Changes in progressive-ratio performance under increased pressures of air.

Rats performed on progressive-ratio schedules that required an increasing number of responses for each successive reinforcement. The number of responses required increased until the subjects failed to complete the next ratio in the sequence within 15 min. Response-ratio increments of two responses, five responses, and 20 responses were investigated. The size of the final completed ratio generally increased with increases in the progressive-ratio step size. Increased pressures of air in a hyperbaric chamber led to both increases and decreases in terminal ratio size, with the differential effects depending on both air pressure and on the size of the progressive-ratio increment. Changes in the number of responses in the final ratio were related to increased pressures of nitrogen, as similar pressures of helium produced few effects.