Handedness and the spatial modulation of drift magnitude
We previously showed that drift is maximal in the habitual action space of the arm, that is, when the hand is near the shoulder of origin (Dempsey-Jones and Kritikos 2017). Here, however, we show a significant modulation of drift with proximity to the action space for the dominant hand alone. In our previous study, we did not identify an effect of handedness on the spatial drift effect because, critically, we did not examine left-handed and right-handed groups separately for the left and right hands. Thus, we were unable to identify that the spatial effect varied between dominant and non-dominant hands. The current study, therefore, extends this previous work to reveal further details about how action may influence multisensory integration in ways that are more specific and nuanced than previously conceptualised. Interestingly, a recent study by Smit et al. (2017) did not find any relationship between handedness and the hand used for RHI on drift magnitude. We suggest the divergence in results may stem from the immersive RHI induction apparatus and real hand images used in the current study providing a stronger illusion, allowing more power to detect this drift effect.
But why might a concentration of hand action around a particular area lead to greater integration of visual and proprioceptive body information in this space? It is known that the integration of multisensory inputs improves accuracy and speed of perception when compared to single sensory input sources (reviewed in Calvert et al. 2004). This is reflected in the properties of ‘superadditive’ multisensory neurons that show larger responses to multisensory stimulation than the sum of the response to individual sensory components alone (Stein and Meredith 1993). It may be that the requirement for precision where the hand most commonly performs manual actions leads to a greater integration in this area, to improve perception and task performance. In the case of the RHI, greater integration of visual and proprioceptive inputs in this space leads to a stronger illusion—due to the spatial displacement of visual and proprioceptive inputs (see above). In this specific and unusual case, it may be that the mechanism is subverted to cause less accurate perception of the hand location in this space (more drift), where typically it would cause enhanced perception by integration.
As stated above, our results suggest this action-based effect is significant for the hand that is used more frequently in action, the dominant hand. This dominance-related finding is consistent with those from the popular hand laterality paradigm (introduced by Parsons 1987, 1994). In this task, identification of the laterality of a hand stimulus (‘left or right?’) is slowed when the stimulus is presented at more extreme rotations, e.g., 0° or 180° (Brady et al. 2011). While various factors have been shown to modulate this laterality effect, these modulatory effects often only hold for the dominant hand, e.g., pinched vs. open hand positions (Meugnot et al. 2015), and other tasks (Ni Choisdealbha et al. 2011; Parsons 1987). Because performance on the task has been suggested to rely on retrieval of the internal motor model for the seen hand (Brady et al. 2011) as well as motor imagery (Parsons 1987, 1994), this suggests activation of dominant hand representations causes greater engagement of sensorimotor processes. Similar effects have been seen in other tasks. For instance, Hoover et al. (2016) showed superior detection of movement for self vs. other hand stimuli, only in the dominant hand. They attributed this to enhanced ownership and embodiment of the dominant hand, suggesting there may also be a role for subjective aspects of body representation in handedness effects. Therefore, a variety of lower- and higher-level factors might combine to cause stronger effects for the dominant hand across paradigms (also see Gandrey et al. 2013 for improved mentally simulated actions in the dominant hand of right-handers).