repeated exposures each in four periods with a total maximum of 25 s
A device to expose the forearm and hand to a local DC orAC electric field was designed and built.
The electric field was generated by a plate suspended in the middle of the enclosure. The ground plate, on which the subject¿s arm rested, was elevated from the bottom of the enclosure, and heated to a comfortable temperature. The radial field intensity distribution (around the arm) was maximum at the highest exposed point in the chamber. Along the arm, field intensity was highest distally (hand), declining proximally (forearm).
During testing, the subject was seated beside the exposure chamber. The left forearm and hand were inserted into the chamber through an aperture in the metal grid that surrounded the apparatus, and rested on the floor of the chamber.
Each signal trial (field present) consisted of four periods: (1) onset period (increase in field from 0 to the desired intensity, constant slope, maximum 7 s); (2) observation period (constant field, 7 s); (3) response period (maximum 4 s, i.e., until the subject responded); and (4) offset period (decrease in field to 0, same slope, maximum 7 s). There was no delay before the beginning of the next trial. Non-signal trials had the same time-course as signal trials, but no electric field was presented.
All subjects were subsequently tested using the SDT rating procedure with two blocks of trials (total of four AC intensities; two intensities/block). For seven subjects, the intensities were chosen on the basis of the results of the psychophysical staircase and spanned the estimated threshold (two above, two below). For nine subjects, four predetermined intensities (8, 17, 26, and 35 kV/m) were used, spanning the entire working range of the exposure chamber. The AC sensitive subjects participated in a further two or three sessions.