Changes in spinal excitability during choice reaction time: The H reflex as a probe of information transmission

The aim of the present study was to investigate the modulations in amplitud e of H reflexes elicited in a hand muscle, the, flexor pollicis brevis, dur ing the performance of a choice reaction time (RT) task in which this muscl e was directly involved. Ten subjects were to choose between a left- or a r ight-thumb key-press according to the lateral location of a flash of light. The stimulus-response mapping was either compatible or incompatible. Hoffm an reflexes were elicited at different times during the RT by stimulation o f the median nerve. Twenty-five milliseconds before the voluntary response, the amplitude of the H reflex suddenly increased when the muscle was invol ved in the response and decreased symmetrically when the muscle was not inv olved in the response. Mapping compatibility exerted no detectable influenc e on the changes in spinal excitability. The latter result supports the ass umptions that are at the core of Sternberg's additive factor method.