A trial began with the onset of the green LED (50 ms);
500 ms later the red LED was lit, either strongly (0.8 mcd) or
weakly (0.2 mcd). Stimulus intensity was varied within blocks.
The response extinguished the stimulus. The next trial started
200 ms after the response. In 20% of the trials, the red LED
was not lit and subjects were not to respond in order to avoid
anticipation. Within a block (68 trials), stimulus intensity (strong
versus weak) was presented equally often according to a pseudorandom
series. The first two blocks were practice blocks and
were discarded. After a warm-up period (5 min), the subjects performed
three consecutive blocks during exercise (15 min while
cycling at 50% of PV˙ O2 max with a freely chosen pedal rate)2
and three consecutive blocks at rest. The order of exercise and
rest was counter-balanced across subjects