In a nutshell: One-way connections between the parts of the brain involved in hearing and seeing help the brain to quickly react to new sensory information.

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Our senses help us perceive the world around us. The human brain takes in sensory information – such as what we hear, see, smell, taste and touch – and processes it, so that we can respond appropriately to our environment.

Until the late twentieth century, scientists believed that each type of sensory input was handled in a different part of the brain, and that the inputs were combined into a single experience of the environment only very late in the chain of processing, leading to consciousness.

This idea was overturned by a report that brain cells in the auditory cortex – the part of the brain that performs the initial processing of sounds – are connected to the primary visual cortex (V1), where visual information is processed. The brain areas seem to be sharing sensory information from the very beginning. However, the existence of these connections remained controversial, because several subsequent studies failed to find them.

Brain Function CoE investigators Marcello Rosa and Ramesh Rajan from Monash University, Piotr Majka from the Nencki Institute of Experimental Biology in Poland, and Partha Mitra from Cold Spring Harbor Laboratory in the USA conducted a systematic search for direct connections between the auditory and visual areas of the cortex in marmoset monkeys.

Along with colleagues from Japan, the researchers injected fluorescent dyes into the brain, which reveal connections even down to the level of single cells. By varying the location of the injections, they confirmed the existence of one-way connections between the auditory and visual areas of the brain: information is sent from the auditory cortex to V1, but not from V1 to the auditory cortex.

Because auditory information is sent to the brain much faster than visual information, auditory perception can influence visual perception. In addition, the researchers found that the auditory connections led only to areas of V1 that are responsible for peripheral vision. This suggests that primates use hearing to supplement their vision when trying to locate objects outside their main field of view.

Next steps:
The researchers hope to learn more about the precise functions of these connections. This knowledge could eventually be used to improve artificial intelligence, enabling machines to react more accurately to new stimuli by integrating different types of sensors.

Majka, P., Rosa, M. G. P., Bai, S., Chan, J. M., Huo, B.-X., Jermakow, N., et al. (2018). Unidirectional monosynaptic connections from auditory areas to the primary visual cortex in the marmoset monkey. Brain Structure and Function. doi: 10.1007/s00429-018-1764-4

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