Auditory Motion Does Not Modulate Spiking Activity in Visual Motion Processing Areas MT and MST

The integration of multiple sensory modalities is one of the key aspects of brain function, allowing animals to take advantage of concurrent sources of information to make more accurate perceptual judgments. For many years, it was thought that multisensory integration in the cerebral cortex only occurs in high-level “polysensory” association areas, but recent studies have demonstrated cross-modal influences in regions that were traditionally designated as unimodal. In particular, several human neuroimaging studies have reported that extrastriate areas involved in visual motion perception are also activated by auditory motion, and may integrate audio-visual motion cues. However, the exact nature and extent of the effects of auditory motion on the visual cortex have not been studied at the single neuron level. We recorded the spiking activity of neurons in the middle temporal (MT) and medial superior temporal (MST) areas of anesthetized marmoset monkeys upon presentation of unimodal stimuli (moving auditory or visual patterns), as well as bimodal stimuli (concurrent audio-visual motion). Despite robust, direction selective responses to visual motion, none of the sampled neurons responded to auditory motion stimuli. Moreover, concurrent moving auditory stimuli had no significant effect on the ability of single MT and MST neurons, or populations of simultaneously recorded neurons, to discriminate the direction of motion of visual stimuli (moving random dot patterns with varying levels of motion noise). Our findings do not support the hypothesis that direct interactions between MT, MST and areas low in the hierarchy of auditory areas underlie audiovisual motion integration. Significance Statement Many studies have demonstrated that brain regions originally thought to be unisensory may play a role in multisensory processing. For example, some neuroimaging studies have found activity in regions involved in the processing of visual motion can be modified by auditory motion. We tested whether the spiking activity of neurons in two visual motion processing areas of the primate brain, areas MT and MST, can be modulated by moving auditory stimuli. Our results revealed that neurons in these areas neither respond to auditory motion, nor change their responses to visual motion according to auditory motion along the frontoparallel plane. These findings call into question the idea that audio-visual integration occurs at early stages of processing in the extrastriate cortex.