Abstract
BackgroundHow does the brain convert sounds and phonemes into comprehensible speech? In the present magnetoencephalographic study we examined the hypothesis that the coherence of electromagnetic oscillatory activity within and across brain areas indicates neurophysiological processes linked to speech comprehension.ResultsAmplitude-modulated (sinusoidal 41.5 Hz) auditory verbal and nonverbal stimuli served to drive steady-state oscillations in neural networks involved in speech comprehension. Stimuli were presented to 12 subjects in the following conditions (a) an incomprehensible string of words, (b) the same string of words after being introduced as a comprehensible sentence by proper articulation, and (c) nonverbal stimulations that included a 600-Hz tone, a scale, and a melody. Coherence, defined as correlated activation of magnetic steady state fields across brain areas and measured as simultaneous activation of current dipoles in source space (Minimum-Norm-Estimates), increased within left- temporal-posterior areas when the sound string was perceived as a comprehensible sentence. Intra-hemispheric coherence was larger within the left than the right hemisphere for the sentence (condition (b) relative to all other conditions), and tended to be larger within the right than the left hemisphere for nonverbal stimuli (condition (c), tone and melody relative to the other conditions), leading to a more pronounced hemispheric asymmetry for nonverbal than verbal material.ConclusionsWe conclude that coherent neuronal network activity may index encoding of verbal information on the sentence level and can be used as a tool to investigate auditory speech comprehension.
Highlights
How does the brain convert sounds and phonemes into comprehensible speech? In the present magnetoencephalographic study we examined the hypothesis that the coherence of electromagnetic oscillatory activity within and across brain areas indicates neurophysiological processes linked to speech comprehension
The present study investigated coherence patterns of the auditory evoked magnetic Steady-State-Field (SSF), coherence among SSF-generators within and across hemispheres, as a measure of neural networks involved in speech comprehension
The present study studied co-activated cortical networks involved in speech comprehension by using auditory steady-state (41.5-Hz amplitude modulated) stimuli and measuring the coherence of generator activity of the magnetic steady state response
Summary
How does the brain convert sounds and phonemes into comprehensible speech? In the present magnetoencephalographic study we examined the hypothesis that the coherence of electromagnetic oscillatory activity within and across brain areas indicates neurophysiological processes linked to speech comprehension. One key function of the cerebral cortex involves the integration of elements into a percept that separates them from the background In this process, changes in cortical networks are formed and modified by experience through the simultaneous excitation of groups of neurons [1,2,3]. Changes in cortical networks are formed and modified by experience through the simultaneous excitation of groups of neurons [1,2,3] These "long-range connections formed by excitatory cortical neurons" [[4] p.3] are considered the anatomical substrate of this integrative capability. BMC Neuroscience 2004, 5:40 http://www.biomedcentral.com/1471-2202/5/40 similar principles should describe other sensory functions such as auditory speech perception and comprehension This assumption was tested in the present study by probing patterns of co-activation within and across hemispheres during the processing of verbal and nonverbal acoustic material. Coherence is defined as the correlated activity between two locations within a distinct frequency range
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