Apparent Negative Interference Due to Variation in Recombination Frequencies

Variation in recombination frequencies may lead to a bias in the estimated interference value in a linkage experiment. Depending on the pattern of variation, the bias may be toward negative interference or toward positive interference, even when there is positive interference at the cytological level. In this paper we have mainly concentrated on the case of negative interference. We use models to quantify this effect when data are derived from a backcross experiment or from the selfing of F1 individuals. The effect is quantitatively similar in the two cases. There is an upper limit to the size the bias may reach for every given level of recombination. Two reported cases of negative interference in Drosophila and cultivated barley fall within this possible parameter range, i.e., the observed negative interference values could--at least in principle--be due solely to a variation in the recombination frequencies in the experiments.     

Visuomotor Adaptation: How Forgetting Keeps Us Conservative

Even when provided with feedback after every movement, adaptation levels off before biases are completely removed. Incomplete adaptation has recently been attributed to forgetting: the adaptation is already partially forgotten by the time the next movement is made. Here we test whether this idea is correct. If so, the final level of adaptation is determined by a balance between learning and forgetting. Because we learn from perceived errors, scaling these errors by a magnification factor has the same effect as subjects increasing the amount by which they learn from each error. In contrast, there is no reason to expect scaling the errors to affect forgetting. The magnification factor should therefore influence the balance between learning and forgetting, and thereby the final level of adaptation. We found that adaptation was indeed more complete for larger magnification factors. This supports the idea that incomplete adaptation is caused by part of what has been learnt quickly being forgotten.     

Reduction in Learning Rates Associated with Anterograde Interference Results from Interactions between Different Timescales in Motor Adaptation

Prior experiences can influence future actions. These experiences can not only drive adaptive changes in motor output, but they can also modulate the rate at which these adaptive changes occur. Here we studied anterograde interference in motor adaptation – the ability of a previously learned motor task (Task A) to reduce the rate of subsequently learning a different (and usually opposite) motor task (Task B). We examined the formation of the motor system''s capacity for anterograde interference in the adaptive control of human reaching-arm movements by determining the amount of interference after varying durations of exposure to Task A (13, 41, 112, 230, and 369 trials). We found that the amount of anterograde interference observed in the learning of Task B increased with the duration of Task A. However, this increase did not continue indefinitely; instead, the interference reached asymptote after 15–40 trials of Task A. Interestingly, we found that a recently proposed multi-rate model of motor adaptation, composed of two distinct but interacting adaptive processes, predicts several key features of the interference patterns we observed. Specifically, this computational model (without any free parameters) predicts the initial growth and leveling off of anterograde interference that we describe, as well as the asymptotic amount of interference that we observe experimentally (R² = 0.91). Understanding the mechanisms underlying anterograde interference in motor adaptation may enable the development of improved training and rehabilitation paradigms that mitigate unwanted interference.     

Neural Substrates Related to Motor Memory with Multiple Timescales in Sensorimotor Adaptation

Recent computational and behavioral studies suggest that motor adaptation results from the update of multiple memories with different timescales. Here, we designed a model-based functional magnetic resonance imaging (fMRI) experiment in which subjects adapted to two opposing visuomotor rotations. A computational model of motor adaptation with multiple memories was fitted to the behavioral data to generate time-varying regressors of brain activity. We identified regional specificity to timescales: in particular, the activity in the inferior parietal region and in the anterior-medial cerebellum was associated with memories for intermediate and long timescales, respectively. A sparse singular value decomposition analysis of variability in specificities to timescales over the brain identified four components, two fast, one middle, and one slow, each associated with different brain networks. Finally, a multivariate decoding analysis showed that activity patterns in the anterior-medial cerebellum progressively represented the two rotations. Our results support the existence of brain regions associated with multiple timescales in adaptation and a role of the cerebellum in storing multiple internal models.     

Mechanisms of Motor Adaptation in Reactive Balance Control

Balance control must be rapidly modified to provide stability in the face of environmental challenges. Although changes in reactive balance over repeated perturbations have been observed previously, only anticipatory postural adjustments preceding voluntary movements have been studied in the framework of motor adaptation and learning theory. Here, we hypothesized that adaptation occurs in task-level balance control during responses to perturbations due to central changes in the control of both anticipatory and reactive components of balance. Our adaptation paradigm consisted of a Training set of forward support-surface perturbations, a Reversal set of novel countermanding perturbations that reversed direction, and a Washout set identical to the Training set. Adaptation was characterized by a change in a motor variable from the beginning to the end of each set, the presence of aftereffects at the beginning of the Washout set when the novel perturbations were removed, and a return of the variable at the end of the Washout to a level comparable to the end of the Training set. Task-level balance performance was characterized by peak center of mass (CoM) excursion and velocity, which showed adaptive changes with repetitive trials. Only small changes in anticipatory postural control, characterized by body lean and background muscle activity were observed. Adaptation was found in the evoked long-latency muscular response, and also in the sensorimotor transformation mediating that response. Finally, in each set, temporal patterns of muscle activity converged towards an optimum predicted by a trade-off between maximizing motor performance and minimizing muscle activity. Our results suggest that adaptation in balance, as well as other motor tasks, is mediated by altering central sensitivity to perturbations and may be driven by energetic considerations.     

