Simultaneity and space representation

Executive SummarySimultaneity and space representation, #2011066, A. Karniel, I. Nisky, and F.A. Mussa-IvaldiWe studied two related hypotheses: (1) that alteration of simultaneity between the visual, haptic, and proprioceptive modalities results in deformations of the proprioceptive space; and that (2) such alteration is responsible for disturbances of space perception, such as hemi-neglect.

In a continuous tracking task, we found evidence suggesting that the motor system is more likely using an equivalent mechanical system rather than time-dependent representations to resolve mismatches between visual and proprioceptive feedback (Leib, et al., submitted). Such representation can also successfully explain space warping effects following a "virtual pong" game with delayed visual and haptic feedback about the movement of the ball, the paddle, and their collision. Following the game, subjects made larger reaching movements, and their blind tracking performance revealed an expanded space effect (Avraham et al., 2014a). Both effects are consistent with a model of internal representation of a mechanical system connecting the paddle to the hand (Bekker et al., 2014, Avraham, et al., 2014b, Avraham, et al., submitted). Interestingly, while the recalibration effects generalized across tasks, the perceived location of the hand remained unaltered (Sulimani, Weiss, et al., 2015), suggesting an implicit rather than explicit recalibration process. On average, the warping effects are similar for small and large delays, and they develop gradually regardless whether delay is introduced gradually or abruptly, further supporting the implicit mechanism hypothesis (Avraham et al., 2015c, Avraham et al., 2015d). The hypermetric movements during the pong game were associated with an increase in movement duration rather than maximum velocity, and this strategy might be beneficial if indeed the mechanical system-based delay representation is used (Avraham et al., in preparation).

In contrast to these findings, we found new evidence overturning our working hypothesis that the only way that the motor system can represent delayed signals is through correlating them with (non-delayed) current state. We found that while delay biases perceived stiffness (Pressman et al., 2007, Nisky et al., 2008, Nisky et al., 2010, Nisky et al., 2011), grip force during probing is adjusted to the correct stiffness level and with correct timing (Leib et al., 2015). Using a similar paradigm we found that non-invasive long lasting stimulation (cTBS) on the posterior parietal cortex (but not premotor cortex) enhances the effect of delay. In the context of previous research, this can only be explained if the parietal cortex is where position and force signals are integrated to form a perception of stiffness, and if cTBS alters the synchronization between the two signals (Leib et al., 2016). Another evidence for using a delayed state information was obtained in an adaptation study to velocity-dependent force fields with delay: we found that the motor system uses a combination of non-delayed and delayed state primitives to predict the perturbing forces and apply the appropriate counteracting forces (Avraham et al., 2015a, Avraham et al., 2015b, Avraham et al., submitted). This representation generalizes to larger velocities than experienced with delays. Further, delay representation is likely used to cope with delayed information when the control task is a newly acquired redundant mapping between complicated hand movements and movement of a cursor. (Farshchiansadegh et al., 2015). The latter findings support an explicit representation of the temporal delay when low-order state-based representations may not be adequate to predict the sensory consequences of motor commands. We further explored the organization of movements in the presence of complex dynamics. These findings demonstrate that the brain can solve a complex optimization problem that takes into account the geometrical and mechanical properties of an external object being moved by the hand and follow energy-optimal trajectories that are unfamiliar (Farshchiansadegh et al., 2016).

To investigate further what kind of spatio-temporal perturbation may lead to spatial distortions, we designed a series of studies with adaptation to asymmetrical delays in visual feedback. We found that following adaptation to delay in half of the space, participants performed asymmetrical drawings, elongated only in one side of the circle, and the transfer effects did not depend on the side where the delay was applied, but rather on the side where the drawing was initiated (Avraham C. et al., 2016a, 2016b, in preparation). This indicates that asymmetrical temporal perturbations can cause asymmetrical movements across space, and may be related to motor disorders with symptoms of asymmetrical movements such as hemispatial neglect.