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Humans use their senses to experience the world around them, as well as themselves and others. Even though the senses are the sole means of accessing the outside world, people rarely examine how accurate they are at representing physical reality. Neuroscience research over the last decade has revealed that the cerebral cortex is always making predictions about what will happen next and that sensory processing neurons only record the difference between our assumptions and reality.
The fundamental channels for mapping our environment to our thoughts are the subcortical sensory pathways. Sensory pathways transport information efficiently by altering neural responses to the local statistics of sensory input. The long-held mechanical explanation for this adaptive behaviour is that when local statistics of inputs become more regular, brain activity decreases. Another theory is that sensory input expectations affect neural coding directly. When disambiguating ambiguous inputs like an item in the dark or spoken phrases in a noisy pub, expectations have measurable influence on human perception. The predictive coding theoretical framework formalises the active function of expectations in perception by proposing that sensory neurons constantly compare incoming inputs to an internal prediction, which is generated from a generative model of sensory input. This method elevates the importance of unexpected occurrences, which are frequently important for behaviour and survival.
Previous research has shown that single neurons and subcortical sensory pathway nuclei exhibit stimulus-specific adaptation (SSA). Neurons and neural populations showing SSA adapt to standards (frequently occurring stimuli) yet show restored responses to deviants (rarely occurring stimuli). SSA is generally evoked in the auditory modality using sequences consisting of repeats of a standard sound (often a pure tone of a particular frequency) with a single, randomly positioned deviant sound (a pure tone of the same duration and loudness but with a different frequency). Although SSA is frequently used to support the predictive coding theory, it may also be explained in terms of habituation, in which neurons become less receptive to higher regularities in their local statistics regardless of prediction. At each layer of the processing hierarchy, habituation optimizes information transmission locally by minimizing reactivity to duplicate input.
To disentangle the habituation and predictive coding viewpoints of redundancy reduction in the auditory subcortical sensory system, researchers developed a new paradigm in conjunction with ultra-high-field fMRI in human participants. The nuclei of the thalamus (medial geniculate body, MGB) and midbrain (inferior colliculus, IC) were chosen because they are the major nuclei of the ascending subcortical pathway that can be consistently studied in human volunteers in vivo . For their study, the researchers used functional magnetic resonance imaging (fMRI) to examine the brain responses of 19 participants while they listened to sound patterns. The participants were given the task of determining which of the sounds in the sequence was different from the others. The participants' expectations were then shifted such that the abnormal sound would appear at specified moments in the sequences. The neuroscientists looked at how the abnormal noises affected the inferior colliculus and the medial geniculate body, which are the two primary nuclei of the subcortical pathway for auditory processing. The subcortical nuclei registered the noises only when they were put in unexpected locations, despite the fact that participants recognised the deviant faster when it was presented in predictable locations.
Their findings are the first to demonstrate that abstract processing takes place in the subcortical sensory pathway. They indicate that even at the most fundamental levels of the processing hierarchy, our prior beliefs impact our neural representation of the outside world. The current study focused on the nuclei of the auditory sensory pathway. However, early evidence of stimulus-specific adaptation has been found in the visual, olfactory, and somatosensory pathways. Predictive coding tries to improve the dynamic range of sensory systems and increase information transmission in the neural code by reducing responses to expected stimuli and redundant portions of the incoming sensory signal. Abstract expectations, according to the researchers, are used in other sensory modalities to facilitate sensory processing in subcortical sensory nuclei.
Considering that predictive coding is very important in sensory processing, abnormal predictive coding in the subcortical sensory pathway is likely to have significant cognitive implications. Individuals with developmental dyslexia, a condition marked by trouble processing speech sounds, have irregular adaption dynamics to stimulus regularities and abnormal cortico- thalamic connections in the left hemisphere. The mechanisms that cause SSA and their links to sensory processing within the subcortical sensory pathways might have therapeutic implications.
https://elifesciences.org/articles/64501 Written by: Reem Alzafiri, BSc, MSc.