Christina Zelano, Heidi Jiang, Guangyu Zhou, Nikita Arora, Stephan Schuele, Joshua Rosenow and Jay A. Gottfried
Journal of Neuroscience 7 December 2016, 36 (49) 12448-12467;
The need to breathe links the mammalian olfactory system inextricably to the respiratory rhythms that draw air through the nose. In rodents and other small animals, slow oscillations of local field potential activity are driven at the rate of breathing (∼2–12 Hz) in olfactory bulb and cortex, and faster oscillatory bursts are coupled to specific phases of the respiratory cycle. These dynamic rhythms are thought to regulate cortical excitability and coordinate network interactions, helping to shape olfactory coding, memory, and behavior. However, while respiratory oscillations are a ubiquitous hallmark of olfactory system function in animals, direct evidence for such patterns is lacking in humans. In this study, we acquired intracranial EEG data from rare patients (Ps) with medically refractory epilepsy, enabling us to test the hypothesis that cortical oscillatory activity would be entrained to the human respiratory cycle, albeit at the much slower rhythm of ∼0.16–0.33 Hz. Our results reveal that natural breathing synchronizes electrical activity in human piriform (olfactory) cortex, as well as in limbic-related brain areas, including amygdala and hippocampus. Notably, oscillatory power peaked during inspiration and dissipated when breathing was diverted from nose to mouth. Parallel behavioral experiments showed that breathing phase enhances fear discrimination and memory retrieval. Our findings provide a unique framework for understanding the pivotal role of nasal breathing in coordinating neuronal oscillations to support stimulus processing and behavior.
SIGNIFICANCE STATEMENT Animal studies have long shown that olfactory oscillatory activity emerges in line with the natural rhythm of breathing, even in the absence of an odor stimulus. Whether the breathing cycle induces cortical oscillations in the human brain is poorly understood. In this study, we collected intracranial EEG data from rare patients with medically intractable epilepsy, and found evidence for respiratory entrainment of local field potential activity in human piriform cortex, amygdala, and hippocampus. These effects diminished when breathing was diverted to the mouth, highlighting the importance of nasal airflow for generating respiratory oscillations. Finally, behavioral data in healthy subjects suggest that breathing phase systematically influences cognitive tasks related to amygdala and hippocampal functions.
The act of breathing results in a cyclical flow of air through the nose, providing an entry point for respiratory entrainment of neural activity. In mammals, local field potential (LFP) responses in olfactory bulb and piriform cortex (PC) oscillate in phase with breathing (Adrian, 1942; Kay and Freeman, 1998; Fontanini et al., 2003), and when breathing is diverted away from the nose, these patterns diminish (Fontanini et al., 2003). Based on such findings, it has been proposed that dynamic oscillatory rhythms regulate cortical excitability, synchronize activity within cell assemblies, and coordinate network interactions, helping to shape olfactory sensory coding, memory, and behavior (Laurent et al., 2001; Kay, 2005; Kepecs et al., 2006; Martin and Ravel, 2014).
Breathing is a vital rhythm of mammalian life, replenishing the bloodstream with oxygen and eliminating carbon dioxide with essential regularity. Although the respiratory drive is generated by conditional bursting pacemaker neurons in the brainstem (Smith et al., 1991, 2009; Garcia et al., 2011), its pace is not fixed: a variety of emotional and cognitive states, including anxiety (Boiten, 1998), stress (Suess et al., 1980), and exploratory behavior (Welker, 1964; Kay and Freeman, 1998; Verhagen et al., 2007; Evans et al., 2009; Vlemincx et al., 2011; Huijbers et al., 2014), can all modify the rate and depth of breathing. The alternative idea, that respiratory phase exerts a direct impact on emotion and cognition, is unknown. The fact that the olfactory system is closely linked with limbic brain regions mediating emotion, memory, and behavior (Carmichael et al., 1994; LeDoux, 2000; Eichenbaum et al., 2007) suggests a robust pathway by which nasal breathing could even shape rhythmic electrical activity in downstream limbic areas, with corresponding effects on cognitive functions.
In line with these ideas, the power of fast cortical oscillations can be modulated by rhythmic external events occurring at lower frequency (Lakatos et al., 2005), with recent animal work revealing that breathing entrains high-frequency oscillations in both olfactory brain regions (Rojas-Líbano et al., 2014; Frederick et al., 2016) and nonolfactory areas, including whisker barrel cortex (Moore et al., 2013; Ito et al., 2014) and hippocampus (Yanovsky et al., 2014; Nguyen Chi et al., 2016). Interestingly, new behavioral data in rodents show that sniffing can even impact nonolfactory behaviors, such as whisking (Cao et al., 2012; Ranade et al., 2013) and ultrasonic vocalizations (Sirotin et al., 2014). Thus, data across a wide range of studies and sensory modalities suggest that respiratory rhythms modulate oscillatory patterns throughout the brain, with potential impact on stimulus sampling behaviors. One broad implication is that breathing subserves more than just supplying oxygen to the body; it can also organize neuronal population activity across brain regions to orchestrate complex behaviors affiliated with orofacial sensation (Kleinfeld et al., 2014). Whether respiratory rhythms have a direct influence on cortical oscillations in the human brain is not well understood.
Here we used intracranial EEG (iEEG) methods to test four inter-related hypotheses about breathing, the brain, and cognition. First, we asked whether respiratory-induced oscillations are present in human PC. Such evidence has been elusive, given that surface EEG approaches lack the spatial resolution and functional MRI (fMRI) approaches lack the temporal resolution to identify oscillatory activity in deeply situated olfactory structures. Second, we asked whether respiratory oscillations propagate to amygdala and hippocampus, and, if so, whether these depend on airflow stimulation through the nose. This question was designed to establish a mechanistic basis for our third hypothesis, examining whether breathing phase systematically influences cognitive tasks related to amygdala and hippocampal functions. Finally, in one patient, we directly tested the idea that respiratory oscillations in the amygdala influence performance on an emotion judgment task, which would imply that the breathing rhythm can exert potent control over learning and behavior.