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要旨:
Sound localization is one of the most fundamental tasks performed by the auditory system. In mammals, the location of a sound source in azimuth is mainly determined by interaural time and intensity differences between sounds reaching the two ears. Although binaural sound processing in subcortical structures is well understood, much less is known about the representation of space at the cortical level. In humans, the left auditory cortex (AC) shows a predominant response to sounds in the right hemifield, while the right AC responds to sounds in both hemifields (Krumbholz et al., 2007), with contrast between the two hemifields revealing activation along the dorsal stream into parietal cortex. In the monkey, selectivity of neurons in primary AC for positions in contralateral space has been observed, albeit with broad spatial tuning (Middlebrooks et al., 1994). Spatial tuning sharpens significantly in the caudal belt regions (Tian et al., 2001; Recanzone Beckerman, 2004), but it is not known whether the preferred azimuth positions form a map of auditory space. Here we attempt to bridge studies across human and nonhuman primates by obtaining a comprehensive overview of the cortical representation of azimuth space in the monkey for the first time using phase-mapping functional magnetic resonance imaging (fMRI).
Sounds were generated in virtual acoustic space and played back via headphones during fMRI. Stimuli consisted of broad-band noise bursts (0.2-16 kHz, 100 ms duration) moving through azimuth in steps of 30° at a rate of 5° per second. They were presented in a sparse-sampling design as a moving wave analogous to methods used in visual field mapping (Wandell Winawer, 2011). We acquired high-resolution images oriented along the superior temporal plane in two anesthetized monkeys. We then analyzed the BOLD signal amplitude modulation at the frequency of stimulus presentation (12 cycles per scan) to determine voxel coherence and phase values corresponding to the stimulus cycle.
In accordance with prior single-unit studies, a robust contralateral response to azimuth position was observed. The left AC represented mainly the anterior contralateral quadrant, including straight-ahead positions, while the right AC represented both ipsilateral and contralateral space. This hemispheric bias supports previous neuroimaging studies in humans. In addition, it may elucidate the hierarchical processing of space from AC into posterior parietal cortex and the sound localization deficits observed in humans with damage to the right temporo-parietal cortex (Spierer et al., 2009, Rauschecker Tian, 2000).
