Demonstrated the cyclical nature of sound perception
A new research, published in Current Biology has proved that the brain samples rhythmically the sensory information that gets to our ears
How do we perceive the sounds? With oscillations that reflect the rhythms of our attention. This was demonstrated by a study published in Current Biology, entitled “Auditory Sensitivity and Decision Criteria Oscillate at Different Frequencies Separately for the Two Ears”, which provided new key evidence of the cyclical nature of perception. The research is the result of an Italian-Australian collaboration involving David Alais and Johahn Leung of the Department of Psychology and Medical Sciences at Sydney University, David Burr of the Department of Neuroscience at the University of Florence and Maria Concetta Morrone and Tam Ho from the Department of Translational Research at the University of Pisa.
With simple behavioural experiments, the authors have shown that sensitivity in detecting the presence of a weak sound is not constant, but swings rhythmically over time. The rhythm of sensitivity induces sequential sampling of the signals from the two ears, at about a tenth of a second, sufficiently fast to act outside of our consciousness.
“Perceptual oscillations have already been described for the visual system, but never before for the auditory system,” explains David Burr. “The difficulty in demonstrating the perceptual rhythm of the auditory system is precisely related to the peculiarity of an oscillating frequency of opposite polarity in the two ears. However, sampling at different times signals from the two ears can offer a number of advantages, not least that of carefully monitoring mono-aural signal, without losing the important information for localizing the sound.”
Even more surprisingly, the authors have shown that cyclicity is not only about the detection capability, but also about the decision-making ability to classify a sound. In this case, the oscillations are much faster, emphasizing how the processes underlying our decisions and our perceptive abilities can use different circuits and be regulated by different rhythms.
What is the benefit for the brain to make a rhythmic sampling of sensory information? Theories in this regard abound, but one of the most successful ones, embraced by the authors of this study, suggests that the phenomenon reflects the action of attention. “When we look at a scene, not all of its parts are equally salient: some receive more attention than others, and these are analyzed with priority,” says Tam Ho, in the photo with David Burr. “This is a very effective strategy: it allows us to focus our attention resources, usually very limited, on specific objects of interest, instead of diluting them on the whole scene. Likewise, attention resources can be concentrated in short periods of time: as a stroboscopic light that binds together objects in a scene that are lit simultaneously ».
Even more interesting it is the use of these rhythms by the time-tracking mechanism of the brain clock: “Neural oscillations may have the purpose of organizing and coding, over different frequencies, information from various senses to make us interact optimally with the outside world,” conclude the authors of the study. “Although these are only hypotheses, one thing is clear: our perception of the world is inherently rhythmic, although we are not completely conscious of it.”
