Inhalation of 7.5% carbon dioxide increases alerting and orienting attention network function

Rationale: The development of experimental models that readily translate between animals and humans is required to better integrate and clarify the biological, behavioural and cognitive mechanisms that underlie normal fear and pathological anxiety. Inhalation of low concentrations of carbon dioxide (CO₂) increases anxiety and autonomic arousal in humans, triggers related behaviours in small animals, and increases selective attention to threat in healthy humans. However the effects on broader cognitive (non-emotional) processes that characterize anxiety are not known. Objectives: To examine the effect of 7.5% CO₂ inhalation (vs. air) on the efficiency of discrete attention networks implicated in anxiety: alerting (maintaining an alert state), orienting (the selection of information from sensory input) and executive control (resolving cognitive conflict). Methods: Twenty-three healthy human participants completed a computerized Attention Network Test (ANT) during inhalation of 7.5% CO₂ enriched and normal/medical air. Gas was administered blind to participants with inhalation order counterbalanced across participants. Measures of heart rate, blood pressure and subjective mood/anxiety were obtained at baseline and following each inhalation period. Results: CO₂ inhalation increased anxiety, autonomic arousal and the efficiency of alerting and orienting attention network function. Autonomic response to CO₂ correlated with increased orienting; and CO ₂-induced anxiety, autonomic arousal and orienting network function increased with chronic (trait) anxiety. Conclusions: Evidence that CO ₂ modulates attention mechanisms involved in the temporal detection and spatial location of salient stimuli converges with evidence that CO ₂ triggers fear behaviour in animals via direct innervation of a distributed neural network that facilitates environmental hypervigilance.