Summary: Theta oscillations appear to play a central role in how the human brain processes smells, a recent study reports.

Source: Northwestern Medicine.

Theta Oscillations Rapidly Convey Odor Information in the Human Brain

Theta oscillations — rhythmic electrical activity that cycles about four to eight times per second — may be a core mechanism for processing odors in the human brain, according to a study published in Neuron.

Using intracranial EEG recordings from patients with medically resistant epilepsy, researchers led by Jay A. Gottfried, MD, PhD, professor of Neurology, were able to examine the time-frequency dynamics of odor processing in the human piriform cortex, the primary cortical region for smell. These rare clinical recordings provided a direct window into the brain’s physiological rhythms during odor perception.

“We wanted to understand what happens at a microstructural level of the human brain when you smell an odor,” Gottfried said. “Recording physiological rhythms with implanted electrodes in these patients gives us access to timing and neural mechanisms that other methods cannot provide.”

Key findings include the rapid emergence of odor-specific neural signals and a prominent role for theta-band activity. Across seven patients, the team found that odors reliably increased theta power in the piriform cortex, whereas inhaling odorless air failed to produce the same effect. Using classification analyses, the researchers were able to decode which odor a person smelled as early as 110 milliseconds after the first sniff.

“Many assume smell is a slow sense, but these results show how quickly odor information becomes available to the brain and how that speed is reflected in its underlying physiology,” Gottfried said.

Graduate student Heidi Jiang, the study’s first author, collected intracranial electrophysiological recordings while patients performed a cued odor detection task. Across trials with four distinct odors, the time-frequency features of theta oscillations in the piriform cortex carried information that differentiated those odors — for example, strawberry, peanut butter, chocolate and garlic — within a rapid temporal window.

Piriform-Hippocampal Coordination

The team also recorded from both piriform cortex and hippocampus in some patients and observed that odor presentation led to increased theta-phase synchrony between these regions. This coordination suggests theta oscillations help coordinate information exchange between the piriform cortex and hippocampus during olfactory processing.

“The hippocampus is a central hub for reactivating and retrieving memories — for example, remembering what and where you ate a food. It’s possible the hippocampus can convey memory-related information to piriform cortex to help interpret odors,” Gottfried explained.

Timing and Functional Significance

The findings also raise questions about how theta rhythms relate to breathing. While rodents show strong coupling between sniffing and theta-band activity at faster breathing rates, humans breathe much more slowly. This suggests that in humans, theta oscillations are not simply a direct reflection of the respiratory cycle but may be a more fundamental timing mechanism for odor processing.

Gottfried proposes that theta rhythms could act as an internal clock, segmenting incoming sensory information into discrete packets that the brain can process and compare. He plans future studies to test whether theta oscillations function as a clock that regulates brain dynamics during odor perception and whether modulation of theta affects odor discrimination and memory.

Study Context and Methods

The participants were patients with medically resistant epilepsy who had electrodes implanted for clinical reasons. With informed consent, the research team recorded intracranial EEG while participants completed an odor detection task. Intracranial recordings offer high temporal and spatial resolution that complements previous functional MRI studies, which identify odor-related brain areas but provide limited insight into millisecond-scale timing and oscillatory mechanisms.

Funding and Publication

Funding: This work was supported by grants from the National Institute on Deafness and Other Communication Disorders (R01DC013243 and R21DC012014) and by a National Science Foundation Graduate Research Fellowship Program training grant (DGE-1324585).

Publication: Heidi Jiang, Stephan Schuele, Joshua Rosenow, Christina Zelano, Josef Parvizi, James X. Tao, Shasha Wu, and Jay A. Gottfried. “Theta Oscillations Rapidly Convey Odor-Specific Content in Human Piriform Cortex.” Published online April 5, 2017 in Neuron. doi:10.1016/j.neuron.2017.03.021

Abstract Highlights

Highlights:

• Odor elicits theta power selectively in human piriform cortex within 500 ms of a sniff.
• Presence of odor (versus absence) enhances piriform-hippocampal theta phase locking.
• Odor-specific content can be decoded from piriform oscillations as early as 110 ms after a sniff.

Summary: Theta oscillations are a distinct electrophysiological signature of olfactory processing in the human brain. Across seven patients, odor stimulation enhanced theta power in piriform cortex with robust single-trial effects. Classification analyses showed that piriform oscillatory activity conveyed odor-specific information that was decodable within 110–518 ms after a sniff and was strongest in the theta band. This temporal window was also associated with increased theta-specific phase coupling between piriform cortex and hippocampus, suggesting rapid, coordinated access to olfactory content in the human brain.

Source: Sarah Plumridge – Northwestern Medicine

Image Source: NeuroscienceNews.com image is cited for illustrative purposes.