Is there a left hemispheric asymmetry for tool affordance processing?

The perception of tools vs. other objects has been shown to activate the left premotor and somatosensory cortex, which represents object affordance associated with tool manipulability (Proverbio, Adorni, & D'Aniello, 2011). The question of whether hemispheric asymmetry depends on right hand use or is linked to a hemispheric functional specialization for fine-grained precision movement is unclear. Thus, in this paper, ERPs were recorded from 128 sites in response to the visual presentation of bidimensional (2D) pictures depicting unimanual (e.g., a hammer) and bimanual (e.g., a handlebar) tools (Study 1). Central N2 and prefrontal N400 components were much larger for bimanual than unimanual tools (over the left hemisphere for N400). SwLORETAs performed for both components showed at first the activation of the left parietal cortex (BA39) and then of the right homologous (BA40) one, for both grips but stronger for the bimanual coordination. At all times and for both grips, the left premotor cortex (BA6) was involved in coding action affordance, while only unimanual tools activated the left postcentral gyrus (BA3). In Study 2, unimanual tools were presented with an orientation congruent (standard) or incongruent to their interaction with the right hand (rotated), to manipulate affordance's quality. Standard objects elicited much larger ERP responses (namely: N1, N2, N400) than rotated tools (over the left hemisphere for N400). At the earliest stage (190-270ms) the significant intracranial sources were of visual nature (mainly the contralateral precuneus). Regions representing motor information were not involved. Rotated tools induced a smaller activation in the STS and parahippocampal regions (possibly coding affordable biological motion and the spatial aspects of hand/object interaction), whereas rotated tools activated to a greater extent the dorsolateral prefrontal cortex (DLPF, BA9). In the later time window standard objects activated the left BA6 and the right BA40 more than rotated objects. Overall, these data suggest that viewing tools automatically activates mental representations associated with their manipulation. The left premotor cortex was found to be involved with any kind of object and grip, as early as 200ms post-stimulus, thus supporting the hypothesis of a LH asymmetry in the neural representation of grasping, within this region. The right supramarginal gyrus was also found to be crucially involved later in time.