Biomedical subjects

Julia Trommershäuser

Publications and source records attributed to Julia Trommershäuser.

9 recordsLinked to original sources

PubMed2006

Limits to human movement planning with delayed and unpredictable onset of needed information.

In motor tasks with explicit rewards and penalties, humans choose movement strategies that nearly maximize expected gain (Trommershäuser et al. in J Opt Soc Am A 20:1419-1433, 2003). Here, we examine whether performance is still close to optimal when information about payoffs is not available prior to movement onset. Subjects rapidly touched a target region while trying to avoid hitting an overlapping penalty region placed randomly to the left or right of the target. Subjects received rewards and incurred penalties for hitting the corresponding regions. Late responses (>700 ms) were heavily penalized. The penalty region was displayed 0, 200 or 400 ms after the reward region and the subject could not know where it would be until then. Reaction times to begin the movement after stimulus appearance were constant across conditions. Median reaction times were approximately 200 ms, i.e., the time the penalty was first displayed in the 200 ms delay condition. Performance was compared to that of an optimal movement planner that chooses mean end points to maximize expected gain despite movement variability. In the 0 and 200 ms delay conditions, subjects selected strategies that did not differ significantly from optimal, indicating that humans are able to plan their movements well despite delayed and unpredictable onset of information. Performance dropped below optimal in the 400 ms delay condition, with mean movement end points closer to the penalty region than predicted by the optimal strategy (in the high-penalty condition). We conclude that relevant information concerning the reward structure is required between 200 and 400 ms prior to the end of the movement, but can still be integrated into the movement plan after movement initiation.

Adult↗

PubMed2006

Limits to human movement planning in tasks with asymmetric gain landscapes.

We studied human movement planning in a task with predefined costs and benefits to movement outcome. Participants pointed rapidly at stimulus configurations consisting of a target region and up to two penalty regions. Hits on the target and penalty regions resulted in monetary gains and losses. In previous studies involving single penalty regions or other symmetric target-penalty configurations, performance was optimal in the sense of maximizing expected gain. In this study, more complex, asymmetric configurations were used in which the two penalty regions carried different penalties. With these configurations, the landscape of expected gain as a function of mean end point (MEP) was spatially asymmetric. Further, the optimal movement plan with these configurations was sometimes counterintuitive (e.g., one should aim slightly inside the lesser penalty region). In one asymmetric condition, four out of six naïve participants' performed suboptimally, indicating that there are limits to human movement planning. Further, the suboptimal performance was inconsistent with a model in which participants misestimate motor variability but otherwise optimally plan their movement.

Adult↗

PubMed2006

Humans rapidly estimate expected gain in movement planning.

We studied human movement planning in tasks in which subjects selected one of two goals that differed in expected gain. Each goal configuration consisted of a target circle and a partially overlapping penalty circle. Rapid hits into the target region led to a monetary bonus; accidental hits into the penalty region incurred a penalty. The outcomes assigned to target and penalty regions and the spatial arrangement of those regions were varied. Subjects preferred configurations with higher expected gain whether selection involved a rapid pointing movement or a choice by key press. Movements executed to select one of two goal configurations exhibited the same movement dynamics as pointing movements directed at a single configuration, and were executed with the same high efficiency. Our results suggest that humans choose near-optimal strategies when planning their movement, and can base their selection of strategy on a rapid judgment about the expected gain associated with possible movement goals.

Adult↗

PubMed2005

Optimal compensation for changes in task-relevant movement variability.

Effective movement planning should take into account the consequences of possible errors in executing a planned movement. These errors can result from either sensory uncertainty or variability in movement planning and production. We examined the ability of humans to compensate for variability in sensory estimation and movement production under conditions in which variability is increased artificially by the experimenter. Subjects rapidly pointed at a target region that had an adjacent penalty region. Target and penalty hits yielded monetary rewards and losses. We manipulated the task-relevant variability by perturbing visual feedback of finger position during the movement. The feedback was shifted in a random direction with a random amplitude in each trial, causing an increase in the task-relevant variability. Subjects were unable to counteract this form of perturbation. Rewards and penalties were based on the perturbed, visually specified finger position. Subjects rapidly acquired an estimate of their new variability in <120 trials and adjusted their aim points accordingly. We compared subjects' performance to the performance of an optimal movement planner maximizing expected gain. Their performance was consistent with that expected from an optimal movement planner that perfectly compensated for externally imposed changes in task-relevant variability. When exposed to novel stimulus configurations, aim points shifted in the first trial without showing any detectable trend across trials. These results indicate that subjects are capable of changing their pointing strategy in the presence of externally imposed noise. Furthermore, they manage to update their estimate of task-relevant variability and to transfer this estimate to novel stimulus configurations.

