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Abstract:
Functional magnetic resonance imaging (fMRI) with non-human primates is invaluable because localized patterns of activity can guide subsequent neurophysiological recordings. However, it is unknown whether fMRI of the macaque monkey can reveal reliable auditory activations consistent with known properties of primate auditory cortical fields (ACFs). We used high-field (4.7- and 7-Tesla) fMRI to image the blood-oxygen level dependent response (BOLD) of auditory cortex in awake and in anesthetized macaques. For awake-animal imaging we trained a macaque to complete long duration trials of visual fixation in combination with minimal body movement. Scanning this animal at 7T during sound presentation revealed robust activity over auditory cortex in the superior temporal plane. A paradigm where stimulation alternated with image acquisition revealed greater auditory activity than continuous imaging where sound stimulation must compete with the scanner noise. Imaging data with more extensive sound stimulation was obtained from anesthetized animals since these experiments allow for quicker data acquisition. Here, we used sounds varying in center frequency and bandwidth as have neurophysiological experiments mapping the basic organizational properties of macaque ACFs. In the antero-posterior direction, regions within the lateral sulcus were selective for sounds with low and high center frequencies, revealing expected frequency selective gradients (tonotopy) with multiple mirror reversals of these gradients. In comparison to tonal stimulation, sounds with greater spectral bandwidth activated more lateral and medial portions of the superior temporal plane, consistent with this activity occurring over non-primary ACFs. In summary, high-field fMRI reveals the global organization of macaque auditory cortex and will be important for helping us to understand how the primate auditory cortex is functionally organized.
