The what and when of olfactory working memory in humans.

Encoding and retaining novel sequences of sensory stimuli in working memory is crucial for adaptive behavior. A fundamental challenge for the central nervous system is to maintain each sequence item in an active and discriminable state, while also preserving their temporal context. Nested neural oscillations have been postulated to disambiguate the "what" and "when" of sequences, but the mechanisms by which these multiple streams of information are coordinated in the human brain remain unclear. Drawing from foundational animal studies, we recorded local field potentials from the human piriform cortex and hippocampus during a working memory task in which subjects experienced sequences of three distinct odors. Our data revealed a unique organization of odor memories across multiple timescales of the theta rhythm. During encoding, odors elicited greater gamma at distinct theta phases in both regions, time stamping their positions in the sequence, whereby the robustness of this effect was predictive of temporal order memory. During maintenance, stimulus-driven patterns of theta-coupled gamma were spontaneously reinstated in piriform cortex, recapitulating the order of the initial sequence. Replay events were time compressed across contiguous theta cycles, coinciding with periods of enhanced piriform-hippocampal theta-phase synchrony, and their prevalence forecasted subsequent recall accuracy on a trial-by-trial basis. Our data provide a novel link between endogenous replay orchestrated by the theta rhythm and short-term retention of sequential memories in the human brain.

Ambulatory and challenge-associated heart rate variability measures predict cardiac responses to real-world acute emotional stress.

BACKGROUND: Heart rate variability (HRV) measures homeostatic regulation of the autonomic nervous system in response to perturbation and has been previously shown to quantify risk for cardiac events. Despite known interactions among stress vulnerability, psychiatric illness, and cardiac health, however, this is the first study to our knowledge to compare directly the value of laboratory HRV in predicting autonomic modulation of real-world emotional stress. METHODS: We recorded electrocardiograms (ECG) on 56 subjects: first, within the laboratory and then during an acute emotional stressor: a first-time skydive. Laboratory sessions included two 5-min ECG recordings separated by one ambulatory 24-hour recording. To test the efficacy of introducing a mild emotional challenge, during each of the 5-min laboratory recordings, subjects viewed either aversive or benign images. Following the laboratory session, subjects participated in the acute stressor wearing a Holter ECG. Artifact-free ECGs (n = 33) were analyzed for HRV then statistically compared across laboratory and acute stress sessions. RESULTS: There were robust correlations (r = .7-.8) between the laboratory and acute stress HRV, indicating that the two most useful paradigms (long-term wake, followed by short-term challenge) were also most sensitive to distinct components of the acute stressor: the former correlated with the fine-tuned regulatory modulation occurring immediately prior and following the acute stressor, whereas the latter correlated with gross amplitude and recovery. CONCLUSIONS: Our results confirmed the efficacy of laboratory-acquired HRV in predicting autonomic response to acute emotional stress and suggest that ambulatory and challenge protocols enhance predictive value.