Many actions involve limb movements toward a target. Visual and proprioceptive estimates are available online, and by optimally combining (Ernst and Banks, 2002) both modalities during the movement, the system can increase the precision of the hand estimate. The notion that both sensory modalities are integrated is also motivated by the intuition that we do not consciously perceive any discrepancy between the felt and seen hand's positions. This coherence as a result of integration does not necessarily imply realignment between the two modalities (Smeets et al., 2006). For example, the two estimates (visual and proprioceptive) might be different without either of them (e.g., proprioception) ever being adjusted after recovering the other (e.g., vision). The implication that the felt and seen positions might be different has a temporal analog. Because the actual feedback from the hand at a given instantaneous position reaches brain areas at different times for proprioception and vision (shorter for proprioception), the corresponding instantaneous unisensory position estimates will be different, with the proprioceptive one being ahead of the visual one. Based on the assumption that the system integrates optimally and online the available evidence from both senses, we introduce a temporal mechanism that explains the reported overestimation of hand positions when vision is occluded for active and passive movements (Gritsenko et al., 2007) without the need to resort to initial feedforward estimates (Wolpert et al., 1995). We set up hypotheses to test the validity of the model, and we contrast simulation-based predictions with empirical data.
It is remarkable that the movement time of a goal-directed movement, the result of complex coordination in the nervous system, can be predicted by a simple mathematical equation. That equation is Fitts' law, and it is one of only a few laws that capture human motor performance. It has recently been shown that reaches to targets with placeholders modulate Fitts' law (e.g. Adam et al. in Psychol Sci 17(9):794-798, 2006). The purpose of this study was to further test whether the modulation to Fitts' law is a result of processes related to movement preparation or movement execution. Preparation and control processes were isolated with trajectory analysis; specifically, the durations of the primary submovement and the secondary submovement were selected to reflect the preparation and control processes, respectively. The time available for movement preparation was also manipulated by precuing the target in some blocks. We found that the modulation to Fitts' law in total movement time with target placeholders occurred during the secondary submovement, suggesting that control processes were the locus of the modulation. However, extending the duration of preparation with a precue eliminated the modulation in total movement time, which suggests that preparation processes were the locus of the modulation. Based on these results, it is premature to isolate unequivocally the modulation to either preparation or control processes. The modulation to Fitts' law during the secondary submovement presents the possibility that facilitated online control may contribute to the modulation.
The goal of this study was to determine the process or processes most likely to be involved in reaction-time costs for spatially cued bimanual reaching. We used reaction time to measure the cost of bimanual symmetric movements compared to unimanual movements (a bimanual symmetric cost) and the cost for bimanual asymmetric movements compared to symmetric movements (a bimanual asymmetric cost). The results showed that reaction times were comparable for all types of movements in simple reaction time; that is, there was neither a bimanual symmetric cost nor an asymmetric cost. Therefore, unimanual, bimanual symmetric, and bimanual asymmetric movements have comparable complexity during response initiation. In choice conditions, there was no bimanual symmetric cost but there was a bimanual asymmetric cost, indicating that the preparation of asymmetric movements is more complex than symmetric movements. This asymmetric cost is likely the result of interference during response programming.
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