Trial-to-Trial Reoptimization of Motor Behavior Due to Changes in Task Demands Is Limited

Each task requires a specific motor behavior that is tuned to task demands. For instance, writing requires a lot of accuracy while clapping does not. It is known that the brain adjusts the motor behavior to different task demands as predicted by optimal control theory. In this study, the mechanism of this reoptimization process is investigated by varying the accuracy demands of a reaching task. In this task, the width of the reaching target (0.5 or 8 cm) was varied either on a trial-to-trial basis (random schedule) or in blocks (blocked schedule). On some trials, the hand of the subjects was clamped to a rectilinear trajectory that ended 2 cm on the left or right of the target center. The rejection of this perturbation largely varied with target width in the blocked schedule but not in the random schedule. That is, subjects exhibited different motor behavior in the different schedules despite identical accuracy demands. Therefore, while reoptimization has been considered immediate and automatic, the differences in motor behavior observed across schedules suggest that the reoptimization of the motor behavior is neither happening on a trial-by-trial basis nor obligatory. The absence of trial-to-trial mechanisms, the inability of the brain to adapt to two conflicting task demands and the existence of a switching cost are discussed as possible sources of the non-optimality of motor behavior during the random schedule.     

Coordinate Representations for Interference Reduction in Motor Learning

When opposing force fields are presented alternately or randomly across trials for identical reaching movements, subjects learn neither force field, a behavior termed ‘interference’. Studies have shown that a small difference in the endpoint posture of the limb reduces this interference. However, any difference in the limb’s endpoint location typically changes the hand position, joint angles and the hand orientation making it ambiguous as to which of these changes underlies the ability to learn dynamics that normally interfere. Here we examine the extent to which each of these three possible coordinate systems—Cartesian hand position, shoulder and elbow joint angles, or hand orientation—underlies the reduction in interference. Subjects performed goal-directed reaching movements in five different limb configurations designed so that different pairs of these configurations involved a change in only one coordinate system. By specifically assigning clockwise and counter-clockwise force fields to the configurations we could create three different conditions in which the direction of the force field could only be uniquely distinguished in one of the three coordinate systems. We examined the ability to learn the two fields based on each of the coordinate systems. The largest reduction of interference was observed when the field direction was linked to the hand orientation with smaller reductions in the other two conditions. This result demonstrates that the strongest reduction in interference occurred with changes in the hand orientation, suggesting that hand orientation may have a privileged role in reducing motor interference for changes in the endpoint posture of the limb.     

Coetzee on South Africa's Past: Remembering in the Time of Forgetting

As an intellectual figure in South Africa, J. M. Coetzee has consistently engaged with the politics of the past, particularly the contemporary ethical ramifications of the colonial past, alongside the more recent and bitter history of oppression under apartheid. As archaeologists, we aim to excavate these engagements through his novels and essays, particularly his long-standing concerns with the Khoekhoe communities of the Cape. In South Africa today, the tensions between remembering and forgetting are palpable and, in the aftermath of the Truth and Reconciliation Commission (TRC), we suggest that not all forgetting can be cast as therapeutic. Throughout his complex relationships with the nation's history, Coetzee foregrounds the colonial past and reiterates the ethical responsibilities of remembering so that the patterns of the past are not repeated.     

Introduction: Aging and the Multifaceted Influences on Adaptation to Working Time

This special issue of Chronobiology International presents a selection of papers originally delivered at the 18th International Symposium on Shift Work and Working Time, held at Yeppoon, Australia, in August 2007. The key theme of the symposium was “Aging and Working Time: Creating Safe Environments.” Older workers are widely believed to experience greater difficulty than younger workers adapting to shift work and irregular work schedules. However, while the three reviews of age effects published here (Costa & Di Milia, ; Folkard, ; Gander & Signal, ) identify evidence that older workers do indeed adapt less well, they also demonstrate that much more research is urgently required. The remaining papers address various aspects of the impact of work schedules on health, safety, sleep, and performance. They can be divided into three broad categories: circadian and other periodic factors; sleep, sleepiness, and fatigue; and other aspects of health and adjustment. This collection of papers showcases the best of contemporary research on the safety and health effects of working hours, continuing the tradition established by the two previous issues of the journal devoted to earlier symposia on shift work and working time.     

Time Course and Action Spectrum of Vibrotactile Adaptation

In a series of experiments designed to explore the processes underlying adaptation of the sense of flutter-vibration, vibrotactile threshold was measured on the pad of the index finger, using Békésy tracking. Unadapted thresholds were first measured, for a number of frequencies (4-90 Hz) and contactor sizes (1-8 mm diameter). As expected, these measurements indicated the presence of (1) a Pacinian system possessing spatial summation and increasing in sensitivity, as frequency was raised, at the rate of 12 dB/octave; and (2) a non-Pacinian system showing little spatial summation, and with a frequency characteristic matching that of the NP I mechanism of Bolanowski et al. (1988). These baseline data of Experiment 1 guided the selection of stimulus parameters for subsequent experiments, in which threshold for a test stimulus was measured before, during, and after periods of vibrotactile adaptation.

