Summary: When we lie down, the brain shifts away from relying on external sensory cues and leans more on internal, body-centered perceptions such as touch.
Source: The Conversation
You’re annoyed by the sound of a mosquito buzzing near your ear. The buzzing stops. You feel a tiny prick, you sense where it landed, and your hand moves to swat it. Done.
That sequence feels simple, but it involves intricate neural processing. How did you pinpoint the insect before you saw it? Our skin covers roughly two square metres, yet without looking you were able to locate the precise contact point. After checking with your eyes, your hand reached the right place and you didn’t hurt yourself in the process.
Despite enormous advances across the sciences, the precise mechanisms that allow perception and thought to arise are still not fully understood. Even basic categories, like the list of human senses, remain debated. Beyond the traditional five senses, many researchers include balance — the system that helps us orient in space — as a fundamental sense.
Balance while lying down
My colleagues and I at McMaster University have recently identified a subtle but meaningful change in how perception works when people lie on their sides. The effect gives insight into how balance contributes to the way we interpret sensory information.
Specifically, when people lie on their side the brain appears to reduce its reliance on information tied to the external world and instead increase the weight given to internal signals generated by the body, particularly touch and pressure. In other words, the brain shifts from an outside-in reference frame toward a body-centered reference frame.
Related research shows that when people cross their arms, they have more difficulty deciding whether a touch was felt first on the left or right hand. Interestingly, blindfolding can improve performance on that task: removing visual reference to the external world allows the body-centered representation to dominate, making judgment about touch timing easier in some conditions.
In our study we found a similar pattern: with hands crossed, participants performed markedly better at localizing touch when they were lying on their sides than when they were upright. Lying on the side reduced the influence of the external spatial reference and allowed participants to rely more on internal, body-centered cues.
Although this difference between upright and side-lying posture may not change everyday behavior directly, it reveals how flexible and context-dependent perceptual processing can be. The finding helps us understand how the brain chooses which reference frame to use and suggests potential links to other domains, including sleep and clinical assessment.
Blindfold experiments
We tested sighted participants who were sometimes blindfolded, asking them to report which hand received a brief touch first while their hands were either crossed or uncrossed. Our lab has run similar experiments for about two decades, and this manipulation built on that work.
Across conditions the results were consistent with previous findings: crossing the hands typically worsened performance when people were upright. The novel and surprising result in this study was that when participants lay on their side, crossing their hands no longer produced the same difficulty — localization improved substantially. Like blindfolding, lying on the side appears to weaken the external reference frame and allow body-centered signals to guide perception.
We reported this comparison between upright and side-lying tasks in Scientific Reports. The pattern of results led us to hypothesize that the brain may intentionally downregulate the most dominant orientation signals coming from the vestibular system when we lie down, which could be a way to prepare for sleep or to reorient attention inward.
Environmental awareness
These findings raise broader questions about the vestibular system’s role in shaping multisensory perception. In the inner ear we carry fluid-filled structures that evolved to tell us which way is up and how our head moves. When this system is disrupted, people can experience conditions such as vertigo.
Our results showing a shift toward interior perception when people lie on their sides suggest the brain actively tunes down vestibular contributions in certain postures. That tuning highlights how important the vestibular system is to our usual sense of space and how changes in body orientation can alter processing across other sensory modalities.
Surprisingly little research has explored how vestibular signals influence other senses. For example, many brain imaging studies are conducted with participants lying down in scanners; if posture changes how the brain balances internal and external reference frames, brain activity measured while supine may not fully represent activity during upright behavior.
These observations touch on deeper questions about how we become aware of the environment and the components that make up conscious experience. We often treat the body as a passive carrier, but our bodily state and orientation actively shape the way we perceive and understand the world.
Next time you lie down, consider how that change in posture might subtly shift what your brain pays attention to and how it interprets the sensations coming from your body.
Funding: David I. Shore consults to The Multisensory Mind Inc. and has received funding, through McMaster University, from the Natural Sciences and Engineering Research Council of Canada.
About this perception research news
Source: The Conversation
Contact: David I. Shore – The Conversation
Image: The image is in the public domain