A research team at the Institute for Basic Science (IBS) has uncovered a fundamental principle of how the brain prioritizes vision and hearing differently depending on whether we are still or in motion. The study, led by Dr. LEE Seung-Hee, Associate Director of the IBS Center for Synaptic Brain Dysfunctions and Associate Professor at KAIST, provides new insights into how movement alters the brain's sensory decision-making process.

In daily life, we constantly process visual (sight) and auditory (sound) information to navigate the world. For instance, when watching a movie, our brain seamlessly integrates images and sounds to create a complete experience. However, when moving—such as when walking on a busy street—our brain may prioritize visual information over sound.

Until now, it was unclear how the brain decides which sense to prioritize in different situations. This is particularly relevant for individuals with sensory processing disorders such as autism or schizophrenia, where the brain may struggle to integrate sensory information correctly. Understanding how the brain naturally shifts between sensory inputs could lead to better treatments for these conditions.

To investigate this phenomenon, the research team conducted behavioral experiments on mice while tracking real-time brain activity using miniature microscopes and optogenetics (a method that uses light to control neurons). The mice were trained to respond to both visual and auditory cues while running on a treadmill.

The researchers found that when the mice were stationary, their brains relied more on sound to make decisions. However, when they were moving, their brains shifted to prioritize vision.

Further analysis revealed the specific brain circuits responsible for this switch:

• - The posterior parietal cortex (PPC), a key region for decision-making, played a central role in prioritizing sensory information. When the PPC was turned off, mice could no longer make decisions based on visual cues and relied more on sound instead.
• - The secondary motor cortex (M2) acted as a "sensory gatekeeper." When the mice were moving, M2 sent inhibitory signals to the auditory cortex, blocking auditory signals from reaching the PPC. This effectively made vision the dominant sense during motion.
• - Despite this suppression, the auditory cortex continued processing sounds, meaning the brain was adjusting how it integrates sensory information rather than completely ignoring sound.

This study demonstrates that the brain does not treat all sensory inputs equally at all times—instead, it dynamically adjusts depending on movement and environmental needs. When stationary, sound is more useful for detecting nearby events, while vision takes priority during movement because it is more reliable for navigation.

This discovery could have important implications for understanding and treating sensory processing disorders, where the brain may struggle to properly filter and prioritize sensory inputs.

Dr. LEE Seung-Hee, the study’s lead researcher, emphasized the significance of the findings, stating, "Our study reveals how the brain flexibly shifts between vision and hearing based on behavior. This natural adaptability is crucial for survival, and understanding it could help us develop better treatments for individuals with sensory integration difficulties."

Figure 1. Flexible Multisensory Processing Based on Behavioral State
(A) Mice are trained to respond to external audiovisual cues by making decisions to receive a water reward while avoiding a mild air puff. When both auditory and visual stimuli are presented together, stationary mice prioritize auditory cues, whereas running mice rely more on visual information for decision-making.
(B) Calcium imaging was used to analyze neuronal activity in the posterior parietal cortex (PPC) in response to auditory, visual, and combined audiovisual stimuli.
(C) A response map showing neural activation in the PPC of a mouse during sensory stimulation. Activated neurons showed increased fluorescence (white to red). Auditory responses were notably reduced when the mouse was running compared to when it was stationary, highlighting the state-dependent nature of sensory integration.

Figure 2. Mechanism of Flexible Multisensory Processing
When the mouse is stationary, auditory signals suppress visual-processing neurons in the posterior parietal cortex (PPC), leading to auditory-dominant decision-making. However, during running, motor-related signals selectively inhibit the neural pathway transmitting auditory information to the PPC. This reduces auditory interference and enhances visual processing, resulting in visual-dominant decision-making. This dynamic mechanism allows the brain to flexibly integrate audiovisual sensory information, adapting to the behavioral state by prioritizing either auditory or visual cues as needed.