Larger Aftereffects Research Articles

Updating of Implicit Adaptation Processes through Erroneous Numeric Feedback

There is debate about how implicit and explicit processes interact in sensorimotor adaptation, implicating how error signals drive learning. Target error information is thought to primarily influence explicit processes, therefore manipulations to the veracity of this information should impact adaptation but not implicit recalibration (i.e. after-effects). Thirty participants across three groups initially adapted to rotated cursor feedback. Then we manipulated numeric target error through knowledge of results (KR) feedback, where groups practised with correct or incorrect (+/-15°) numeric KR. Participants adapted to erroneous KR, but only the KR + 15 group showed augmented implicit recalibration, evidenced by larger after-effects than before KR exposure. In the presence of sensory prediction errors, target errors modulated after-effects, suggesting an interaction between implicit and explicit processes.

Interlimb differences in visuomotor and dynamic adaptation during targeted reaching in children.

While a number of studies have focused on movement (a)symmetries between the arms in adults, less is known about movement asymmetries in typically developing children. The goal of this study was to examine interlimb differences in children when adapting to novel visuomotor and dynamic conditions while performing a center-out reaching task. We tested 13 right-handed children aged 9-11years old. Prior to movement, one of eight targets arranged radially around the start position was randomly displayed. Movements were made either with the right (dominant) arm or the left (nondominant) arm. The children participated in two experiments separated by at least one week. In one experiment, subjects were exposed to a rotated visual display (30° about the start circle); and in the other, a 1kg mass (attached eccentrically to the forearm axis). Each experiment consisted of three blocks: pre-exposure, exposure and post-exposure. Three measures of task performance were calculated from hand trajectory data: hand-path deviation from the straight target line, direction error at peak velocity and final position error. Results showed that during visuomotor adaptation, no interlimb differences were observed for any of the three measures. During dynamic adaptation, however, a significant difference between the arms was observed at the first cycle during dynamic adaptation. With regard to the aftereffects observed during the post-exposure block, direction error data indicate considerably large aftereffects for both arms during visuomotor adaptation; and there was a significant difference between the arms, resulting in substantially larger aftereffects for the right arm. Similarly, dynamic adaptation results also showed a significant difference between the arms; and post hoc analyses indicated that aftereffects were present only for the right arm. Collectively, these findings indicate that the dominant arm advantage for developing an internal model associated with a novel visuomotor or dynamic transform, as previously shown in young adults, may already be apparent at 9 to 11-year old children.

Augmenting propulsion demands during split-belt walking increases locomotor adaptation of asymmetric step lengths

BackgroundPromising studies have shown that the gait symmetry of individuals with hemiparesis due to brain lesions, such as stroke, can improve through motor adaptation protocols forcing patients to use their affected limb more. However, little is known about how to facilitate this process. Here we asked if increasing propulsion demands during split-belt walking (i.e., legs moving at different speeds) leads to more motor adaptation and more symmetric gait in survivors of a stroke, as we previously observed in subjects without neurological disorders.MethodsWe investigated the effect of propulsion forces on locomotor adaptation during and after split-belt walking in the asymmetric motor system post-stroke. To test this, 12 subjects in the chronic phase post-stroke experienced a split-belt protocol in a flat and incline session so as to contrast the effects of two different propulsion demands. Step length asymmetry and propulsion forces were used to compare the motor behavior between the two sessions because these are clinically relevant measures that are altered by split-belt walking.ResultsThe incline session resulted in more symmetric step lengths during late split-belt walking and larger after-effects following split-belt walking. In both testing sessions, subjects who have had a stroke adapted to regain speed and slope-specific leg orientations similarly to young, intact adults. Importantly, leg orientations, which were set by kinetic demands, during baseline walking were predictive of those achieved during split-belt walking, which in turn predicted each individual’s post-adaptation behavior. These results are relevant because they provide evidence that survivors of a stroke can generate the leg-specific forces to walk more symmetrically, but also because we provide insight into factors underlying the therapeutic effect of split-belt walking.ConclusionsIndividuals post-stroke at a chronic stage can adapt more during split-belt walking and have greater after-effects when propulsion demands are augmented by inclining the treadmill surface. Our results are promising since they suggest that increasing propulsion demands during paradigms that force patients to use their paretic side more could correct gait asymmetries post-stroke more effectively.

