to thought processes and a decrease in the diver's ability to make judgements or calculations. It can also decrease motor skills, and worsen performance in tasks requiring manual dexterity. As depth increases, so does the pressure and hence the severity of the narcosis. The effects may vary widely from individual to individual, and from day to day for the same diver. Because of the perception-altering effects of narcosis, a diver may not be aware of the symptoms, but studies have shown that impairment occurs nevertheless. The narcotic effects dissipate without lasting effect as the pressure decreases during ascent. High-pressure nervous syndrome Helium is the least narcotic of all gases, and divers may use breathing mixtures containing a proportion of helium for dives exceeding about deep. In the 1960s it was expected that helium narcosis would begin to become apparent at depths of . However, it was found that different symptoms, such as tremors, occurred at shallower depths around . This became known as high-pressure nervous syndrome, and its effects are found to result from both the absolute depth and the speed of descent. Although the effects vary from person to person, they are stable and reproducible for the individual. Oxygen toxicity Although oxygen is essential to life, in concentrations significantly greater than normal it becomes toxic, overcoming the body's natural defences (antioxidants), and causing cell death in any part of the body. The lungs and brain are particularly affected by high partial pressures of oxygen, such as are encountered in diving. The body can tolerate partial pressures of oxygen around indefinitely, and up to for many hours, but higher partial pressures rapidly increase the chance of the most dangerous effect of oxygen toxicity, a convulsion resembling an epileptic seizure. Susceptibility to oxygen toxicity varies dramatically from person to person, and to a smaller extent from day to day for the same diver. Prior to convulsion, several symptoms may be present – most distinctly that of an aura. Treatments Treatment of diving disorders depends on the specific disorder or combination of disorders, but two treatments are commonly associated with first aid and definitive treatment where diving is involved. These are first aid oxygen administration at high concentration, which is seldom contraindicated, and generally recommended as a default option in diving accidents where there is any significant probability of hypoxia, and hyperbaric oxygen therapy, which is the definitive treatment for most conditions