Functional neuroimaging of sound motion in the macaque dorsal stream

The macaque ventral intraparietal area (VIP), located in the fundus of the intraparietal sulcus (IPS), is considered a polymodal association area that responds to visual, tactile, vestibular and auditory stimuli (Schlack et al., 2005). In particular, VIP neurons are responsive to moving visual and auditory stimuli. VIP receives projections from multiple visual areas (especially from the middle temporal area [MT] and the medial superior temporal complex [MST]) and from auditory regions in the posterior superior temporal (pST) cortex (Lewis & Van Essen, 2000). Neurons in pST, in particular the caudolateral area (CL), show selective responses to particular sound locations regardless of sound type (Tian et al., 2001; Recanzone, 2001). In humans, several studies have reported activation of the pST and IPS to sound source motion (Warren et al., 2000; Krumbholz et al., 2005), confirming the existence of a dorsal processing stream for spatial aspects of sound in humans. In order to bridge the gap between single-unit recordings in monkeys and neuroimaging studies in humans, we used high-resolution fMRI in monkeys to further investigate these results. First, we created a virtual auditory space environment using binaural sound recording techniques with miniature microphones inserted into a macaque head cast. We validated the technique by measuring saccadic eye movements to sound sources in different locations during playback. We then performed fMRI to identify cortical areas sensitive to sound motion in azimuth of the left and right hemifields. All fMRI data were pre-processed and aligned with the 112RM-SL_T1 rhesus monkey template for identification of cortical fields (McLaren et al., 2009). Preliminary results showed that all moving sounds activated areas MT, MST and the IPS. Contrasting left and right sound-motion conditions against center (i.e. no motion) yielded greater activation in contralateral VIP. These results suggest that interaural information induced by lateralized sounds is processed along a dorsal cortical processing stream comprising VIP in the respective contralateral hemisphere