Visuomotor adaptation and proprioceptive recalibration in older adults

Previous studies have shown that both young and older subjects adapt their reaches in response to a visuomotor distortion. It has been suggested that one’s continued ability to adapt to a visuomotor distortion with advancing age is due to the preservation of implicit learning mechanisms, where implicit learning mechanisms include processes that realign sensory inputs (i.e. shift one’s felt hand position to match the visual representation). The present study examined this proposal by determining if changes in sense of felt hand position (i.e. proprioceptive recalibration) follow visuomotor adaptation in older subjects. As well, we examined the influence of age on proprioceptive recalibration by comparing young and older subjects’ estimates of the position at which they felt their hand was aligned with a visual reference marker before and after aiming with a misaligned cursor that was gradually rotated 30° clockwise of the actual hand location. On estimation trials, subjects moved their hand along a robot-generated constrained pathway. At the end of the movement, a reference marker appeared and subjects indicated if their hand was left or right of the marker. Results indicated that all subjects adapted their reaches at a similar rate and to the same extent across the reaching trials. More importantly, we found that both young and older subjects recalibrated proprioception, such that they felt their hand was aligned with a reference marker when it was approximately 6° more left (or counterclockwise) of the marker following reaches with a rotated cursor. The leftward shift in both young and older subjects’ estimates was in the same direction and a third of the extent of adapted movement. Given that the changes in the estimate of felt hand position were only a fraction of the changes observed in the reaching movements, it is unlikely that sensory recalibration was the only source driving changes in reaches. Thus, we propose that proprioceptive recalibration combines with adapted sensorimotor mappings to produce changes in reaching movements. From the results of the present study, it is clear that changes in both sensory and motor systems are possible in older adults and could contribute to the preserved visuomotor adaptation.     

Visuomotor adaptation and intermanual transfer under different viewing conditions

Does the brain use a separate internal model for cursor mechanics during visuomotor adaptation? We compared the amount of adaptation and transfer to the opposite arm when subjects reached the targets under different viewing conditions of the arm during reaching. If the brain forms separate models, we predict a difference in the amount of adaptation and transfer for each viewing condition. If the brain forms one model, we predict equivalent amounts of adaptation and transfer between the two hands for each viewing condition. Separate groups of subjects performed a reaching task with either a rotated view of cursor motion representing their unseen hand or a rotated view of their actual hand. The two groups were further divided so that the magnitude of the rotation was either 45° or 75° counter-clockwise. After adapting to the rotation with one hand, subjects reached the same targets under the same viewing condition but with the opposite hand. Similar amounts of adaptation and intermanual transfer were found across the different magnitudes of rotation and across patterns of hand-order. Our results suggest that the brain may not be learning a distinct model for cursor mechanics, or if it is, it must be equivalent or overlapping with the arm model.     

Cognitive adaptation training for outpatients with schizophrenia

Schizophrenia is a complex neurodevelopmental disorder characterized by cognitive deficits. These deficits in cognitive functioning have been shown to relate to a variety of functional and treatment outcomes. Cognitive adaptation training (CAT) is a home‐based, manual‐driven treatment that utilizes environmental supports and compensatory strategies to bypass cognitive deficits and improve target behaviors and functional outcomes in individuals with schizophrenia. Unlike traditional case management, CAT provides environmental supports and compensatory strategies tailored to meet the behavioral style and neurocognitive deficits of each individual patient. The case of Ms. L. is presented to illustrate CAT treatment. © 2009 Wiley Periodicals, Inc. J Clin Psychol: In Session 65:1–12, 2009.     

Asymmetric generalization between the arm and leg following prism-induced visuomotor adaptation

We have previously shown an asymmetric generalization following a prism-induced visuomotor adaptation. Subjects who adapt to laterally deviating prism lenses during walking show a broad generalization to an arm pointing task, while subjects who adapt to prisms during arm pointing do not show generalization to walking. It is not known whether this broad generalization persists with other movements outside of walking or what specific features of the walking task, e.g. lower extremity involvement, allow it to be so broadly generalizable. In the current study, we tested healthy adult subjects performing one of three forms of prism adaptation and subsequently measured generalization. In Experiment 1 we tested whether a seated arm pointing prism adaptation would generalize to the leg. In Experiment 2 we tested whether a seated leg pointing prism adaptation would generalize to the arm. In Experiment 3 we tested whether standing influenced the extent of generalization from leg to arm. Results were surprising. We found a clear and consistent generalization from arm to leg, but much less so from leg to arm during either the seated or the standing task. These findings indicate that prism adaptations during arm movements are not limb-specific, as has been previously suggested. Further, the lack of generalization from leg to arm suggests that neither the adaptation of leg movements specifically, nor standing posture, nor the bilateral component of walking could be the salient feature allowing for its broad generalization across body parts.     

