Gerome Manson

On the relationship between the execution, perception, and imagination of action

Humans can perform, perceive, and imagine voluntary movement. Numerous investigations of these abilities have employed variants of goal-directed aiming tasks because the Fittss law equation reliably captures the mathematical relationship between movement time (MT) and accuracy requirements. The emergence of Fittss speed-accuracy relationship during movement execution, perception, and imagination has led to the suggestion that these processes rely on common neural codes. This common coding account is based on the notion that the neural codes used to generate an action are tightly bound to the codes that represent the perceptual consequences of that action. It is suggested that during action imagination and perception the bound codes are activated offline through an action simulation. The present study provided a comprehensive testing of this common coding hypothesis by examining the characteristics of the Fitts relationship in movement execution, perception, and imagination within the same individuals. Participants were required to imagine and perceive reciprocal aiming movements with varying accuracy requirements before and after actually executing the movements. Consistent with the common coding account, the Fitts relationship was observed in all conditions. Critically, the slopes of the regression lines across tasks were not different suggesting that the core of the speed-accuracy trade-off was consistent across conditions. In addition, it was found that incidental limb position variability scaled to the amplitude of imagined movements. This motor overflow suggests motor system activation during action imagination. Overall, the results support the hypothesis that action execution, perception, and imagination rely on a common coding system.

Action Possibility Judgments of People with Varying Motor Abilities Due to Spinal Cord Injury

Predictions about ones own action capabilities as well as the action capabilities of others are thought to be based on a simulation process involving linked perceptual and motor networks. Given the central role of motor experience in the formation of these networks, ones present motor capabilities are thought to be the basis of their perceptual judgments about actions. However, it remains unknown whether the ability to form these action possibility judgments is affected by performance related changes in the motor system. To determine if judgments of action capabilities are affected by long-term changes in ones own motor capabilities, participants with different degrees of upper-limb function due to their level (cervical vs. below cervical) of spinal cord injury (SCI) were tested on a perceptual-motor judgment task. Participants observed apparent motion videos of reciprocal aiming movements with varying levels of difficulty. For each movement, participants determined the shortest movement time (MT) at which they themselves and a young adult could perform the task while maintaining accuracy. Participants also performed the task. Analyses of MTs revealed that perceptual judgments for participants own movement capabilities were consistent with their actual performance- people with cervical SCI had longer judged and actual MTs than people with below cervical SCI. However, there were no between-group differences in judged MTs for the young adult. Although it is unclear how the judgments were adjusted (altered simulation vs. threshold modification), the data reveal that people with different motor capabilities due to SCI are not completely biased by their present capabilities and can effectively adjust their judgments to estimate the actions of others.

Auditory gating during visually-guided action?

We recently used an audiovisual illusion (Shams et al., 2000) during fast and accurate reaching movements and showed that susceptibility to the fusion illusion is reduced at high limb velocities (Tremblay and Nguyen, 2010). This study aimed to determine if auditory information processing is suppressed during voluntary action (Chapman and Beauchamp, 2006), which could explain reduced fusion during reaching movements. Instead of asking our participants ( n = 10 ) to report the number of flashes, we asked them to report the number of beeps (Andersen et al., 2004). Before each trial, participants were asked to fixate on a target LED presented on a horizontal reaching surface. The secondary stimuli combined 3 flash (0, 1, 2) by 2 beep (1, 2). During control tests, the secondary stimuli were presented at rest. In the experimental phase, stimuli were presented 0, 100 or 200 ms relative to the onset of a fast and accurate movement. Participants reported the number of beeps after each trial. A 3 flash × 2 beep × 4 presentation condition (0, 100, 200 ms + Control) ANOVA revealed that participants were less accurate at perceiving the actual number of beeps during the movement as compared to the control condition. More importantly, the number of flashes influenced the number of perceived beeps during the movement but not in the control condition. Lastly, no relationship was found between limb velocity and the number of perceived beeps. These results indicate that auditory information is significantly suppressed during goal-directed action but this mechanism alone fails to explain the link between limb velocity and the fusion illusion.

Effects of robotic guidance on sensorimotor control: Planning vs. online control?

BACKGROUND Robotic guidance has been shown to facilitate motor skill acquisition, through altered sensorimotor control, in neurologically impaired and healthy populations. OBJECTIVE To determine if robot-guided practice and online visual feedback availability primarily influences movement planning or online control mechanisms. METHODS In this two-experiment study, participants first performed a pre-test involving reaches with or without vision, to obtain baseline measures. In both experiments, participants then underwent an acquisition phase where they either actively followed robot-guided trajectories or trained unassisted. Only in the second experiment, robot-guided or unassisted acquisition was performed either with or without online vision. Following acquisition, all participants completed a post-test that was the same as the pre-test. Planning and online control mechanisms were assessed through endpoint error and kinematic analyses. RESULTS The robot-guided and unassisted groups generally exhibited comparable changes in endpoint accuracy and precision. Kinematic analyses revealed that only participants who practiced with the robot exhibited significantly reduced the proportion of movement time spent during the limb deceleration phase (i.e., time after peak velocity). This was true regardless of online visual feedback availability during training. CONCLUSION The influence of robot-assisted motor skill acquisition is best explained by improved motor planning processes.