A study funded by the SNSF highlights previously unknown links between the body and the brain. The findings of this research carried out at the University of Fribourg show how our bodily rhythms affect our visual perception.

Taking a deep breath to get a clearer picture is perhaps more effective than you might think. Breathing has a physiological impact on the perception of visual stimuli. This is what emerges from the work of Juliane Britz, a psychologist and neuroscientist at the University of Fribourg. She has set up an experiment demonstrating that cardiac and respiratory rhythms have an impact on how we become conscious 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 realising it
For this study, volunteers observed a screen on which grey squares appeared briefly, either cross-hatched diagonally in one direction or the other. After each visual stimulus, the subjects were asked two questions: Had they seen the patterns or not, and what was their orientation? The contrast of the crosshatching was calibrated so that participants consciously saw it 50% of the time. However, even though it was only consciously perceived one time out of two, the orientation indicated 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 therefore seems that the volunteers sometimes processed the patterns without realising it. 

Throughout the experiment, electrodes were used to measure the electrical activity in the participants' brains (electroencephalogram, EEG) and hearts (electrocardiogram, ECG). Their breathing was also tracked using a belt that measured the abdominal volume. Comparing the brain's electrical signals between cases where the subjects had seen the crosshatching or not allowed 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 was displayed while the heart was relaxed, the markers of consciousness appeared around 150 milliseconds earlier than if the image was displayed while the heart was contracting. Breathing has a similar impact on visual perception, with the same delay when the image appears during exhalation rather than inhalation. “This influence of bodily rhythms on the brain is mediated by pressure receptors in the arteries,” explains Juliane Britz. These remain silent when the heart is relaxed and when we inhale but are activated when the heart contracts and we exhale. They then cause a sort of neuronal traffic jam that delays the brain's processing of visual stimuli.

Two distinct circuits towards consciousness
This discovery has also shown that different areas of the brain are engaged 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 regions of the brain before reaching the point where it activates conscious thought – the moment when the subject realises that he or she has seen the patterns. The new results reveal that at the end of this cerebral processing pathway, visual information can follow two different routes. In the presence of bodily signals, it passes through the parietal cortex, while in their absence, it passes through the neighbouring frontal cortex. The existence of these two parallel circuits provides a new explanation to reconcile the debate between the specialists who located the emergence of consciousness in the frontal cortex and those who preferred the parietal cortex. “It's simple,” says the researcher. “It's as if there are two possible modes of activation for a visual stimulus, depending on whether there are signals coming from the body or not.” Ultimately, it is the cardiac and breathing rhythms that determine, via pressure receptors in the arteries, which cerebral pathway visual perception will take.

Juliane Britz insists on the fundamental nature of the study and prefers not to speculate about why bodily cycles affect the neural markers of consciousness. “These results resolve the controversy surrounding the neurophysiological markers of awareness and the area of the brain in which they occur.” And for her, the main takeaway is simple: we need to be less ‘brain-centred’ in neuroscience. “Bodily signals, which are often discarded and considered as noise, should no longer be dismissed in this way. The brain is not independent from 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)

Source: Press Release from the Swiss National Science Foundation