Assertion training outcome and generalization effects under didactic vs. facilitative training conditions

One hundred and eleven college undergraduates (25% male) participated in one of two cognitive behavioral training programs, which included behavior rehearsal, modelling, coaching, cognitive-restructuring, and emphasis on the transferability of skills. Neither self-report nor behavioral outcome differences between the facilitative vs. the didactic approaches were noted. Both programs produced impressive self-report and behavioral gains in comparison to pretest scores; these gains were maintained 1 to 2 years after the program. There was also evidence of verbal and nonverbal bilateral transfer between request behavior and refusal behavior in the short term. The generalization results were interpreted on the basis that the distinctive components of request behavior (such as enunicating a statement of a problem and a specific request) are more difficult than those of refusal behavior, such than there is transfer of request training to refusal behavior, but not vice versa. Request behavior involves the mobilization and personal initiation of a complex interaction with another person rather than the more reactive non-compliance with the initiative of another, as is the case in refusal behavior.     

Visuomotor tracking with delayed visual feedback

A rhesus monkey and five human subjects used a hand-held joystick to track unpredictable continuously moving targets. Both monkey and human respond by making discrete ("step-and-hold") corrections of positional error, at an average frequency of 1.33 and 2.26 movements/second, respectively. By delaying visual feedback of joystick position, we could reduce these frequencies in a predictable manner. These results imply that the primate visuomotor system probably does not operate as a "sampled-data mechanism" governed by an asynchronous clock, but that inevitable delays in visuomotor feedback control determine the frequency of corrective movements.     

Cardiorespiratory adaptation with short term training in older men

Summary The purpose of this study was to assess the rate of training-induced cardiorespiratory adaptations in older men [mean (SD), 66.5 (1.2) years]. The eight subjects trained an average of 4.3 (0.3) times each week. The walk/jog training was in two phases with 4 weeks (phase 1) at a speed to elicit 70% of pre-training maximal oxygen consumption ( ), and 5 weeks (phase 2) at 80%. Maximal exercise treadmill tests and a standardized submaximal protocol were performed prior to training, at weekly intervals during the training programme, and after training. (ml·kg^–1·min^–1) increased significantly over both phases: 6.6% after the first 4 weeks, and an additional 5.2% after the final 5 weeks. The weekly changes in over phase 1 were well fitted by an exponential association curve (r=0.75). The half-time for the rate of adaptation was 13.8 days, or 8.3 training sessions. Over phase 2, the change in did not plateau and a time course could not be determined. Submaximal exercise heart rate (f _c ) was reduced a significant 10 beats · min^–1 after the first 4 weeks, and a further 6 beats · min^–1 over the final 5 weeks. Thef _c reductions showed half-times of 9.1 days (phase 1) and 9.8 days (phase 2) (or 5-6 training sessions). The anaerobic ventilation threshold was increased 13.9% over the 9 weeks of training and the respiratory exchange ratio during constant load heavy exercise was significantly reduced; however, these changes could not be described by an exponential time course. Thus, short-term exercise training of older men resulted in significant and rapid cardiorespiratory improvements.     

The moving platform aftereffect: limited generalization of a locomotor adaptation

We have recently described a postural after-effect of walking onto a stationary platform previously experienced as moving, which occurs despite full knowledge that the platform will no longer move. This experiment involves an initial baseline period when the platform is kept stationary (BEFORE condition), followed by a brief adaptation period when subjects learn to walk onto the platform moving at 1.2 m/s (MOVING condition). Subjects are clearly warned that the platform will no longer move and asked to walk onto it again (AFTER condition). Despite the warning, they walk toward the platform with a velocity greater than that observed during the BEFORE condition, and a large forward sway of the trunk is observed once they have landed on the platform. This aftereffect, which disappears within three trials, represents dissociation of knowledge and action. In the current set of experiments, to gain further insight into this phenomenon, we have manipulated three variables, the context, location, and method of the walking task, between the MOVING and AFTER conditions, to determine how far the adaptation will generalize. It was found that when the gait initiation cue was changed from beeps to a flashing light, or vice versa, there was no difference in the magnitude of the aftereffect, either in terms of walking velocity or forward sway of the trunk. Changing the leg with which gait was initiated, however, reduced sway magnitude by approximately 50%. When subjects changed from forward walking to backward walking, the aftereffect was abolished. Similarly, walking in a location other than the mobile platform did not produce any aftereffect. However, in these latter two experiments, the aftereffect reappeared when subjects reverted to the walking pattern used during the MOVING condition. Hence, these results show that a change in abstract context had no influence, whereas any deviation from the way and location in which the moving platform task was originally performed profoundly reduced the size of the aftereffect. Although the moving platform aftereffect is an example of inappropriate generalization by the motor system across time, these results show that this generalization is highly limited to the method and location in which the original adaptation took place.     

