Abstract
The aim of this study was to characterize, using fMRI, the functional asymmetries of hand laterality task (HLT) in a sample of 295 participants balanced for handedness. During HLT, participants have to decide whether the displayed picture of a hand represent a right or a left hand. Pictures of hands’ back view were presented for 150 ms in the right or left hemifield. At the whole hemisphere level, we evidenced that the laterality of the hand and of the hemifield in which the picture was displayed combined their effects on the hemispheric asymmetry in an additive way. We then identified a set of 17 functional homotopic regions of interest (hROIs) including premotor, motor, somatosensory and parietal regions, whose activity and asymmetry varied with the laterality of the presented hands. When the laterality of a right hand had to be evaluated, these areas showed stronger leftward asymmetry, the hROI located in the primary motor area showing a significant larger effect than all other hROIs. In addition a subset of six parietal regions involved in visuo-motor integration together with two postcentral areas showed a variation in asymmetry with hemifield of presentation. Finally, while handedness had no effect at the hemispheric level, two regions located in the parietal operculum and intraparietal sulcus exhibited larger leftward asymmetry with right handedness independently of the hand of presentation. The present results extend those of previous works in showing a shift of asymmetries during HLT according to the hand presented in sensorimotor areas including primary motor cortex. This shift was not affected by manual preference. They also demonstrate that the coordination of visual information and handedness identification of hands relied on the coexistence of contralateral motor and visual representations in the superior parietal lobe and the postcentral gyrus.
Highlights
The hand laterality task (HLT) consists in identifying if the hand presented in a picture is a left or a right one
Given that motor activity is one of the most strongly lateralized function in the brain, the left hand being controlled by the right motor cortex and viceversa, it could be hypothesized that the asymmetry of activation in HLT varies according to the left or right handedness of the presented hand
The results showed a main effect of Orientation (F(1,283.2) = 5.2, p = 0.02, η2 = 2%) medial orientation being identified faster than lateral, as well as an interaction between the hand laterality (Hand) and Orientation factors (F(1,274.8.1) = 99.1, p < 0.0001, η2 = 27%): corrected post hoc showed that medial orientation led to shorter reaction times (RT) than lateral for left hands while the opposite was observed for right hands (p < 0.05 for both)
Summary
The hand laterality task (HLT) consists in identifying if the hand presented in a picture is a left or a right one. Cortical Asymmetries during Hand Laterality Task including, premotor and superior parietal regions (Parsons et al, 1995; Kosslyn et al, 1998; Vingerhoets et al, 2002; Wraga et al, 2003; Seurinck et al, 2004; de Lange et al, 2005, 2006; Ferri et al, 2012; Zapparoli et al, 2014) Among these studies, only Kosslyn et al (1998) reported an activation in primary motor cortex, while Berneiser et al (2016) described that activity increased in the right primary motor cortex after an intensive training to HLT. Such an observation would be a convincing hint that one mentally moves the same hand than the one to be identified during this task, supporting the embodied nature of HLT
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