Adaptation, Physiological↗

PubMed2004

Immediate spatial distortions of pointing movements induced by visual landmarks.

We tested the influence of two horizontally aligned visual landmarks on pointing movements to memorized targets, to investigate whether the visuomotor system can make use of an egocentric representation unaffected by visual context. The endpoints of pointing movements were systematically distorted toward the nearest visual landmark, indicating that spatial representations included both target and nontarget information. These distortions were not due to the presence of the landmarks during the movement but, rather, to their presence in the encoding phase. Qualitatively similar distortions were present even with the shortest possible retention phase, when the target was extinguished at movement onset. Finally, we found the same pattern of distortion when participants were forced to remember the target within an allocentric frame of reference. We argue that even early memory representations for pointing movements are influenced by visual information in the surrounding visual field.

Adolescent↗

PubMed2003

The consistency of bisection judgments in visual grasp space.

We study whether bisection in visual grasp space (the region over which eye and hand can work together to grasp or touch objects) depends on fixation or on the method of judgment employed (the task). We determined observer bias and sensitivity for bisection judgments (in a fronto-parallel plane as well as along contours slanted in depth). Significant biases were found that varied across observers both qualitatively and quantitatively. These biases were stable for a given individual (across a year between data collection intervals) and across tasks (method of adjustment vs. forced-choice). When observers maintained fixation (on an endpoint or in the neighborhood of the bisection point), fixation location had a small but significant effect on bias, although those effects were small compared with bisection uncertainty. We conclude that bisection judgments differ significantly between fixations, but that the effect of fixation location on bisection is not large enough to be detected reliably by the observer moving his or her eyes during a judgment.

Depth Perception↗

PubMed2003

Heterogeneous presynaptic release probabilities: functional relevance for short-term plasticity.

We discuss a model of presynaptic vesicle dynamics, which allows for heterogeneity in release probability among vesicles. Specifically, we explore the possibility that synaptic activity is carried by two types of vesicles; first, a readily releasable pool and, second, a reluctantly releasable pool. The pools differ regarding their probability of release and time scales on which released vesicles are replaced by new ones. Vesicles of both pools increase their release probability during repetitive stimulation according to the buildup of Ca(2+) concentration in the terminal. These properties are modeled to fit data from the calyx of Held, a giant synapse in the auditory pathway. We demonstrate that this arrangement of two pools of releasable vesicles can account for a variety of experimentally observed patterns of synaptic depression and facilitation at this synapse. We conclude that synaptic transmission cannot be accurately described unless heterogeneity of synaptic release probability is taken into account.

Action Potentials↗

PubMed2003

Statistical decision theory and trade-offs in the control of motor response.

We present a novel approach to the modeling of motor responses based on statistical decision theory. We begin with the hypothesis that subjects are ideal motion planners who choose movement trajectories to minimize expected loss. We derive predictions of the hypothesis for movement in environments where contact with specified regions carries rewards or penalties. The model predicts shifts in a subject's aiming point in response to changes in the reward and penalty structure of the environment and with changes in the subject's uncertainty in carrying out planned movements. We tested some of these predictions in an experiment where subjects were rewarded if they succeeded in touching a target region on a computer screen within a specified time limit. Near the target was a penalty region which, if touched, resulted in a penalty. We varied distance between the penalty region and the target and the cost of hitting the penalty region. Subjects shift their mean points of contact with the computer screen in response to changes in penalties and location of the penalty region relative to the target region in qualitative agreement with the predictions of the hypothesis. Thus, movement planning takes into account extrinsic costs and the subject's own motor uncertainty.

Adult↗

PubMed2003

Statistical decision theory and the selection of rapid, goal-directed movements.

We present two experiments that test the range of applicability of a movement planning model (MEGaMove) based on statistical decision theory. Subjects attempted to earn money by rapidly touching a green target region on a computer screen while avoiding nearby red penalty regions. In two experiments we varied the magnitudes of penalties, the degree of overlap of target and penalty regions, and the number of penalty regions. Overall, subjects acted so as to maximize gain in a wide variety of stimulus configurations, in good agreement with predictions of the model.

Adult↗