Heliox at 4 ATA Reduces Error Rates Compared to Trimix and Air, but It Does Not Affect Short-Term Memory in Hyperbaric Conditions

Cognitive performance during diving is challenged by hyperbaric conditions, where inert gas narcosis, oxygen partial pressure, and carbon dioxide retention may impair accuracy and reaction time. Breathing gas mixtures such as heliox and trimix are proposed to reduce these effects, but controlled evidence at recreational depths remains limited. Our study investigated short-term effects of hyperbaric exposure at 4 ATA (equivalent to 30 m of seawater) on cognitive functioning and whether the severity of impairment varies across breathing gas mixtures: air, trimix and heliox. Each participant completed neuropsychological testing at three stages: before exposure (1 ATA), during exposure (4 ATA), and after decompression (1 ATA). Our findings concerned accuracy in the Simon task: the participants breathing heliox had a significantly lower error rate at 4 ATA (1.12%) compared with baseline (1.89%), whereas the error rate nearly doubled for the participants breathing air (from 2.3% to 4.38%). This brings us to the conclusion that breathing heliox minimises errors under cognitive conflict, supporting an accuracy-preserving strategy, while breathing air promotes speed at the cost of mistakes. Given that operational safety depends more on preventing errors than on preserving speed, heliox emerges as the preferable breathing gas mixture for diving, even at recreational depths.