Partial Inactivation of the Primary Motor Cortex Hand Area: Effects on Individuated Finger Movements

Abstract

After large lesions of the primary motor cortex (M1), voluntary movements of affected body parts are weak and slow. In addition, the relative independence of moving one body part without others is lost; attempts at individuated movements of a given body part are accompanied by excessive, unintended motion of contiguous body parts. The effects of partial inactivation of the M1 hand area are comparatively unknown, however. If the M1 hand area contains the somatotopically ordered finger representations implied by the classic homunculus or simiusculus, then partial inactivation might produce weakness, slowness, and loss of independence of one or two adjacent digits without affecting other digits. But if control of each finger movement is distributed in the M1 hand area as many studies suggest, then partial inactivation might produce dissociation of weakness, slowness, and relative independence of movement, and which fingers movements are impaired might be unrelated to the location of the inactivation along the central sulcus.To investigate the motoric deficits resulting from partial inactivation of the M1 hand area, we therefore made single intracortical injections of muscimol as trained monkeys performed visually cued, individuated flexion-extension movements of the fingers and wrist. We found little if any evidence that which finger movements were impaired after each injection was related to the injection location along the central sulcus. Unimpaired fingers could be flanked on both sides by impaired fingers, and the flexion movements of a given finger could be unaffected even though the extension movements were impaired, or vice versa. Partial inactivation also could produce dissociated weakness and slowness versus loss of independence in a given finger movement. These findings suggest that control of each individuated finger movement is distributed widely in the M1 hand area.Key words: cortex; dexterity; finger; individuation; macaque; motor; muscimol; response time, somatotopy; weaknessThe classic homunculus and simiusculus implied that a separate region of the primary motor cortex (M1) moves each digit of the hand (Penfield and Rasmussen, 1950;Woolsey et al., 1951). Evidence now suggests, however, that control of any finger movement is distributed throughout the M1 hand area (see Discussion), calling into question the degree to which a somatotopic map of the fingers mediates the crucial contribution of M1 to performance of individuated finger movements. Clearly, lesions of lateral M1 impair voluntary movement of the face much more than the leg and vice versa for medial M1 lesions. If such somatotopic organization extends to the level of different fingers, then lateral lesions within the hand area should impair thumb movements more than little finger movements and vice versa for medial lesions. We therefore sought to reversibly inactivate only part of the M1 hand area, because such within-hand somatotopic effects would not have been evident in previous studies of lesions in M1 or the corticospinal tract. These ...

Dissociation of Sensorimotor Deficits After Rostral Versus Caudal Lesions in the Primary Motor Cortex Hand Representation

“…Because in most primate species, M1c is buried in the anterior bank of the central sulcus, selective injury or inactivation has rarely been attempted. However, observations of M1 stroke patients (Kim 2001;Schieber 1999) and M1 inactivation in monkeys (Brochier et al 1999;Fogassi et al 2001;Kubota 1996;Schieber and Poliakov 1998) have demonstrated that caudal inactivation results in selective deficits in individuation of finger movements and relative preservation of reaching.…”

Role of primary motor cortex in the control of manual dexterity assessed via sequential bilateral lesion in the adult macaque monkey: A case study

“…Rizzolatti and Luppino, 2001;Shelton and Reding, 2001;Olivier et al, 2007;Brown and Teskey, 2014). In monkeys, alterations of hand movements have been reported after unilateral transient inactivation or permanent lesion of M1 followed by the occurrence of compensatory strategies, affecting motor parameters such as force (Brochier et al, 1999), trajectory (Cirstea and Levin, 2000), precision grip (Brochier et al, 1999;Darling et al, 2009Darling et al, , 2011bDarling et al, , 2013Darling et al, , 2014Kermadi et al, 1997;Liu and Rouiller, 1999;Kaeser et al, 2010;Hoogewoud et al, 2013;Morecraft et al, 2015Morecraft et al, , 2016Wyss et al, 2013;Murata et al, 2015), flexion-extension (Schieber and Poliakov, 1998) and wrist movement (Hoffman and Strick, 1995). Depending on the size and location of the injury, spontaneous recovery occurs to a variable extent, though it is generally incomplete.…”

Dissociating the Role of Ventral and Dorsal Premotor Cortex in Precision Grasping

“…Because lesions of the corticospinal (CS) tract often impair irremediably dexterous finger movements (Forssberg et al, 1999;Duque et al, 2003;Hermsdorfer et al, 2003), the contribution of the CS system [and that of its main cortical area of origin, the primary motor cortex (M1)] to such movements has been extensively studied (Muir and Lemon, 1983;Schieber and Poliakov, 1998;Brochier et al, 1999).…”