Behavioral and physiological adaptation to repeated chair restraint in rhesus macaques

Physical restraint is a commonly used procedure when working closely with nonhuman primates. Nonhuman primates show rapid behavioral changes when learning the restraint procedure, and these changes have been taken to reflect behavioral and physiological habituation to the procedure. This study examined the behavioral and adrenocortical responses to repeated physical restraint in a large sample of adult male rhesus monkeys. Subjects showed a decline in behavioral agitation and cortisol concentrations across seven consecutive days of restraint. The changes in adrenocortical responsiveness were also coincident with an increased sensitivity to dexamethasone and a change in early morning basal cortisol secretion. The subjects were restrained for a single session 6 months later, and while the reduction in behavioral agitation was still present, the majority of changes in adrenocortical responsiveness were no longer present. These data show that behavior is not necessarily an indicator of underlying physiological processes and that the reduction of hypothalamic-pituitary-adrenal (HPA) activity with repeated restraint is due to physiological adaptation to high glucocorticoid concentrations and not to psychological habituation to the restraint procedures.     

Associative processes in adaptation to repeated cold exposure in rats

Learning processes have been implicated in drug tolerance, but the role of associative mechanisms in adaptation to stressors has not previously been determined. Rats that received daily brief cold exposures demonstrated adaptation to the cold as measured by an attenuation of hypothermia. Tolerance to the cold was disrupted by changing the context in which the subject experienced the cold. These findings provide evidence of associative processes in adaptation to cold exposure and illustrate that these processes are not limited to drug tolerance.     

Multiple immediate-early gene expression during physiological and endocrine adaptation to repeated stress

This study had three objectives: (i) to determine whether there were individual differences in the activation and adaptation of a range of immediate-early genes to repeated restraint stress, (ii) to monitor physiological responses (endocrine, cardiovascular and core temperature) and their adaptation with repeated presentations of the stressor, and (iii) to determine whether any of these indices were altered by dehydroepiandrosterone, an anti-glucocorticoid steroid known to be reduced in humans by stress. Four groups of male rats were implanted subcutaneously with either dehydroepiandrosterone or control (paraffin) pellets. They were then subjected to either a single or 14 days of restraint (60 min/day) or transferred to the testing room (unstressed). Repeatedly stressed animals and their controls were also implanted with intra-abdominal telemetric transmitters to record heart rate and core temperature. Protein products for c-fos,fos-b, c-jun and jun-b were displayed by immunocytochemistry. Areas examined included the ventrolateral septum, hypothalamic paraventricular nucleus, amygdala, locus coeruleus and nucleus of the solitary tract. Acute restraint increased Fos immunoreactivity in all of the areas examined, with the exception of the medial amygdala. The pattern of induction for Fos-B and Jun-B was similar, while c-Jun was only increased in the septum (though constitutive levels were high in most structures compared to the other proteins examined). After 14 days of restraint, immediate-early gene immunostaining was reduced in all of the areas examined, though the extent of adaptation depended on the area and immediate-early gene. In the forebrain, Fos expression adapted in the paraventricular nucleus, amygdala and septum, whereas Fos-B and c-Jun adapted incompletely in the septum. In contrast, Jun-B behaved like Fos. In the brainstem, Fos, Fos-B and Jun-B expression adapted in the nucleus of the solitary tract (but not the locus coeruleus). Corticosterone levels were still raised above baseline, but the response was blunted compared to acute stress. There was marked stress-induced hypothermia which did not adapt during the restraint session, but this returned to baseline during restraint after about five days. In contrast, stress-induced tachycardia did not change during repeated restraint. Dehydroepiandrosterone implants had no clear-cut effects on any immunostaining following acute stress, though there was a trend towards lessened adaptation of the Fos response in the septum after steroid treatment. Dehydroepiandrosterone also did not affect the cardiovascular or endocrine responses to repeated restraint. These experiments show that adaptation of the expression of multiple immediate-early genes occurs during repeated restraint, but in a site-specific pattern in the brains of male rats.     

