Gamma activity temporally dissociates sensory encoding from perceptual decision-making in human olfactory circuits

Smell is intimately linked to emotion, memory, hazard detection, and social communication, yet the neural mechanisms by which human olfactory circuits encode chemosensory stimuli and transform them into conscious percepts remain poorly understood. We combined stereotactic electroencephalography (SEEG) with a passive, inspiration-triggered olfactometer that delivers odorized air without voluntary sniffing, attentional cuing, or motor engagement, enabling clean separation of sensory, perceptual, and motor contributions to chemosensory processing. Across 19 participants (two subjects had two study sessions), we identified two temporally and functionally distinct gamma-band components within human olfactory circuits. An early, stimulus-locked gamma response reflects primary olfactory encoding: it persists on trials in which participants fail to report odor detection, demonstrating that sensory processing proceeds independently of conscious awareness. A later gamma component is time-locked to behavioral report and emerges hierarchically across limbic structures, reflecting perceptual decision-making and response preparation. Gamma responses also encode odor intensity through two dissociable mechanisms: graded, dose-dependent modulation and switch-like, report-dependent dynamics, consistent with a signal-detection framework for perceptual threshold. These findings establish that gamma activity dissociates primary sensory encoding from higher-order perceptual decisions in human olfactory circuits, demonstrating that the computational principles governing sensory awareness in vision and touch extend to human olfaction.