Speech perception and its temporal dynamic

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13, Number

6, 2001,

Part 2 of 2 Parts 1 D E tl@

LANGUAGE

Speech perception and its temporal dynamic Karsten Specht*t,

N. Jon Shah+, Lutz JIncket

*Medicine Centre Bonn, Germany tlnstitute of Gemeral Psychology, Otto-von-Guericke

University Magdeburg, Germany

Slnstitute of Medicine, Research Centre Jiilich, Germany Introduction In this fMRI study, we investigated in six subjects the auditory perception of speech, using a single session event-related paradigm, containing the following conditions (beside null events): 1. passive perception of nonwords, i.e. one- and two syllable words, played backwards, 2. internal repetition of one- and two syllable words and 3. internal naming after definitions. All stimuli were presented aurally via shielded headphones. According to a previous, blocked design study, we expected different levels of speech perception and different temporal dynamics within the temporal and prefrontal, language related areas.

We found bilateral activations of the primary and secondary auditory cortex, with varying significance levels, according to the complexity of the stimuli. The smallest activations were found during passive perception of nonwords. This activation pattern extended bilaterally into the inferior frontal gyms during the internal repetition of real words. The naming task resulted in an activation pattern, involving also the left and right primary and secondary auditory cortex and a larger extension into the left inferior frontal gyms, including Broca’s area and to more posterior regions of the left superior temporal lobe, including Wemicke’s area. Analysing the temporal dynamics, using the time derivative basic function, we found in the comparison between the nonwords and repetition task, the main difference of the temporal dynamic in the left superior temporal gyms (BA 22), where the activation was longer and stronger, as in the passive perception task. Comparing the dynamic of the HRF (hemodynamic response function) in the naming and repetition task, we found again a significant difference in the left superior temporal gyms and in the left inferior frontal gyms (Broca), as well as a small region in the medial part of the temporal transverse gyms of the right hemisphere. Comparing the two active tasks, i.e. naming and repetition, we found significant differences within the left primary and secondary auditory cortex and in the left inferior frontal gyms (Broca)

Discussion One of the main findings of this study is the detection of laterality, induced by the different presentation length. The presentation of the definitions were two times longer than the nonwords or realwords. Nevertheless, this temporal extended auditory presentation leads to a temporal change of the BOLD response only in the left auditory cortex, whereas the auditory cortex of the right hemisphere activated in the same way, like in the perception of nonwords or realwords. The stronger activation of BA 22 in the repetition against the passive nonword task is in a good agreement to this. This study demonstrated, that hemispheric dominance for speech perception can also be detected by analysing the temporal behaviour of the evoked hemodynamic responses.

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