Biomedical subjects

Tamer Demiralp

Publications and source records attributed to Tamer Demiralp.

8 recordsLinked to original sources

PubMed2007

The effects of interstimulus interval on sensory gating and on preattentive auditory memory in the oddball paradigm. Can magnitude of the sensory gating affect preattentive auditory comparison process?

P50, and mismatch negativity (MMN) are components of event-related potentials (ERP) reflecting sensory gating and preattentive auditory memory, respectively. Interstimulus interval (ISI) is an important determinant of the amplitudes of these components and N1. In the present study the interrelation between stimulus gating and preattentive auditory sensory memory were investigated as a function of ISI in 1.5, 2.5 and 3.5s in 15 healthy volunteered participants. ISI factor affected the N1 peak amplitude significantly. MMN amplitude in 2.5s ISI was significantly smaller compared to 1.5 and 3.5s ISI. ISI X stimuli interaction on P50 amplitude was statistically significant. P50 amplitudes to deviant stimuli in 2.5s ISI were larger than the P50 amplitudes in other ISIs. P50 difference (P50d) waveform amplitude correlated significantly with MMN amplitude. The results suggest that: (i) auditory sensory gating could affect preattentive auditory sensory memory by supplying input to the comparator mechanism; (ii) 2.5s ISI is important in displaying the sensory gating and preattentive auditory sensory memory relation.

Acoustic Stimulation↗

PubMed2006

Gamma amplitudes are coupled to theta phase in human EEG during visual perception.

Human subjects typically keep about seven items (plus or minus two) in short-term memory (STM). A theoretical neuronal model has been proposed to explain this phenomenon with physiological parameters of brain oscillations in the gamma and theta frequency range, i.e., roughly 30-80 and 4-8 Hz, respectively. In that model, STM capacity equals the number of gamma cycles (e.g., 25 ms for 40 Hz), which fit into one theta cycle (e.g., 166 ms for 6 Hz). The model is based on two assumptions: (1) theta activity should modulate gamma activity; and (2) the theta/gamma ratio should correlate with human STM capacity. The first assumption is supported by electrophysiological data showing that the amplitude of gamma oscillations is modulated by the phase of theta activity. However, so far, this has only been demonstrated for intracranial recordings. We analyzed human event-related EEG oscillations recorded in a memory experiment in which 13 subjects perceived known and unknown visual stimuli. The paradigm revealed event-related oscillations in the gamma range, which depended significantly on the phase of simultaneous theta activity. Our data are the first scalp-recorded human EEG recordings revealing a relationship between the gamma amplitude and the phase of theta oscillations, supporting the first assumption of the above-mentioned theory. Interestingly, the involved frequencies revealed a 7:1 ratio. However, this ratio does not necessarily determine human STM capacity. Since such a correlation was not explicitly tested in our paradigm, our data are not conclusive about the second assumption. Instead of theta phase modulating gamma amplitude, it is also conceivable that focal gamma activity needs to be downsampled to theta activity, before it can interact with more distant brain regions.

Adult↗

PubMed2006

Comparative analysis of event-related potentials during Go/NoGo and CPT: decomposition of electrophysiological markers of response inhibition and sustained attention.

Neuropsychological tests target specific cognitive functions; however, numerous cognitive subcomponents are involved in each test. The aim of this study was to decompose the components of two frontal executive function tests, Go/NoGo (GNG) and cued continuous performance task (CPT), by analyzing event-related potentials (ERPs) of 24 subjects both in time and time-frequency domains. In the time domain, P1, N1, P2, N2 and P3 peak amplitudes and latencies and mean amplitudes of 100 ms time windows of the post-P3 time period were measured. For GNG, the N1 amplitude and for both GNG and CPT N2 amplitudes were significantly higher in the NoGo condition compared with the Go condition. P3 had a central maximum in the NoGo conditions of both paradigms in contrast to a parietal maximum in the Go conditions. All peaks except P1 and mean amplitudes of the post-P3 period were more positive in CPT compared to those of GNG. N1, N2 and P3 latencies were longer for the NoGo condition than the Go condition in the CPT. In time-frequency analyses, the NoGo condition evoked higher theta coefficients than the Go condition, whereas the CPT and GNG paradigms differed mainly in the delta band. These results suggest that theta component reflects response inhibition in both GNG and CPT, whereas delta component reflects the more demanding sustained attention requirement of the CPT. The latency prolongation observed with the NoGo condition of the CPT paradigm was thought to be due to perseverance/inhibition conflict enhanced by the primer stimuli in CPT.

Adult↗

PubMed2006

DRD4 and DAT1 polymorphisms modulate human gamma band responses.

Gamma oscillations (30-80 Hz) have been demonstrated to be important for perceptual and cognitive processes. Animal and in vitro studies have revealed possible underlying generation mechanisms of the gamma rhythm. However, little is known about the neurochemical modulation of these oscillations during human cognition. Schizophrenia and Attention Deficit Hyperactivity Disorder, which lead to failure of attentional modulation and working memory, introduce significant changes in gamma responses and have significant associations with genetic polymorphisms of dopamine receptor D4 (DRD4), dopamine transporter (DAT), and catechol-O-methyltransferase (COMT). Therefore, the presence of direct relations between these polymorphisms and gamma oscillations was investigated in human subjects using an auditory target detection paradigm. The 7-repeat isoform of the DRD4 polymorphism that produces a subsensitive variant of the D4 receptor enhanced the auditory evoked and induced gamma responses to both standard and target stimuli. The 10/10 genotype of the DAT1 polymorphism, which reduces DAT expression and hence yields an increase in extracellular dopamine, specifically enhanced evoked gamma responses to target stimuli. The COMT polymorphism did not significantly change gamma responses. It seems plausible to assume that the modulation pattern of the evoked gamma response by DRD4 polymorphism relates to reduced inhibition via the D4 receptor, whereas the DAT1 effect is related to the target detection mechanism probably mediated by the D1 receptor.

