The effect of listening tasks and motor responding on activation in auditory cortex

Previous human functional magnetic resonance imaging (fMRI) research has shown that activation in the auditory cortex (AC) is strongly modulated by motor influences. Other fMRI studies have indicated that the AC is also modulated by attention-engaging listening tasks. How these motor- and task-related activation modulations relate to each other has, however, not been previously studied. The current understanding of the functional organization of the human AC is strongly based on primate models. However, some authors have recently questioned the correspondence between the monkey and human cognitive systems, and whether the monkey AC can be used as a model for the human AC. Further, it is unknown whether active listening modulates activations similarly in the human and nonhuman primate AC. Thus, non-human primate fMRI studies are important. Yet, such fMRI studies have been previously impeded by the difficulty in teaching tasks to non-human primates. The present thesis consists of three studies in which fMRI was used both to investigate the relationship between the effects related to active listening and motor responding in the human AC and to investigate task-related activation modulations in the monkey AC. Study I investigated the effect of manual responding on activation in the human AC during auditory and visual tasks, whereas Study II focused on the question whether auditory-motor effects interact with those related to active listening tasks in the AC and adjacent regions. In Study III, a novel paradigm was developed and used during fMRI to investigate auditory task-dependent modulations in the monkey AC. The results of Study I showed that activation in the AC in humans is strongly suppressed when subjects respond to targets using precision or power grips during both visual and auditory tasks. AC activation was also modulated by grip type during the auditory task but not during the visual task (with identical stimuli and motor responses). These manual-motor effects were distinct from general attention-related modulations revealed by comparing activation during auditory and visual tasks. Study II showed that activation in widespread regions in the AC and inferior parietal lobule (IPL) depends on whether subjects respond to target vowel pairs using vocal or manual responses. Furthermore, activation in the posterior AC and the IPL depends on whether subjects respond by overtly repeating the last vowel of a target pair or by producing a given response vowel. Discrimination tasks activated superior temporal gyrus (STG) regions more strongly than 2-back tasks, while the IPL was activated more strongly by 2-back tasks. These task-related (discrimination vs. 2-back) modulations were distinct from the response type effects in the AC. However, task and motor-response-type effects interacted in the IPL. Together the results of Studies I and II support the view that operations in the AC are shaped by its connections with motor cortical regions and that regions in the posterior AC are important in auditory-motor integration. Furthermore, these studies also suggest that the task, motor-response-type and vocal-response-type effects are caused by independent mechanisms in the AC. In Study III, a novel reward-cue paradigm was developed to teach macaque monkeys to perform an auditory task. Using this paradigm monkeys learned to perform an auditory task in a few weeks, whereas in previous studies auditory task training has required months or years of training. This new paradigm was then used during fMRI to measure activation in the monkey AC during active auditory task performance. The results showed that activation in the monkey AC is modulated during this task in a similar way as previously seen in human auditory attention studies. The findings of Study III provide an important step in bridging the gap between human and animal studies of the AC.