The effect of handedness on spinal and supra-spinal reflex excitability.

Twenty-four subjects, 8 right-handers, 8 left-handers, and 8 familial left-handers, participated in a study aimed at understanding the relations between handedness, cerebral asymmetry, and a possible asymmetry in spinal excitability. This was accomplished by employing 3 different techniques of investigating the H reflex, measuring 3 different parameters of spinal excitability. The recovery curve height (RCH) of left-handers was significantly higher than the RCH of right-handers, and, as well, familial left-handers exhibited higher RCH than non-familial left-handers. The recovery ability was not found to be significantly more inhibited on the dominant side, as would be expected from previous work. Instead, recovery ability was associated with the fractional involvement of the motoneuron pool in the H process (H/M ratio). RCH was highly correlated with both hand preference and hand performance. The above results suggest involvement of a long-loop transcortical reflex in lateral asymmetries.