Animals can use different sensory signals to localize objects in the environment. Depending on the situation, the brain either integrates information from multiple sensory sources or it chooses the modality conveying the most reliable information to direct behavior. This suggests that somehow, the brain has access to a modality-invariant representation of external space. Accordingly, neural structures encoding signals from more than one sensory modality are best suited for spatial information processing. In primates, the posterior parietal cortex (PPC) is a key structure for spatial representations. One substructure within human and macaque PPC is the ventral intraparietal area (VIP), known to represent visual, vestibular, and tactile signals. In the present study, we show for the first time that macaque area VIP neurons also respond to auditory stimulation. Interestingly, the strength of the responses to the acoustic stimuli greatly depended on the spatial location of the stimuli [i.e., most of the auditory responsive neurons had surprisingly small spatially restricted auditory receptive fields (RFs)].Given this finding, we compared the auditory RF locations with the respective visual RF locations of individual area VIP neurons. In the vast majority of neurons, the auditory and visual RFs largely overlapped. Additionally, neurons with well aligned visual and auditory receptive fields tended to encode multisensory space in a common reference frame. This suggests that area VIP constitutes a part of a neuronal circuit involved in the computation of a modality-invariant representation of external space.

In monkeys, posterior parietal and premotor cortex play an important integrative role in polymodal motion processing. In contrast, our understanding of the convergence of senses in humans is only at its beginning. To test for equivalencies between macaque and human polymodal motion processing, we used functional MRI in normals while presenting moving visual, tactile, or auditory stimuli. Increased neural activity evoked by all three stimulus modalities was found in the depth of the intraparietal sulcus (IPS), ventral premotor, and lateral inferior postcentral cortex. The observed activations strongly suggest that polymodal motion processing in humans and monkeys is supported by equivalent areas. The activations in the depth of IPS imply that this area constitutes the human equivalent of macaque area VIP.

Navigation in space requires the brain to combine information arising from different sensory modalities with the appropriate motor commands. Sensory information about self-motion in particular is provided by the visual and the vestibular system. The macaque ventral intraparietal area (VIP) has recently been shown to be involved in the processing of self-motion information provided by optical flow, to contain multimodal neurons and to receive input from areas involved in the analysis of vestibular information. By studying responses to linear vestibular, visual and bimodal stimulation we aimed at gaining more insight into the mechanisms involved in multimodal integration and self-motion processing. A large proportion of cells (77%) revealed a significant response to passive linear translation of the monkey. Of these cells, 59% encoded information about the direction of self-motion. The phase relationship between vestibular stimulation and neuronal responses covered a broad spectrum, demonstrating the complexity of the spatio-temporal pattern of vestibular information encoded by neurons in area VIP. For 53% of the direction-selective neurons the preferred directions for stimuli of both modalities were the same; they were opposite for the remaining 47% of the neurons. During bimodal stimulation the responses of neurons with opposite direction selectivity in the two modalities were determined either by the visual (53%) or the vestibular (47%) modality. These heterogeneous responses to unimodal and bimodal stimulation might be used to prevent misjudgements about self- and/or object-motion, which could be caused by relying on information of one sensory modality alone.

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