The training effect of repeated isometric muscle contractions

Conclusions A training program of repeated isometric contractions with submaximal tension of the elbow flexors, increased to a striking degree the ability to perform isometric contractions until fatigue, while the ability to perform dynamic contractions of the same muscle group remained almost unaltered.This is interpreted as a change in the function of the parts of the central nervous system involved, summarized in the term motor learning. It is stressed that motorlearning is a major factor when increase of the endurance is the aim of the training.Isometric contractions with submaximal tension do not seem to be an optimal stimulus if an increase of the isometric strength is required.     

Progressive adaptation of the soleus H-reflex with daily training at walking backward

When untrained subjects walk backward on a treadmill the amplitude of the soleus H-reflex in midswing is equal to or exceeds the value in stance. This is a surprising result because during the swing phase of backward walking the soleus is inactive and its antagonist, the tibialis anterior, is active. We suggested that the high amplitude of the soleus H-reflex in late swing reflects task uncertainties, such as estimating the moment of foot contact with the ground and losing balance. In support of this idea we show that when untrained subjects held on to handrails the unexpected high-amplitude H-reflex during midswing was no longer present. We therefore asked whether daily training at this task without grasping the handrails would adaptively modify the H-reflex modulation pattern. In this event, within 10 days of training for 15 min daily, the anticipatory reflex activity at the beginning of training was gradually abated as the subjects reported gaining confidence at the task. However, when adapted subjects were made to walk backward with their eyes shut, the anticipatory reflex activity in midswing returned immediately. The reflex changes as a result of training were not due to changes in the motor activity or kinematics; they are likely part of the motor program controlling backward walking. This adaptive phenomenon may prove to be a useful model for studying the neural mechanisms of motor learning and adaptive plasticity in humans and may be relevant to rehabilitation programs for neurological patients.     

A dissociation between visual and motor workspace inhibits generalization of visuomotor adaptation across the limbs

We have previously shown that the pattern of interlimb transfer following visuomotor adaptation depends on whether the two arms share task-space at a given workspace location: when the two arms adapted to a novel visuomotor rotation in unshared, lateral workspaces, transfer of movement direction information occurred symmetrically (i.e., from dominant to nondominant arm, and vice versa). When the two arms shared the same task-space, however, transfer of the same information became asymmetric (i.e., only from dominant to nondominant arm). In the present study, I investigated the effect of a conflict between visual and proprioceptive information of task-space on the pattern of interlimb transfer, by dissociating visual and motor workspaces. I hypothesized that the pattern of interlimb transfer would be determined by the way the motor control system uses available sensory information, and predicted that depending on whether the system relied more on vision or proprioception, transfer would occur either symmetrically or asymmetrically. Surprisingly, the results indicated that despite substantial adaptation to a novel visuomotor rotation, no transfer occurred across the arms when the visual and motor workspaces were dissociated in space. Based on this finding, I suggest that when a conflict exists between visual and proprioceptive information with respect to the sharing of the given task-space by the two arms, it interferes with executive decisions made by the motor control system in determining hand dominance at a given workspace, which results in a lack of transfer across the arms.     

Short-term training: when do repeated bouts of resistance exercise become training?

Chronic resistance training induces increases in muscle fibre cross-sectional area (CSA), otherwise known as hypertrophy. This is due to an increased volume percentage of myofibrillarproteins within a given fibre. The exact time-course for muscle fibre hypertrophy is not well-documented but appears to require at least 6-7 weeks of regular resistive training at reasonably high intensity before increases in fibre CSA are deemed significant. Proposed training-induced changes in neural drive are hypothesized to increase strength due to increased synchrony of motor unit firing, reducedant agonist muscle activity, and/or a reduction in any bilateral strength deficit. Nonetheless, increases in muscle protein synthesis were observed following an isolated bout of resistance exercise. In addition, muscle balance was positive, following resistance exercise when amino acids were infused/ingested. This showed that protein accretion occurred during the postexercise period. The implications of this hypothesis for training-induced increases in strength are discussed.