Phonological working memory with auditory presentation of pseudo-words -- an event related fMRI Study

Phonological working memory (PWM) tasks consist of a sequence of stimulus-encoding, maintenance and response. Few previous functional magnetic resonance imaging (fMRI) studies of PWM have employed event-related designs that make it possible to analyze the activations associated with each phase of such a task. The exploration of the cortical activation that takes place during the maintenance of PWM has been of particular interest to us. It has been suggested that temporary storage is served by the inferior parietal cortex. However, to the best of our knowledge earlier studies have used visual stimuli that might generate different cortical activations than auditory stimuli, which, presumably, would be more dependent on the auditory association cortex in the temporal lobe. This study involved an auditory stimulus presentation and a forced two-choice task. We used a parametric event-related design with fMRI. The stimuli consisted of pseudo-words of five, seven and nine syllables in length. In the control task, acoustically analogous stimuli without phonetic or linguistic content were used for passive listening. The left inferior frontal gyrus was activated during the stimulus-encoding, maintenance and response, in agreement with previous studies. However, in contrast to previous studies using visual presentation of verbal material, the inferior parietal cortex was not activated during the maintenance in the present study, but only active in the comparison and decision phase. Significant activation of the auditory cortex in the middle temporal gyrus was observed during the maintenance phase, which, together with the inferior frontal gyrus activation might underly the maintenance of phonological information.