In Experiment 2, test stimuli of 10 H_z and 50 H_z were combined factorially with 30-dB SL adapting stimuli of the same two frequencies. When the test stimulus was 10 Hz, the two adapting frequencies were equally effective in raising threshold; however, when the 50-Hz test stimulus was used, the 50-Hz adapting stimulus raised threshold by a greater amount than did the 10-Hz adapter. These results confirm on the finger the independence of adaptation in Pacinian and non-Pacinian channels, a result previously established on the thenar by other workers. For all four frequency combinations, threshold rose exponentially with a time constant of 1.5-2 min.

In Experiment 3, an action spectrum was determined, showing the adapting amplitude needed at each of a series of frequencies to raise the threshold of a 10-Hz stimulus by 10 dB; this spectrum was essentially flat from 30 to 90 Hz. The results, taken in conjunction with what is known about rapidly adapting cutaneous mechanoreceptors, imply that the effectiveness of an adapting stimulus is not determined solely by the amount of activity it generates in first-order afferents.     

Distinct Motor Strategies Underlying Split-Belt Adaptation in Human Walking and Running

The aim of the present study was to elucidate the adaptive and de-adaptive nature of human running on a split-belt treadmill. The degree of adaptation and de-adaptation was compared with those in walking by calculating the antero-posterior component of the ground reaction force (GRF). Adaptation to walking and running on a split-belt resulted in a prominent asymmetry in the movement pattern upon return to the normal belt condition, while the two components of the GRF showed different behaviors depending on the gaits. The anterior braking component showed prominent adaptive and de-adaptive behaviors in both gaits. The posterior propulsive component, on the other hand, exhibited such behavior only in running, while that in walking showed only short-term aftereffect (lasting less than 10 seconds) accompanied by largely reactive responses. These results demonstrate a possible difference in motor strategies (that is, the use of reactive feedback and adaptive feedforward control) by the central nervous system (CNS) for split-belt locomotor adaptation between walking and running. The present results provide basic knowledge on neural control of human walking and running as well as possible strategies for gait training in athletic and rehabilitation scenes.     

Environmental Consistency Determines the Rate of Motor Adaptation

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Affective and Personality Characteristics in Function of Nightmare Prevalence, Nightmare Distress, and Interference Due to Nightmares.

This study analyzes the relationships among nightmare frequency, nightmare distress, and interference due to nightmares, sleep quality, psychopathology (anxiety and depression), and personality (neuroticism, psychoticism, and boundaries). The nightmare parameters were evaluated with the Nightmare Frequency Questionnaire (B. Krakow et al., 2000; see also record 2000-16731-002), the Nightmare Distress Questionnaire (K. Belicki, 1992b; see also record 1993-08389-001), and the Nightmare Effects Survey (Krakow et al., 2000). The sample was composed of 147 healthy students. The results demonstrated that the 2 measurements of distress (subjective distress associated with nightmares and, especially, deterioration of psychosocial functioning) were the dimensions that best related to the variables studied, although the relationships were modulated by nightmare frequency. The parameter of nightmare frequency was found to be useful as an indicator of certain personality characteristics rather than psychological perturbation. Having nightmares on a weekly basis was strongly associated with depressed mood. The knowledge about these relations is relevant not only theoretically but also as a practical guide. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Polygenic Adaptation and Clonal Interference Enable Sustained Diversity in Experimental Pseudomonas aeruginosa Populations

How biodiversity arises and can be maintained in asexual microbial populations growing on a single resource remains unclear. Many models presume that beneficial genotypes will outgrow others and purge variation via selective sweeps. Environmental structure like that found in biofilms, which are associated with persistence during infection and other stressful conditions, may oppose this process and preserve variation. We tested this hypothesis by evolving Pseudomonas aeruginosa populations in biofilm-promoting arginine media for 3 months, using both a bead model of the biofilm life cycle and planktonic serial transfer. Surprisingly, adaptation and diversification were mostly uninterrupted by fixation events that eliminate diversity, with hundreds of mutations maintained at intermediate frequencies. The exceptions included genotypes with mutator alleles that also accelerated genetic diversification. Despite the rarity of hard sweeps, a remarkable 40 genes acquired parallel mutations in both treatments and often among competing genotypes within a population. These incomplete soft sweeps include several transporters (including pitA, pntB, nosD, and pchF) suggesting adaptation to the growth media that becomes highly alkaline during growth. Further, genes involved in signal transduction (including gacS, aer2, bdlA, and PA14_71750) reflect likely adaptations to biofilm-inducing conditions. Contrary to evolution experiments that select mutations in a few genes, these results suggest that some environments may expose a larger fraction of the genome and select for many adaptations at once. Thus, even growth on a sole carbon source can lead to persistent genetic and phenotypic variation despite strong selection that would normally purge diversity.     

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

Predictions about one''s 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, one''s 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 one''s 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 participant''s 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.