Adaptation, Physiological↗

PubMed2004

Alpha rhythm of the EEG modulates visual detection performance in humans.

The effects of the changes in the frequency spectrum of the electroencephalogram (EEG) on the perception of near-threshold visual stimuli and on the event-related potentials (ERPs) produced by these stimuli were investigated on 12 healthy volunteers. The stimulus intensity, at which each subject could detect 50% of the presented stimuli, was defined as the sensory threshold for that subject. Single ERP trials were separated into two groups: trials with detected and undetected stimuli. The ERPs and the average power spectra of the 1 s prestimulus periods were computed for both conditions. P300 amplitudes of the ERPs, and total power and relative band powers of the delta (0.5-4 Hz), theta (4-7.5 Hz), alpha (7.5-13 Hz), beta (13-30 Hz), and gamma (30-70 Hz) frequency bands of the prestimulus power spectra were measured. Between the two conditions, a specific difference was observed in the relative power of the alpha band, which was significantly lower before detected stimuli (p < 0.01) in line with significantly higher amplitudes of the ERPs (p < 0.001). These results show that short-lasting changes in brain's excitability state are reflected the relative alpha power of the EEG, which may explain significant variability in perceptual processes and ERP generation especially at boundary conditions such as sensory threshold.

Adult↗

PubMed2002

N2 and P3 components of event-related potential in first-episode schizophrenic patients: scalp topography, medication, and latency effects.

Auditory N2 and P3 components of event-related potentials were assessed in first-episode schizophrenic and normal control subjects (n=12/group). P3 amplitude was decreased in the patients most prominently over the frontal areas in contrast to a widespread P3 amplitude decrease reported in chronic schizophrenia. Moreover, frontal attenuation of P3 amplitude was greater in the non-medicated compared with medicated patients, a finding that suggests frontal areas are primarily affected at the onset of the first schizophrenic episode. Prolongation of N2 and P3 latencies was also observed in the patients, which indicates that stimulus classification and memory updating processes were slowed even in early stages of schizophrenia. These findings indicate that first-episode schizophrenic patients produce N2 and P3 abnormalities that are distinct from those in chronic patients, and that psychotropic medication can attenuate event-related potential effects in specific ways.

Acoustic Stimulation↗

PubMed2002

Cognitive impairment in amyotrophic lateral sclerosis: evidence from neuropsychological investigation and event-related potentials.

The presence of subclinical cognitive impairment in patients with amyotrophic lateral sclerosis (ALS) is investigated using neuropsychological assessment and event-related potential recordings (ERP). An extensive battery of neuropsychological tests assessing the domains of attention, memory, language, visuo-spatial and executive functions were administered to 20 non-demented patients with sporadic ALS and 13 age- and education-matched healthy control subjects. Mismatch negativity (MMN), P3b, P3a (novelty P300) and contingent negative variation (CNV) were recorded. ALS patients were significantly impaired in tests of working memory, sustained attention, response inhibition, naming, verbal fluency and complex visuo-spatial processing. The memory impairment seemed to be secondary to deficits in forming learning strategies and retrieval. In ERP recordings, P3a and P3b amplitudes of ALS patients were lower compared with the controls, P3a latencies were significantly longer and mean CNV amplitudes were higher. These results indicate subclinical impairment of cognitive functions in patients with ALS. The pattern of cognitive impairment suggests the dysfunction of the frontal network.

Adult↗

PubMedDate not supplied

Evaluation of the functions of the parvocellular and magnocellular pathways in strabismic amblyopia.

PURPOSE: To evaluate the functions of the magnocellular and parvocellular pathways in strabismic amblyopia. METHODS: Visual evoked potentials produced by achromatic and chromatic stimuli in 14 children with strabismic amblyopia were compared with those in 14 age-matched control subjects. The achromatic sine-wave grating with low spatial frequency was used to stimulate the magnocellular pathway, whereas the parvocellular pathway was stimulated by the chromatic sine-wave grating (isoluminant red-green) with low and high spatial frequencies. RESULTS: When the achromatic stimuli with low spatial frequency were used, the latencies of the N1 and P1 components and the amplitude of the N1-P1 complex were similar in the children with strabismic amblyopia and the control subjects. There also were no differences between the amblyopic and non-amblyopic eyes of the children with strabismic amblyopia. When chromatic stimuli with low and high spatial frequencies were used, the latencies of the N1 and P1 components were significantly longer and the amplitude of the N1-P1 complex was significantly smaller in the children with strabismic amblyopia than in the control group. When the amblyopic and non-amblyopic eyes of the children with strabismic amblyopia were compared, the latencies of the N1 and P1 components were longer and the amplitude of the N1-P1 complex was smaller in the amblyopic eyes. CONCLUSION: In strabismic amblyopia, the cells in the parvocellular pathway may be more involved, whereas the magnocellular pathway may be relatively spared.

Amblyopia↗