Motor errors lead to enhanced performance in older adults.

Young individuals make larger and faster forearm movements when visual feedback about the movement is not available, compared to when it is. We set out to test whether this behavior persists with aging. We tested 40 participants, 20 in each age group - young and old, on a task that required making rhythmic movements of the forearm with and without visual feedback. Surprisingly, we found that older adults increased the speed and the amplitude of their movements to an even greater extent than did the young adults. Furthermore, we found that the increase in speed and amplitude during the non-vision trial segments improved their performance on the task, and they were able to leverage the change in these movement parameters (speed and amplitude) to improve their performance during subsequent trial segments that did include visual feedback. The improvement in accuracy on the task was accompanied by a decrease in path variability. The results indicate that older adults can adapt their movement parameters to enhance performance following a motor perturbation. They further suggest that motor variability in old age can be advantageous under certain circumstances.

Aging and Sensory Substitution in a Virtual Navigation Task.

Virtual environments are becoming ubiquitous, and used in a variety of contexts-from entertainment to training and rehabilitation. Recently, technology for making them more accessible to blind or visually impaired users has been developed, by using sound to represent visual information. The ability of older individuals to interpret these cues has not yet been studied. In this experiment, we studied the effects of age and sensory modality (visual or auditory) on navigation through a virtual maze. We added a layer of complexity by conducting the experiment in a rotating room, in order to test the effect of the spatial bias induced by the rotation on performance. Results from 29 participants showed that with the auditory cues, it took participants a longer time to complete the mazes, they took a longer path length through the maze, they paused more, and had more collisions with the walls, compared to navigation with the visual cues. The older group took a longer time to complete the mazes, they paused more, and had more collisions with the walls, compared to the younger group. There was no effect of room rotation on the performance, nor were there any significant interactions among age, feedback modality and room rotation. We conclude that there is a decline in performance with age, and that while navigation with auditory cues is possible even at an old age, it presents more challenges than visual navigation.

Fast, accurate reaching movements with a visual-to-auditory sensory substitution device.

PURPOSE: Visual sensory substitution devices (SSDs) use sound or touch to convey information that is normally perceived by vision. The primary focus of prior research using SSDs was the perceptual components of learning to use SSDs and their neural correlates. However, sensorimotor integration is critical in the effort to make SSDs relevant for everyday tasks, like grabbing a cup of coffee efficiently. The purpose of this study was to test the use of a novel visual-to-auditory SSD to guide a fast reaching movement. METHODS: Using sound, the SSD device relays location, shape and color information. Participants were asked to make fast reaching movements to targets presented by the SSD. RESULTS: After only a short practice session, blindfolded sighted participants performed fast and accurate movements to presented targets, which did not differ significantly from movements performed with visual feedback in terms of movement time, peak speed, and path length. A small but significant difference was found between the endpoint accuracy of movements under the two feedback conditions; remarkably, in both cases the average error was smaller than 0.5 cm. CONCLUSIONS: Our findings combine with previous brain-imaging studies to support a theory of a modality-independent representation of spatial information. Task-specificity, rather than modality-specificity, of brain functions is crucially important for the rehabilitative use of SSDs in the blind and the visually impaired. We present the first direct comparison between movement trajectories performed with an SSD and ones performed under visual guidance. The accuracy level reached in this study demonstrates the potential applicability of using the visual-to-auditory SSD for performance of daily tasks which require fast, accurate reaching movements, and indicates a potential for rehabilitative use of the device.

Rhythmic movements are larger and faster but with the same frequency on removal of visual feedback.

The brain controls rhythmic movement through neural circuits combining visual information with proprioceptive information from the limbs. Although rhythmic movements are fundamental to everyday activities the specific details of the responsible control mechanisms remain elusive. We tested 39 young adults who performed flexion/extension movements of the forearm. We provided them with explicit knowledge of the amplitude and the speed of their movements, whereas frequency information was only implicitly available. In a series of 3 experiments, we demonstrate a tighter control of frequency compared with amplitude or speed. We found that in the absence of visual feedback, movements had larger amplitude and higher peak speed while maintaining the same frequency as when visual feedback was available; this was the case even when participants were aware of performing overly large and fast movements. Finally, when participants were asked to modulate continuously movement frequency, but not amplitude, we found the local coefficient of variability of movement frequency to be lower than that of amplitude. We suggest that a misperception of the generated amplitude in the absence of visual feedback, coupled with a highly accurate perception of generated frequency, leads to the performance of larger and faster movements with the same frequency when visual feedback is not available. Relatively low local coefficient of variability of frequency in a task that calls for continuous change in movement frequency suggests that we tend to operate at a constant frequency at the expense of variation in amplitude and peak speed.

Rhythmic movement in Parkinson's disease: effects of visual feedback and medication state.

Previous studies examining discrete movements of Parkinson's disease (PD) patients have found that in addition to performing movements that were slower than those of control participants, they exhibit specific deficits in movement coordination and in sensorimotor integration required to accurately guide movements. With medication, movement speed was normalized, but the coordinative aspects of movement were not. This led to the hypothesis that dopaminergic medication more readily compensates for intensive aspects of movement (such as speed), than for coordinative aspects (such as coordination of different limb segments) (Schettino et al., Exp Brain Res 168:186-202, 2006). We tested this hypothesis on rhythmic, continuous movements of the forearm. In our task, target peak speed and amplitude, availability of visual feedback, and medication state (on/off) were varied. We found, consistent with the discrete-movement results, that peak speed (intensive aspect) was normalized by medication, while accuracy, which required coordination of speed and amplitude modulation (coordinative aspect), was not normalized by dopaminergic treatment. However, our findings that amplitude, an intensive aspect of movement, was also not normalized by medication, suggests that a simple pathway gain increase does not act to remediate all intensive aspects of movement to the same extent. While it normalized movement peak speed, it did not normalize movement amplitude. Furthermore, we found that when visual feedback was not available, all participants (PD and controls) made faster movements. The effects of dopaminergic medication and availability of visual feedback on movement speed were additive. The finding that movement speed uniformly increased both in the PD and the control groups suggests that visual feedback may be necessary for calibration of peak speed, otherwise underestimated by the motor control system.

Early switching between movement types: indication of predictive control?

In everyday life, we frequently alternate between performing discrete and rhythmic movements. When performing a periodic movement, two distinct movement types can be distinguished: highly harmonic vs. discrete-like. The harmonicity of the movement is used to classify it as one or the other. We asked: (1) whether the frequency at which a periodic movement is performed affects the harmonicity of the resultant movement; and (2) what underlies switching between these movement types. To answer these questions, we studied horizontal flexion/extension forearm movements in 13 young adults over a wide range of frequencies. Movements were performed either at a fixed frequency, or at gradually increasing or decreasing target frequencies. We found movement harmonicity to depend on the frequency of the movement. Furthermore, we found a reverse hysteresis behavior, where participants switched movement type in anticipation of the future-required frequency. These findings suggest that predictive control is employed in switching between movement types.

Non-monotonicity on a spatio-temporally defined cyclic task: evidence of two movement types?

We tested 23 healthy participants who performed rhythmic horizontal movements of the elbow. The required amplitude and frequency ranges of the movements were specified to the participants using a closed shape on a phase-plane display, showing angular velocity versus angular position, such that participants had to continuously control both the speed and the displacement of their forearm. We found that the combined accuracy in velocity and position throughout the movement was not a monotonic function of movement speed. Our findings suggest that specific combinations of required movement frequency and amplitude give rise to two distinct types of movements: one of a more rhythmic nature, and the other of a more discrete nature.