October 2011
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Prediction of smells

ScienceDaily reports on a recent paper in Neuron by Zelano, Mohanty and Gottfried, Olfactory Predictive Codes and Stimulus Templates in Piriform Cortex. (here)

In the moments before you “stop and smell the roses,” it’s likely your brain is already preparing your sensory system for that familiar floral smell. New research from Northwestern Medicine offers strong evidence that the brain uses predictive coding to generate “predictive templates” of specific smells — setting up a mental expectation of a scent before it hits your nostrils…”Our study confirmed the existence of predictive coding mechanisms in olfaction,” said Gottfried, senior author of the study. “We found that the entirety of the olfactory cortex we looked at did form predictive templates that were very specific to the targeted smell.”… In the study, subjects performed “odor search tasks” while being monitored inside an MRI scanner. The two scents used in the study were a watermelon smell and a Play-Doh-like smell. … Before each trial began, subjects were told which of two target smells they should try to identify. A visual countdown, informing the subjects that they should get ready to receive a specific odor was administered and then, after smelling the odor, subjects indicated by pressing a button whether they thought the target smell was present. Sometimes the target scent administered was the same as the subject was foretold, sometimes it was different, and sometimes the target scent was hidden in a mixture of other scents. … The researchers were able to look at the activity pattern of the brain before any odor arrived and found that, for trials where the target was the same, the activity pattern was more correlated than when the target was different.

When I saw this item, I immediately thought that this might have a bearing in how close might the connection between consciousness and prediction be. The picture was that our consciousness awareness is based on a prediction so that it matches in time what is happening at the time. The fraction of a second that it takes to create the conscious experience is hidden by the experience being based on a similar fraction of a second prediction.

If smell has similar prediction to vision and sound but is not usually raised to conscious awareness, then that prediction need not be an aspect of consciousness but of perception. On a closer look, the experiment only dealt with consciously registered and reported smells. It appears to still be an open question: how bound is near-future prediction to consciousness?

For a look at this prediction in more detail, here is the abstract:

Neuroscientific models of sensory perception suggest that the brain utilizes predictive codes in advance of a stimulus encounter, enabling organisms to infer forthcoming sensory events. However, it is poorly understood how such mechanisms are implemented in the olfactory system. Combining high-resolution functional magnetic resonance imaging with multivariate (pattern-based) analyses, we examined the spatiotemporal evolution of odor perception in the human brain during an olfactory search task. Ensemble activity patterns in anterior piriform cortex (APC) and orbitofrontal cortex (OFC) reflected the attended odor target both before and after stimulus onset. In contrast, prestimulus ensemble representations of the odor target in posterior piriform cortex (PPC) gave way to poststimulus representations of the odor itself. Critically, the robustness of target-related patterns in PPC predicted subsequent behavioral performance. Our findings directly show that the brain generates predictive templates or “search images” in PPC, with physical correspondence to odor-specific pattern representations, to augment olfactory perception.

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