Background: Controversy still remains on whether phonological dyslexia underlies a general auditory process- ing defect or is a speech specific defect. Aims: To compare processing of non-verbal auditory stimuli by dyslexic and non-dyslexic children. Patient and methods: Thirty-nine individuals (17 dyslex- ics children plus 22 normal controls) were studied and the power values of the Haar wavelet transform (WT) were cal- culated for signals at Fz, Cz, Pz, T5 and T6 elicited as stan- dard and target responses in the P300 auditory oddball paradigm. Results: The mean power of the wavelet-transformed 40- Hz oscillation at 25 ms after stimulus onset (either 1000-Hz standard tone or 2000-Hz target tone, thought to be a per- ceptual time event, is significantly different in dyslexia as compared with controls. Correlation of values obtained at each derivation point (Fz, Cz, Pz, T5 and T6) was r = 0.88 in controls; no significant correlation (r = À0.27) was found in dyslexics. Discussion: Three parameters of the WT, namely mean power, correlation and standard/target ratio (S/T) are con- stantly and significantly different (p < 0.05) to normal controls. Conclusions: Significant differences of the mean power of 40-Hz oscillations detected at very early stages of auditory processing and the S/T differences in dyslexic children, as well as dispersion of the WT mean values, reveals a defec- tive processing of non-verbal sounds in dyslexia. Our results also support recent findings using both behavioral and electrophysiological methods that interpret dyslexia as a general auditory deficit instead of a speech specific deficit. doi:10.1016/j.clinph.2006.06.414 P21.3 Transcranial direct current stimulation applied over the somatosensory cortex – Differential effect on low and high frequency SEPs R. Gobbele ´ 1 , A. Dieckho ¨fer 1 , M. Nitsche 2 , W. Paulus 2 , H. Buchner 3 , T.D. Waberski 1 1 University Hospital Aachen, Department of Neurology, Germany 2 University Hospital Go ¨ ttingen, Department of Clinical Neurophysiology, Germany 3 Knappschaftskrankenhaus Recklinghausen, Department of Neurology, Germany Background: Transcranial direct current stimulation (tDCS) has an influence on the excitability of the human motor cortex measured by the amplitude of motor evoked potentials (MEPs) after transcranial magnetic stimulation. Objectives: High and low frequency components of somatosensory evoked potentials (SEPs) were studied questioning whether a comparable effect can be observed after applying tDCS to the human somatosensory cortex. Subjects and methods: Multichannel (96-electrodes) median nerve SEPs were recorded in ten healthy volun- teers before and after applying tDCS of 1 mA over a period of 9 min with the cathode placed over the somatosensory cortex and the anode over the contralat- eral forehead and vice versa in a second session. Recording time of the SEPs after tDCS was 1 h. The maximum amplitude of the source activity of the N20 and N30 components was evaluated before and after application of tDCS. The high frequency oscillations (HFOs) of the SEPs were extracted by application of a digital high-pass filter to the data and equally analysed. Results: After application of cathodal tDCS to the somatosensory cortex we found a significant reduction of the N20 source amplitude while there was no effect after anodal stimulation. For the N30 and the HFOs no change in source activity was observed. Discussion: Corresponding to the results for the motor cortex a sustained reduction of the excitability of the somatosensory cortex after cathodal tDCS could be shown. Furthermore we demonstrated differential effects on the high and low frequency components of SEPs confirming the hypothesis of locally and functionally distinct genera- tors of these two components. doi:10.1016/j.clinph.2006.06.415 P21.4 Gamma EEG in the evaluation of persistent vegetative state S. Balazs 1 , I.G. Bodis-Wollner 2 , H. Binder 1 1 Sozial-medizinisches Zentrum, Otto Wagner Spital, Neurological Center, Austria 2 Downstate Medical Center, State University of New York, Brooklyn, Department of Neurology and Department of Ophthalmology, USA Background: Power in the c band EEG increases during saccades in normal subjects. The only motor activity which occurs with some regularity in patients in persistent vega- tive state (PVS), are slow ballistic eye movements (SBEM). Objectives: To quantify early signs of cortical respon- siveness in PVS we studied c range EEG (40 Hz) in associ- ation with SBEM. Methods: The EEG and the simultaneous electro-oculo- gram were recorded in 14 PVS patients. Clinical scoring was based on the Glasgow-Coma-Scale and Coma- Remission-Scale. The Wavelet transform followed by Hilbert transform was applied to the EEG and c power distribution was quantified relative to the timing of an eye movement. We correlated the clinical and the neuro- physiological measures. Results: c Activity was present in all PVS patients. Its power was modulated in association with eye movements only in less severely affected patients, with minimum power Posters / Clinical Neurophysiology 117 (2006) S121–S336 S217