1. Neural control of Movement Laboratory This work has been partially supported by the European Commission with the Collaborative Project no. 248587, “THE Hand Embodied”, within the FP7- ICT-2009-4-2-1 program “Cognitive System and Robotics”. SymmetricAsymmetric Contralateral Ipsilateral Frequency 0 100 50 0 100 50 0100 50 0 100 50 -30 0 30-300 30 Reference hand d y (mm) Absolute error (mm) 40 0 30 20 10 0 4040 20 3030 10 0 4040 20 3030 10 40 0 30 20 10 Absolute error (mm) Sym.Asym. P = 0.001 P = 0.014 PostureHand B Contra.Ipsi. Matching conditions Contra- Sym Contra- Asym Ipsi- Asym Ipsi- Sym C P = 0.001 Reference hand d y (mm) P = 0.001 P = 0.012 A * SymmetricAsymmetric Contralateral Ipsilateral Relative error (mm) Reference hand d y (mm) -30030-30030 060 0 30 060 30 0 60 30 Frequency -10 10 20 30 40 -40 -30 -20 0 -30 -10 10 40 -40 0 30 20 -40 -10 40 -30 -20 0 10 30 20 -10 40 -40 -30 -20 0 10 30 20 Relative error (mm) Reference hand d y (mm) ** -30030 Matching conditions Contra-SymContra-AsymIpsi-AsymIpsi-Sym AB * ** * R hand d y : +30 or –30 mm SymmetricAsymmetric |R hand d y | > |T hand d y |–– |R hand d y | < |T hand d y |++ R hand d y : 0 mm SymmetricAsymmetric R hand d y > T hand d y –– R hand d y < T hand d y ++ B frontal view 80 mm a side view 30 mm z y Matching errora would be greater (a) in the collinear than non-collinear digits positions, (b) when the postures of the R hand and T hand were not congruent, and (c) when subjects reproduced d y using the contralateral hand. Background Dexterous manipulation relies on the ability to coordinate digit forces 1 and positions 2-7. These findings indicate that the central nervous system (CNS) integrates the sense of digit position with motor commands responsible for digit forces. However, little is known about the sense of vertical digits position relative to each other without vision of the hands under controlled force magnitude conditions. Furthermore, the ability to accurately reproduce perceived digit position using the same or the contralateral hand is not clear. In addition, it remains unclear whether congruence of hand posture affects the ability to match digit positions. Hypotheses The present errors associated with somatosensory-based reproduction of finger pad distance indicates that the CNS must implement mechanisms to sense and compensate for finger pad distance to ensure that digit forces are distributed according to the required manipulation constraints. The extent to which these mechanisms include vision of the hand and/or force cues is the subject of ongoing investigation. To determine the factors that affect subjects’ ability to match perceived vertical distance between the thumb and index finger pads (d y ) of the right hand (“reference” hand, R hand ) using the ipsilateral or contralateral hand (“test” hand, T hand ) without vision of the hands. Purpose Experimental setup Experimental conditions Subjects were right-handed 10males and 5 females (23.53 ± 4.5 yrs). The thumb and index finger of R hand were passively placed to one of three d y s (C). Subjects were asked to match the d y of R hand. Matching error was defined as d y of R hand minus d y of T hand and was computed as either absolute or relative error (E abs and E rel, respectively). Protocol Definition of relative matching errors (Erel) Subjects made significantly greater absolute errors (E abs ) in a)collinear (d y = 0 mm) than non-collinear d y s (d y = ± 30mm) (A), b)the non-congruent posture of T hand and R hand (Asymmetric, Posture, B), c)The contralateral hand (Hand, B). During the collinear d y, the E abs in the asymmetric condition was significantly greater than that in the symmetric condition (asterisk, A). The smaller E abs in the symmetric than asymmetric conditions was prominent in the ipsilateral hand (C). Relative error (E rel ) was significantly different from zero in a)The non-collinear conditions (d y = ± 30mm, A) b)Contralateral-Symmetrical, Contralateral-Asymmetrical, and Ipsilateral-Asymmetrical (B). No clear directional bias was found when the R hand d y was collinear (A). Subjects tended to underestimate R hand d y (as indicated by the negative E rel ) in all conditions with the exception of Ipsilateral Symmetrical. 1.Johansson RS, Flanagan JR. (2009). Coding and use of tactile signals from the fingertips in object manipulation tasks. Nature Rev Neurosci. 10:345-359. 2.Crajé C, Lukos JR, Gordon AM, Ansuini C, Santello M. (2011). Planning of multi-digit contact points as a function of task. Exp Brain Res. 212: 119- 124. 3.Fu Q, Zhang W, Santello M. (2010). Anticipatory planning and control of grasp positions and forces for dexterous two-digit manipulation. J Neurosci. 31: 9117-9126. 4.Fu Q, Hasan Z, Santello M. (2011). Transfer of Learned Manipulation following Changes in Degrees of Freedom. J Neurosci. 31: 13576-13584. 5.Lukos J, Ansuini C, Santello M. (2007). Choice of contact points during multidigit grasping: effect of predictability of object center of mass location. J Neurosci 27:3894-3903. 6.Lukos J, Ansuini C, Santello M. (2008). Anticipatory control of grasping: independence of sensorimotor memories for kinematics and kinetics. J Neurosci 28:12765-12774. 7.Zhang W, Gordon AM, Fu Q, Santello M. (2010). Manipulation after object rotation reveals independent sensorimotor memory representations of digit positions and forces. J Neurophysiol. 103: 2953-2964. 8.Adamo DE, Scotland S, Martin BJ. (2012 ). Asymmetry in grasp force matching and sense of effort. Exp Brain Res. 217: 273-285. 9.Gordon AM, Forssberg H, Iwasaki N. (1994).Formation and lateralization of internal representations underlying motor commands during precision grip. Neuropsychologia. 32: 555-568. t = 10 s thumb index finger Contralateral Ipsilateral Symmetric R hand T hand t = 0 st = 10 s R hand T hand t = 0 st = 10 s time R hand T hand Asymmetric R hand T hand t = 0 s t = 10 s time Collinear R hand T hand R hand T hand t = 0 st = 10 s t = 0 st = 10 s time R hand d y : −30 mm R hand d y : +30 mm C R hand d y : 0 mm thumb index finger z y Left handle Right handle monitor “Reference” hand “Test” hand A x y Collinear vs. non-collinear digit positions: The smaller error found for non-collinear digit positions might have resulted primarily from the integration of sensory inputs from skin and joint receptors. Congruent vs. inverse hand configurations: The Asymmetric condition is a more reliable measure of subjects’ ability to integrate sensory feedback to estimate d y regardless of postural sensory cues. It follows that higher-level processing of sensory inputs to estimate fingertip distance leads to greater haptic-motor transformation errors. Contralateral vs. ipsilateral hand: Asymmetric activation of hemispheres 8-9 might have contributed to the greater error found for the contralateral condition. Directionality of matching errors: We speculate that higher-level processing is the primary source of underestimation error when the matching task was performed asymmetrically using the ipsilateral hand. Digit position sensing for dexterous manipulation: For greater digit positions errors that might occur when contacts of one or more digits are blocked from view by the object (i.e., a scenario similar to our present study), greater and more detrimental manipulation performance errors can be expected. Negative relative errors (E rel ) in most conditions with one exception Greater absolute errors (E abs ) in the Collinear, Asymmetric, and Contralateral conditions