CHANGES IN SPINAL REFLEX EXCITABILITY IN A COUNTERMANDED TIMED RESPONSE TASK

Subjects (N = 8) performed a timed response task in which they attempt ed to synchronize an impulsive foot-press response with the last in a series of four regularly spaced tones. In Experiment 1, the response w as countermanded on one third of the trials (stop trials) by a stop si gnal that appeared at a predetermined delay after the third tone. No s top signal appeared on the remaining trials (go trials). All subjects showed a systematic transition from withholding the response on stop t rials in which the stop signal appeared shortly after the third tone t o executing the response on trials in which the stop signal appeared a fter longer delays. In Experiment 2, the same subjects performed the t imed response task for which a single stop signal delay had been chose n so that a response would be made on about 50% of the stop trials. We elicited Hoffmann (H) reflexes from the soleus muscle on all trials t o determine whether the reflexes were augmented on occasions on which a response was prepared but withheld. Mean H-reflex amplitudes on go t rials and on stop trials on which the response was executed were simil ar and showed a marked augmentation beginning about 250 ms before resp onse onset: mean H-reflex amplitudes on stop trials on which the respo nse was withheld showed less pronounced augmentation. Inspection of in dividual H-reflex amplitudes revealed that on stop trials on which the response was withheld the reflexes could be augmented to the same ext ent as on trials on which the response was executed. This dissociation of H-reflex augmentation and response execution shows that H-reflex a ugmentation reflects a controlled response process. Ballistic response processes therefore must be limited to a brief duration.