Brain research. Cognitive brain research最新文献

The present study demonstrated that the mismatch negativity (MMN), generated by the brain's preattentive detection of a sound change, is elicited by infrequent reversals of two consecutive tones differing in intensity. When tones were presented in a pairwise manner, the MMN was time-locked to the onset of the intensity reversal. When the tones were continuously presented, the MMN was elicited by an irregular loud tone succeeding a regular loud tone but not by an irregular soft tone following a regular soft tone. Results suggest that the preattentive construction of auditory units is primarily governed by the timing of tone presentation but that it may also utilize intensity information, when no sufficient timing information is available.

Event-related oscillatory brain activity during language perception differs from activity occurring during the processing of comparable non-language stimuli. This fact became apparent in the observation of changes in the normalized spectral power of magnetoencephalographic (MEG) signals during the subject's processing of these stimuli. MEG was recorded over the left and right hemispheres of 12 right-handed subjects. During the experimental session, bisyllablic content words and physically similar non-language stimuli were presented with equal probability in a randomized order in either the visual or auditory modality. Approximately 15% of these stimuli were marked and the subject's task was to detect these marked stimuli. As a major characteristic of language vs. non-language processing, we obtained an enhancement of the normalized spectral power around 240 ins in the 60-65-Hz band over the left hemisphere for the language condition and over the right hemisphere for the non-language condition, independent of the modality of stimulus presentation. Starting at approximately the same latency but in lower-frequency bands (15-45-Hz), an extended (250-600 ms) reduction of normalized spectral power was observed. This reduction, although it generally confirmed previous results, differed in the no hemisphere-specific reduction was found for the processing of words. A domain-specific enhancement of normalized spectral power was also evident around 800-1200 ms in the 15-30-Hz band. In the auditory condition, this enhancement of the normalized spectral power was larger after the presentation of language stimuli whereas in the visual condition a larger enhancement of the normalized spectral power was obtained after presentation of non-language stimuli. As this latter effect appears relatively late after the stimulus onset and differs in expression for both modalities of stimulus presentation, a simple relationship between language perception and oscillatory brain dynamics can be excluded for this enhancement. In contrast, the left hemispheric enhancement of the normalized spectral power present around 240 ms in the 60-65-Hz band seems to reflect oscillatory pattern specific to the processing of words.

The objective of the present study was to test if and to what extent phasic and tonic event-related potentials of the human EEG may reflect phenomena of cortical plasticity. In particular, it was tested if the occipital cortex of blind subjects participates in the processing of non-visual stimuli. To this end, 12 blind and 12 blindfolded sighted subjects were tested in an auditory and a somatosensory discrimination task with 2 levels of discrimination difficulty. Slow and fast event-related potentials were recorded from 18 scalp electrodes. In addition to the negative slow waves found in sighted subjects over frontal and central sites during auditory and somatosensory discrimination, a pronounced negative wave was revealed in the blind also over occipital brain areas. These negative shifts were time-locked to the train of stimuli which had to be monitored with sustained attention, i.e. they rised and resolved with the beginning and the end of a 20-s discrimination time epoch. The P300 complex, on the other hand, which is a slow positive deflection over the posterior part of the scalp and which follows rare and task-relevant events 200-800 ms after stimulus onset was significantly smaller at occipital electrodes in the blind than in the sighted subjects. Combined with neurophysiological and neuronanatomical evidence originating from studies with visually deprived animals, these data suggest that the occipital cortex of blind human subjects is coactivated whenever the system is engaged in a task which requires sustained attention and is less effectively inhibited at the end of a perceptual time epoch. In total, the data cast doubt on the hypothesis that the occipital cortex of blind subjects participates in modality-specific non-visual information processing.

Age-related changes in semantic context effects were examined using late event-related brain potentials (ERPs). Auditory ERPs to semantically congruous and incongruous final words in spoken sentences were recorded in 16 children (aged 5-11 years) and 16 adults. Previous findings concerning age-related effects on N400 were replicated: the N400 effect was significantly larger in children than in adults. The main new finding was that a late positive component (LPC) following N400 and modulated by semantic context in adults was not found in children. Thus, the common generalization that semantic context effects decline with age holds only for ERP components occurring in the N400 time window or earlier. The cognitive function reflected by the semantic LPC we observed is not clear, but it seems to have a role different from that of the N400, although in adults the components often co-exist as an N400-LPC complex.