Progressive suppression of intracortical inhibition during graded isometric contraction of a hand muscle is not influenced by hand preference

• Published in: Experimental brain research 2007-02, Vol.177 (2), p.266-274
• Main Authors: ZOGHI, Maryam, NORDSTROM, Michael A
• Format: Article
• Language: English
• Subjects: Adult; Biological and medical sciences; Brain research; Electromyography; Evoked Potentials, Motor - physiology; Eye and associated structures. Visual pathways and centers. Vision; Fingers & toes; Functional Laterality - physiology; Fundamental and applied biological sciences. Psychology; gamma-Aminobutyric Acid - metabolism; Hand - innervation; Hand - physiology; Humans; Interneurons - physiology; Isometric Contraction - physiology; Motor control and motor pathways. Reflexes. Control centers of vegetative functions. Vestibular system and equilibration; Motor Cortex - anatomy & histology; Motor Cortex - physiology; Motor Neurons - physiology; Muscle Strength - physiology; Muscle, Skeletal - innervation; Muscle, Skeletal - physiology; Neural Inhibition - physiology; Neural Pathways - anatomy & histology; Neural Pathways - physiology; Neuropsychological Tests; Pyramidal Tracts - physiology; Transcranial Magnetic Stimulation; Vertebrates: nervous system and sense organs
• ISSN: 0014-4819; 1432-1106
• DOI: 10.1007/s00221-006-0669-2

GABAergic intracortical inhibition (ICI) in human motor cortex (M1) assists fractionated activation of muscles, and it has been suggested that hemispheric differences in ICI may contribute to hand preference. Previous studies of this issue have all been conducted at rest, with conflicting results. Testing during voluntary activation may reveal functionally relevant differences. In normal subjects, we assessed (1) operation of ICI circuits during selective activation of an intrinsic hand muscle at different forces, and (2) whether this differs between right and left hemispheres. Surface EMG was recorded bilaterally from abductor pollicis brevis (APB), first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles in eleven right-handed subjects. A circular coil applied paired transcranial magnetic stimulation (TMS) with posteriorly directed current in the brain. Conditioning intensity was 0.8 x active threshold and interstimulus interval was 3 ms. TMS was applied to right or left M1 while subjects were at rest or performing isometric thumb abduction at different forces (0.5, 1, 2, 3, 5 and 10 N) with the contralateral hand. Conditioning TMS was less effective at suppressing the muscle evoked potential in APB during 2-10 N thumb abduction (P < 0.0001) versus rest, but not with lower target forces (0.5, 1 N). Conditioning TMS was less effective for FDI and ADM only during 10 N thumb abduction. We conclude that differential modulation of ICI in M1 during selective muscle activation is a function of target isometric force level. At low forces (