Prolonged microwave irradiation of rats: Effects on concurrent operant behavior

Two measures of performance were used to study the effects of pulse‐modulated microwave radiation (PM MWR) on schedule‐controlled operant behavior of rats: 1) cued (S D ), fixed‐ratio (FR) bar pressing for food reinforcement; and 2) noncued (S d ) bar pressing in the absence of food reinforcement. The animals were irradiated and the behavioral data were obtained concurrently, during daily three‐hour sessions, five days per week for six to nine weeks. Each experiment began with a two to three‐week baseline interval of sham irradiation; a two to three‐week interval of sham irradiation followed the irradiation phase. The irradiated animals were exposed to 1.3‐Ghz PM MWR (pulse width of 1 microsecond at 600 pulses per second) at whole‐body, average specific absorbed‐dose rates of from 1.5–6.7 mW/g. Control and irradiated animals were tested in identical, cylindrical waveguide exposure/behavioral assemblies; different groups of irradiated and sham‐irradiated animals were used for each dose rate. At 1.5 mW/g, the levels of S D operant responding by control and irradiated animals were comparable, and showed similar progressive diminutions over the course of each daily session. S d operant responding was more variable, but again comparable, with both groups showing similar, progressive declines in rate of responding during each session. At 3.6 mW/g, no specific effects on S D operant response rates were observed. However, there was an initial and transient increase in the rate of extinction of S d responding. At 6.7 mW/g, S D response rates were slightly reduced, whereas there was a major reduction in noncued (S d ) operant responding followed by a sharp rebound during the first post‐MWR week. This marked reduction in S d operant responding at MWR onset was in contrast to the relative stability and persistence of FR responding for food reinforcement.