, which is in a contralateral relation with the movement, but from secondary motor areas (SMA and premotor areas). The changes in the voltage levels and slopes of this phase due to the changes in laterality, complexity and practice level of the movement show the relation of this activity with the abstract characteristics of the movement. The decrease of voltages and slope values in the later phases of MRCP in the complex task, which is replaced by higher voltages and slopes after a certain learning period was evaluated as a result of inhibition of associative movements via reafferent feedback signals occurring often in the first stages of learning period.

Adult↗