Does the Integration of Haptic and Visual Cues Reduce the Effect of a Biased Visual Reference Frame on the Subjective Head Orientation?

The selection of appropriate frames of reference (FOR) is a key factor in the elaboration of spatial perception and the production of robust interaction with our environment. The extent to which we perceive the head axis orientation (subjective head orientation, SHO) with both accuracy and precision likely contributes to the efficiency of these spatial interactions. A first goal of this study was to investigate the relative contribution of both the visual and egocentric FOR (centre-of-mass) in the SHO processing. A second goal was to investigate humans' ability to process SHO in various sensory response modalities (visual, haptic and visuo-haptic), and the way they modify the reliance to either the visual or egocentric FORs. A third goal was to question whether subjects combined visual and haptic cues optimally to increase SHO certainty and to decrease the FORs disruption effect.Thirteen subjects were asked to indicate their SHO while the visual and/or egocentric FORs were deviated. Four results emerged from our study. First, visual rod settings to SHO were altered by the tilted visual frame but not by the egocentric FOR alteration, whereas no haptic settings alteration was observed whether due to the egocentric FOR alteration or the tilted visual frame. These results are modulated by individual analysis. Second, visual and egocentric FOR dependency appear to be negatively correlated. Third, the response modality enrichment appears to improve SHO. Fourth, several combination rules of the visuo-haptic cues such as the Maximum Likelihood Estimation (MLE), Winner-Take-All (WTA) or Unweighted Mean (UWM) rule seem to account for SHO improvements. However, the UWM rule seems to best account for the improvement of visuo-haptic estimates, especially in situations with high FOR incongruence. Finally, the data also indicated that FOR reliance resulted from the application of UWM rule. This was observed more particularly, in the visual dependent subject. Conclusions: Taken together, these findings emphasize the importance of identifying individual spatial FOR preferences to assess the efficiency of our interaction with the environment whilst performing spatial tasks

Differential exploitation of the inertia tensor in multi-joint arm reaching

International audienceThe identification of the kinaesthetic information used for directing 3D multi-joint arm movements toward a target remains an open question. Several psychophysical studies have suggested that the ability to perceive and control the spatial orientation of our limbs depends on the exploitation of the eigenvectors (e (3)) of the inertia tensor (I ( ij )), which correspond to the arm rotational inertial axes. The present experiment aimed at investigating whether e (3) was used as a collective variable to direct the masses toward the target and hence to control the spatial accuracy of the final hand position. Natural, unconstrained, three-dimensional multi-joint reaching movements were submitted to alterations of forearm mass distribution. Given the existence of several "sensorimotor strategies" for the control of arm movements, the participants were a priori contrasted and ranged in groups according to their reliance on either visual or kinaesthetic information. The results indicated (1) the dependency of the arm's directional control on I ( ij ) parameters, (2) a non-linear relationship between the performance predicted by the inertia tensor and the observed performance, depending on the deviation amplitude and (3) the presence of a large inter-individual variability suggesting the existence of different strategies, including proprioceptive compensation mechanisms. This study validates in unconstrained multi-joint arm movements the exploitation of the inertia tensor by the central nervous system, thus simplifying the coordination of the segments' masses during reaching. The results also provide evidence for the existence of motor alternatives in exploiting proprioceptive information that may depend on spatial referencing modes