Smith et al.
[ 19941 concluded that their experiment demonstrated that rats could detect, or were sensitive to, magnetic fields over five frequency-flux density pairs ranging from 1900 pT at 7 Hz to 200 pT at 65.1 Hz. The absence of exposure to sham-field conditions precludes such a conclusion.
Five female food-deprived Long-Evans rats were trained to press a lever to obtain food intermittently under a variable-ratio schedule of food reinforcement. Under these conditions, the rats lever pressed at a high, steady rate during daily sessions. The 60-min session was segmented into six sequences, each consisting of an intertrial interval that ranged between 3 and 12 min, with the last 3 min used as a control period, each then being followed by a 3-min warning period. Throughout each warning period, the rat was exposed to a magnetic field. At the end of the warning period, the grid floor of the behavioral test chamber was energized with a 10 pA shock for 0.5 sec. Over a 5 day period, each rat was exposed to one of the field conditions during the "warning periods." A Latin-square design was used to balance the order of the five field conditions across blocks (weeks) among the rats.
This general paradigm of superimposing such a sequence of trials upon a baseline of responding has been called "conditioned suppression" (Blackman [ 19771 provides a useful introduction). If the subject can detect a signal during the warning period, under many conditions there will be a disruption in the behavioral performance, the alteration frequently being suppression of responding during the warning period compared to the preceding control period. That outcome was seen during the warning period, most reliably later in the daily session, in the Smith et al. experiment, with no differences among the different field conditions. The