New research in the journal eLife indicates that expectations about the physical world impact how we perceive reality at a fundamental level. The study provides evidence for predictive coding, a theory of information processing that describes perception as a process of hypothesis testing, in the brain’s sensory pathways.
“We rely solely on our senses to know what is out there, but sensory input arriving in our eyes and ears is heavily processed by our brains to conform our experience of others and our environment,” said study author Alejandro Tabas, the chair of Cognitive and Clinical Neuroscience at Technische Universtät Dresden.
“Our research is focus on understanding the general principles underlying this processing that maps the physical world to our minds. Understanding how this works is the only way we have to know how much we can rely on our senses to accurately describe what we see and hear. I find our field of research fascinating because it makes me rethink over and over sensations whose objectivity I’ve always given for granted.”
To examine how expectations influenced human perception, the researchers used ultra-high-resolution functional magnetic resonance imaging (fMRI) to measure the neural activity of 19 participants as they listened to different sequences of auditory tones.
The participants were instructed to listen to the sequence and respond as fast and accurately as possible when they heard a deviant tone. Then, the participants’ expectations were manipulated so that they would expect the deviant tone in certain positions of the sequences.
The neuroscientists examined the responses elicited by the deviant tones in the two brain regions related to auditory processing: the inferior colliculus and the medial geniculate body. Although participants recognized the deviant tone faster when it was placed on positions where they expected it, the subcortical nuclei encoded the sounds only when they were placed in unexpected positions.
The findings indicate that the neurons in charge of sensory processing only encode the difference between internal expectations and the actual reality.
“Our most surprising finding in this study is that even at very low processing centers of the auditory system information is not encoded for what it is, but for how much it differs from what we believe it is,” Tabas told PsyPost.
“This means that the brain of people who have different expectations on what they are hearing encode information differently. The idea that information coding is subjective in the brain is not new, but it was previously assumed that our previous beliefs influenced how we see the world only in higher order cognitive areas.”
“We have shown that subjective encoding goes as far as the auditory midbrain, a very early processing stage in the auditory processing pipeline. My take home message is that maybe we should not be too sure that what we perceive is an objective gist of the underlying reality,” Tabas explained.
But the study — like all research — includes some limitations.
“We investigated only the auditory system, so whether this organization extends to other senses is still unknown,” Tabas said. “Even within the auditory system, we only studied responses to pure tones, the simplest possibly conceivable sound there is, and used simple expectations that were defined very precisely. Further research is needed to clarify if these principles extend also to more complex and uncertain scenarios like the perception of spoken language.”
The study, “Abstract rules drive adaptation in the subcortical sensory pathway“, was authored by Alejandro Tabas, Glad Mihai, Stefan Kiebel, Robert Trampel, and Katharina von Kriegstein.