Left visual field preference for a bimanual grasping task with ecologically valid object sizes.

Grasping using two forelimbs in opposition to one another is evolutionary older than the hand with an opposable thumb (Whishaw and Coles in Behav Brain Res 77:135-148, 1996); yet, the mechanisms for bimanual grasps remain unclear. Similar to unimanual grasping, the localization of matching stable grasp points on an object is computationally expensive and so it makes sense for the signals to converge in a single cortical hemisphere. Indeed, bimanual grasps are faster and more accurate in the left visual field, and are disrupted if there is transcranial stimulation of the right hemisphere (Le and Niemeier in Exp Brain Res 224:263-273, 2013; Le et al. in Cereb Cortex. doi: 10.1093/cercor/bht115, 2013). However, research so far has tested the right hemisphere dominance based on small objects only, which are usually grasped with one hand, whereas bimanual grasping is more commonly used for objects that are too big for a single hand. Because grasping large objects might involve different neural circuits than grasping small objects (Grol et al. in J Neurosci 27:11877-11887, 2007), here we tested whether a left visual field/right hemisphere dominance for bimanual grasping exists with large and thus more ecologically valid objects or whether the right hemisphere dominance is a function of object size. We asked participants to fixate to the left or right of an object and to grasp the object with the index and middle fingers of both hands. Consistent with previous observations, we found that for objects in the left visual field, the maximum grip apertures were scaled closer to the object width and were smaller and less variable, than for objects in the right visual field. Our results demonstrate that bimanual grasping is predominantly controlled by the right hemisphere, even in the context of grasping larger objects.