Sense of Smell Relies on Predictive Coding More Than Vision

Summary: Our sense of smell operates on predictive coding more so than vision, challenging previous notions of smell as a primitive sense. Through a series of experiments, including behavioral studies and fMRI brain imaging, the study highlights smell’s sophistication in reacting to unexpected stimuli by engaging not just the olfactory but also the visual brain regions.

This unique sensory processing underscores the proactive nature of smell in navigating our environment, relying heavily on predictions and cues from other senses to identify odors accurately.

Key Facts:

1. Smell is more dependent on predictions for processing sensory information than vision, indicating its complex and proactive nature.
2. Unexpected odors activate both the olfactory and visual brain regions, suggesting a unique multisensory processing strategy.
3. The study utilized behavioral experiments and fMRI to demonstrate how predictive coding theory applies to olfactory perception, revealing the advanced capabilities of the human sense of smell.

Source: Stockholm University

A popular theory of the brain holds that its main function is to predict what will happen next, so it reacts mostly to unexpected events. Most research on this topic, called predictive coding, has only focused on what we see, but no one knows if the different senses, such as smell, work in the same way.

To figure out more about how smell relates to how we handle different sensory impressions, the researchers conducted a study with three experiments, two behavioural experiments, and one experiment using the brain imaging method fMRI at Stockholm University Brain Imaging Centre (SUBIC).

“The main finding is that smelling was much more dependent on predictions than vision was. This is interesting because many people think that smell is primitive and reactive, when our research shows it is in fact quite sophisticated and proactive,” says Stephen Pierzchajlo, PhD Student at the Department of Psychology, and main author of the study.

The study shows how important it is for our different senses to be able to use correct cues when we classify different sensory impressions.

“We have all experienced that we react to when an unexpected smell appears, for example when we enter someone’s flat and encounter a new smell. Our research shows that the sense of smell is highly influenced by the cues from other senses, while the sense of sight and hearing are affected to a much lesser extent,” says Jonas Olofsson, professor at the Department of Psychology, and co-author of the study.

The researchers also show that when the brain tries to identify odors that it had not expected, both the olfactory and visual brains are activated, despite the absence of visual cues in the task.

“The olfactory brain thus has a completely unique way of processing smells and it is about whether the smells are expected or not. The sense of smell warns us of smells that we had not expected, and engages the visual brain, perhaps to be able to see what it is that smells. It’s a smart function because we humans are so bad at recognizing smells if we don’t get clues,” says Jonas Olofsson.

In the experiments, participants listened to spoken word cues, such as “lemon”, and then received a picture or smell, and participants quickly decided whether it matched with the cue, for example with a lemon picture or smell, or did not match, for example with a rose picture or smell.

“We noticed that overall, the expected pictures and smells led to quicker decisions, which fits well with predictive coding theory. We used the difference in response speed to compare the senses with each other – a bigger delay for unexpected stimuli means that the sense relies more on predictions,” says Stephen Pierzchajlo. 

The study is the first concluded part of his PhD research.

“The human sense of smell is not a reactive, but a proactive sense. It uses a unique brain strategy to process unexpected smells in order to understand what the smells are,” says Stephen Pierzchajlo. 

Facts about the study

• Three experiments were conducted in the study, two behavioral experiments and one fMRI-experiment using the brain imaging method fMRI at Stockholm University Brain Imaging Centre (SUBIC).
• Sixty-nine participants completed the first behavioral experiment. Fifty participants completed the second behavioral study.
• For the fMRI-experiment, data for 15 participants were first collected and analyzed. Then, 32 healthy volunteers participated in the fMRI portion of the study.
• In all three experiments, the reseachers used a set of four familiar stimuli (lavender, lilac, lemon and pear) that were repeatedly presented as smells, pictures or spoken words, in order to achieve high and comparable accuracy rates and thus unbiased response-time assessments.

About this olfaction research news

Author: Gunilla Nordin
Source: Stockholm University
Contact: Gunilla Nordin – Stockholm University
Image: The image is credited to Neuroscience News

Original Research: Open access.
“Olfactory categorization is shaped by a transmodal cortical network for evaluating perceptual predictions” by Stephen Pierzchajlo et al. Journal of Neuroscience

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Abstract

Olfactory categorization is shaped by a transmodal cortical network for evaluating perceptual predictions

Creating and evaluating predictions are considered important features in sensory perception. Little is known about processing differences between the senses and their cortical substrates.

Here, we tested the hypothesis that olfaction, the sense of smell, would be highly dependent on (non-olfactory) object-predictive cues and involve distinct cortical processing features. We developed a novel paradigm to compare prediction error processing across senses.

Participants listened to spoken word cues (e.g. “lilac”) and determined whether target stimuli (odors or pictures) matched the word cue or not. In two behavioral experiments (total n = 113; 72 female), the disparity between congruent and incongruent response-times was exaggerated for olfactory relative to visual targets, indicating a greater dependency on predictive verbal cues to process olfactory targets.

A pre-registered fMRI study (n = 30; 19 female) revealed the anterior cingulate cortex (a region central for error detection) being more activated by incongruent olfactory targets, indicating a role for olfactory predictive error processing.

Additionally, both the primary olfactory and visual cortices were significantly activated for incongruent olfactory targets, suggesting olfactory prediction errors are dependent on cross-sensory processing resources, whereas visual prediction errors are not.

We propose that olfaction is characterized by a strong dependency on predictive (non-olfactory) cues, and that odors are evaluated in the context of such predictions by a designated transmodal cortical network.

Our results indicate differences in how predictive cues are used by different senses in rapid decision-making.