Finger flexion contrasted to extension activates higher-order motor circuitry

Abstract

Purposeful movement such as grasping implies tuning of object shape and hand posture. This is organised by circuitry distributed over parietal and premotor regions. Within this circuitry, a sharp spatial segregation between the perceptual and motor representations does not exist. E.g., only observing objects to be grasped activates premotor cortex. Making hand postures, independent from target shape, is related to parietal activation. Moreover, the finding of mirror neurons shows that the observation of grasping by others, induces the activation of premotor regions mimicking one's own grasping. Even indirect stimuli such as the noise of hammering, elicits premotor activations reflecting the potential of using a hammer oneself. Finally, parieto-premotor circuitry is implicated in imagining task-related movements. In this study, we posed 2 questions. (1) Is simple finger flexion specifically associated with activation of higher-order motor circuitry, reflecting a connection like the above listed perceptual entrances? We hypothesised an intimate relation between such circuitry and finger flexion, because finger extension only provides an initial aperture, whereas subsequent flexion scales handmovement, enabling adequate grasping. (2) Over the motor cortex, the topographic representation of fingers is well documented. Here, we looked for segregation between flexion and extension movements of the same digits along the central sulcus, equivalent to the described proximal-distal representation of hand sensation. Cerebral BOLD responses were studied in twelve right-handed subjects (3 T fMRI, Philips). Subjects listened by headphone to random beeps (20 per min). In the flexor movement condition, responses were made to each beep by 2 flexion movements with digits 2–5 (left hand). In the extensor condition, extension movements were similarly made. The control condition was listening to beeps. Data were acquired in four sessions, each constituted by four 30 s lasting motor-task blocks. Each motor block was preceded by a control block. The order of the two motor conditions was balanced. In each block, 11 brain volumes of 46 slices were obtained. Analysis was done by Statistical Parametric Mapping (SPM2, London). The 2 movement conditions were contrasted with control and with each other. Comparing left-hand finger flexion with extension showed activation of the left parietal cortex (x -32, y -60, z 56) and posterior parts of the insula, bilaterally (P<0.05, corrected whole brain volume; smoothing filter 10 mm). Parietal activation supported our hypothesis, and points at an association with praxis. Activation of the posterior insula is consistent with its supporting role in skeletomotor control. Compared to rest, the motor tasks showed overlapping activation along the contralateral central sulcus, at the hand representation. The centre of gravity for flexion was more superficial than for extension. Contrasted to each other (P<0.05, uncorrected; smoothing 4 mm), finger flexion activated a lateral part of the motor cortex (Brodmann's Area BA 4), extending over the surface of the precentral gyrus (BA 6). Extension movement was related to activation deep in the central sulcus. In conclusion, the relation between finger flexion and parieto-premotor circuitry, indicates that these simple movements may be regarded as building-stones for complex movements such as grasping.