Functional imaging reveals visual modulation of specific fields in auditory cortex

Merging information from different senses is essential for successfully interacting with real world situations. Indeed, sensory integration can reduce perceptual ambiguity, speed reactions or change the qualitative sensory experience. It is widely held that integration occurs at later processing stages and mostly in higher association cortices. However, recent studies suggest that sensory convergence can already occur in primary sensory cortex. A good model for early convergence proved to be the auditory cortex, which can be modulated by visual and tactile stimulation. However, given the large number and small size of auditory fields, neither microelectrode recordings, nor human imaging have systematically identified which fields are susceptible to multisensory influences. To reconcile findings from human imaging with anatomical knowledge from non-human primates, we exploited high-resolution imaging (fMRI) of the macaque monkey to study the modulation of auditory processing by visual stimulation. Using a func tional parcellation of auditory cortex, we localized modulations to individual fields. Our results demonstrate that both primary (core) and non-primary auditory fields (belt) can be activated by mere presentation of visual scenes. Audio-visual convergence was restricted to caudal fields (prominently core field: A1, and belt fields CM, CL and MM) and continued in the auditory parabelt and the superior temporal sulcus. The same fields exhibited enhancement of auditory activation by visual stimulation and showed stronger enhancement for less effective stimuli, two characteristics of sensory integration. Altogether, these findings reveal multisensory modulation of auditory processing prominently in caudal fields but also at the lowest stages of auditory cortical processing