Study reveals new links between body and brain. The findings, conducted at the University of Fribourg, show how our body cycles modify our visual perception.
Taking a deep breath to see more clearly may be more concrete than we thought. Breathing has a physiological impact on the perception of visual stimuli. This is the finding of Juliane Britz, a psychologist and neuroscientist at the University of Freiburg, who has set up an experiment showing that heart and breathing cycles influence the way we become aware of a visual stimulus. The results of this study, funded by the Swiss National Science Foundation (SNSF), have just been published in the journal PNAS (*).
Seeing an image without realizing it
For this research, volunteers observed a screen on which gray squares appeared briefly, cross-hatched diagonally in one direction and then in the other. After each visual stimulus, subjects were asked two questions: had they seen the patterns? What was their orientation? The contrast of the crosshatching was calibrated so that 50% of participants consciously saw it. And yet, even though they were only consciously perceived one time out of two, the indicated orientation was correct 85% of the time. This means that by following their intuition - even without knowing where it came from - the subjects were right more often than if they had answered randomly. It would seem, then, that the volunteers sometimes processed the patterns without realizing it.
Throughout the experiment, electrodes were used to measure the participants’ cerebral electrical activity (electroencephalogram, EEG) and cardiac activity (electrocardiogram, ECG). Their breathing was also tracked using a belt that measured the volume of the abdomen. Comparing the brain’s electrical signals between cases where the subjects had and had not seen the hatchings enabled the psychologist to identify "neuronal markers of consciousness".
She and her team then compared these neural markers according to cardiac phase. The analysis showed that if the image appeared when the heart relaxed, the markers of consciousness appeared around 150 milliseconds earlier than if it appeared when the heart contracted. Breathing has a similar impact on visual perception, which is similarly delayed when the image appears during exhalation rather than inspiration. "This influence of body rhythms on the brain is mediated by pressure receptors in the arteries", explains Juliane Britz. These remain silent when the heart relaxes and breathes in, and are activated when the heart contracts and breathes out. In this way, they create a kind of neuronal plug that delays the brain’s processing of visual stimuli.
Two distinct circuits to consciousness
The discovery also revealed that different brain areas are involved, depending on the signals sent by the body. Until now, it was known that an image is first captured by the visual cortex and then passes through other brain regions before reaching the point where it activates conscious thought - the moment when the subject realizes he or she has seen the patterns. The new results reveal that, at the end of its cerebral processing path, visual information can follow two different routes. In the absence of body signals, it passes through the frontal cortex, while in the presence of body signals, it passes through the adjacent parietal cortex. The existence of these two parallel circuits reconciles specialists who placed the emergence of consciousness in the frontal cortex and others who preferred the parietal cortex. It’s simple," says the researcher. It’s as if there were two possible modes of activation during a visual stimulus, depending on whether or not there are signals coming from the body." Ultimately, then, it’s heartbeat and breathing that determine, via arterial pressure receptors, which cerebral pathway visual perception will take.
As to why, from an evolutionary point of view, cyclical body signals have such an impact on markers of consciousness, Juliane Britz prefers not to speculate. She insists on the fundamental nature of the study. "These results resolve controversies about the neurophysiological markers of awareness and the brain area where it occurs." And for her, the main lesson is simple: we should be less "brain-centric" in neuroscience. "Body signals, which are often dismissed as noise, should no longer be dismissed in this way. The brain is not independent of the body."
Study
V. Leupin and J. Britz: Interoceptive signals shape the earliest markers and neural pathway to awareness at the visual threshold. PNAS (2024)
