628 Occipital P300 responses to parathreshold visual stimuli

June 4, 2017 | Autor: Adnan Kurt | Categoria: Psychophysiology
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628 OCCIPITAL P300 RESPONSES TO PARATHRESHOLD VISUAL STIMULI M. Devrim’, A. Ademoglu2, T. Demiralp’*, A. Kurt’ ‘Electra-Neuro-Physiology Research and Application Center, University of Istanbul, Fizyoloji AD, Capa-Istanbul, 34390 Turkey; ‘Institute of Biomedical Enginering, Bogaxici University, Istanbul, Turkey Parathreshold and suprathreshold visual ERPs and their frequency components in delta, theta and alpha frequency ranges were compared. The time-varying frequency Components were analyzed by using the Wavelet Transform (WT). The parathreshold stimulus for each subject corresponded to the stimulus intensity where the subject could detect 90% of the presented stimuli. The suprathreshold stimulus was far above the energy level, where the subjects could easily detect all the stimuli. The P300 potential constituted the major component of the parathreshold ERPs, whereas earlier components almost disappeared. The striking finding was the significant increase of the P300 amplitude in the occipital area to parathreshold stimuli compared with those to suprathreshold stimuli, whereas P300 amplitudes in central and frontal regions decreased significantly and those in parietal region remained constant. The same topographical change as for the P300 amplitude was also observed for the positive delta wave corresponding to the P300 latency range and for the theta response, whereas alpha responses to parathreshold stimuli were significantly smaller compared to those to suprathreshold stimuli in all leads. The Frequency range of the P300 wave corresponds mainly to the delta. activity which is shown to emerge by the synchronized cortical networks. Considering the presence of a dominant P300 response to the low-energy parathreshold stimuli in the occipital area, we propose a tentative mechanism on P300 generation: P300 wave may indicate a transient, widespread closure of the cortical neurons including those in the visual sensory area to afferent inputs from subcortical structures. However, if the visual inputs to primary visual area through lateral geniculate nuclei are of suprathreshold strength, they override the effect of the general closure of the cortex to subcortical inputs specifically in the primary visual area and disrupt the corticocortical interactions of the occipital cortex. Then, normal topographical pattern of the P300 response is obtained, where the primaty sensory area displays a slight P300 component whereas P300 appears dominantly on parietal and frontal association areas. This research is supported by Istanbul University Research Fond project YP8/160997.

629 EFFECTS OF SLOW SHIFTS ON P300 RESPONSE T. Ergenoglu’, T. Demiralp’*, M. Devrimi, N. Ermutlu’

CORTICAL

POTENTIAL

H. Beydagi*, S. Karamiirsel’,

of Psychophysiology

30 (1998)

95-271

239

‘Department of Physiology, Istanbul University, Istanbul Medical Faculty, Fixyoloji AD, 34390 Turkey 2Department of Physiology, Gaziantep University, Medical Faculty Effects of the negative and positive 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 SCP shifts induced significant topographical changes on P200 and P300 amplitudes. The maximal amplitudes of both waves shifted from fronto-central region during positive SCPs to centro-parietal region during negative SCPs. Hence, the P300 topography resembled a P3a like distribution during positive SCPs, whereas a typical P3b type distribution was observed during negative SCPs. The results show that the SCP shifts, which are considered to be correlated with the arousal level, are responsible at least for part of the inter-trial variability of P300 response in that they induce significant amplitude and topography changes of P300 responses.

630 MATURATION OF A’I-I’ENTIONAL AND MOTOR PREPARATION MECHANISMS. A STUDY USING THE ‘POSNER’ ATTENTION SHIFTING PARADIGM L. Garcia-Larrea* , C. Perchet UPRES EA1880, University Claude’ Bernard, Hopital Neurologique, CERMEP, 59 Bd Pinel, 69003 Lyon, France Aim of the study. To gain insight into the maturation of attentional and motor preparation mechanisms by comparing performance, reaction times and ERP measures in children and adults during the ‘Posner’ attention shifting paradigm. Subjects and Methods. 32 subjects were investigated, namely 10 children (7-9 y.), 12 young US-30 yl and 10 middle-aged adults (30-45 y). Subjects responded to left and right visual targets by pressing the corresponding (left or right) mouse button (reaction times were measured). 80% of targets were preceded by a spatial cue, either valid (same side of the target) or invalid (opposite side). ERPs to both cues and targets were obtained by offline averaging of BEG epochs. Results. Results in children are described in the companion poster (Perchet and Garcia&urea). ERPs from young and middle-aged adults were essentially identical. The most striking differences between ERPs of children and adults concerned responses to the cue stimuli: in children these evoked a first exogenous potential (Pl, 120-150 ms) followed by a N2-P3 complex (250-350 ms), while in adults the early PI (110 ms) was followed by a long lasting negative wave, the characteristics of which were consistent with a CNV and/or readiness potential (RP). By contrast, responses to the visual

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