Top-down task effects overrule automatic multisensory responses to letter-sound pairs in auditory association cortex

Nienke van Atteveldt, Maastricht University, Faculty of Psychology, Dept. of Cognitive Neuroscience

Abstract
In alphabetic scripts, letters and speech sounds are the basic elements of correspondence between spoken and written language. In two previous fMRI studies using blocked stimulus presentation and passive perception, we found a cross-modal modulation of the response to speech sounds in the auditory association cortex by letters, expressed as response enhancement by congruent letters and suppression by incongruent letters. Interestingly, temporal proximity was critical for this congruency effect to occur. In the present study, we used fMRI to investigate the effect of stimulus presentation mode (blocked vs. event-related) and task instruction (passive perception vs. active matching) on the neural integration of letters and speech sounds. The principle findings are 1) a replication of the previous results on passive integration using event-related fMRI, and 2) the absence of the effects of congruency and temporal proximity in the auditory association cortex during active matching. Finding 1 shows the suitability of event-related fMRI for studying letter-sound integration. Finding 2 indicates that the task demands overruled the automatic multisensory responses to letters and speech sounds, most likely because the task changed the behavioral relevance of the stimuli.

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