The Thin White Line: Adaptation Suggests a Common Neural Mechanism for Judgments of Asian and Caucasian Body Size.

Visual adaptation has been proposed as a mechanism linking viewing images of thin women’s bodies with body size and shape misperception (BSSM). Non-Caucasian populations appear less susceptible to BSSM, possibly because adaptation to thin Caucasian bodies in Western media may not fully transfer to own-race bodies. Experiment 1 used a cross-adaptation paradigm to examine the transfer of body size aftereffects across races. Large aftereffects were found in the predicted directions for all conditions. The strength of aftereffects was statistically equivalent when the race of test stimuli was congruent vs. incongruent with the race of adaptation stimuli, suggesting complete transfer of aftereffects across races. Experiment 2 used a contingent-adaptation paradigm, finding that simultaneous adaptation to wide Asian and narrow Caucasian women’s bodies (or vice versa) results in no significant aftereffects for either congruent or incongruent conditions and statistically equivalent results for each. Equal and opposite adaptation effects may therefore transfer completely across races, canceling each other out. This suggests that body size is encoded by a race-general neural mechanism. Unexpectedly, Asian observers showed reduced body size aftereffects compared to Caucasian observers, regardless of the race of stimulus bodies, perhaps helping to explain why Asian populations appear less susceptible to BSSM.

Large Propulsion Demands Increase Locomotor Adaptation at the Expense of Step Length Symmetry.

There is an interest to identify factors facilitating locomotor adaptation induced by split-belt walking (i.e., legs moving at different speeds) because of its clinical potential. We hypothesized that augmenting braking forces, rather than propulsion forces, experienced at the feet would increase locomotor adaptation during and after split-belt walking. To test this, forces were modulated during split-belt walking with distinct slopes: incline (larger propulsion than braking), decline (larger braking than propulsion), and flat (similar propulsion and braking). Step length asymmetry was compared between groups because it is a clinically relevant measure robustly adapted on split-belt treadmills. Unexpectedly, the group with larger propulsion demands (i.e., the incline group) changed their gait the most during adaptation, reached their final adapted state more quickly, and had larger after-effects when the split-belt perturbation was removed. We also found that subjects who experienced larger disruptions of propulsion forces in early adaptation exhibited greater after-effects, which further highlights the catalytic role of propulsion forces on locomotor adaptation. The relevance of mechanical demands on shaping our movements was also indicated by the steady state split-belt behavior, during which each group recovered their baseline leg orientation to meet leg-specific force demands at the expense of step length symmetry. Notably, the flat group was nearly symmetric, whereas the incline and decline group overshot and undershot step length symmetry, respectively. Taken together, our results indicate that forces propelling the body facilitate gait changes during and after split-belt walking. Therefore, the particular propulsion demands to walk on a split-belt treadmill might explain the gait symmetry improvements in hemiparetic gait following split-belt training.

Aftereffects of Spectrally Similar and Dissimilar Spectral Motion Adaptors in the Tritone Paradox.

Shepard tones consist of octave-spaced components, whose amplitudes are generated under a fixed bell-shaped spectral envelope. They are well defined in pitch chroma, but generate octave confusions that in turn can produce ambiguities in the perceived relative pitch heights when their chromas are exactly a tritone apart (the tritone paradox). This study examined the effects of tonal context on relative pitch height judgments using adaptor sequences followed by target sequences (pairs of Shepard tones of different chromas separated by a tritone). Listeners judged whether the second target Shepard tone was higher or lower than the first. Adaptor sequences consisted of rising or falling scales (43 s at the beginning of each block, 4 s before each target sequence). Two sets of Shepard tones were used for adaptors and targets that were generated under spectral envelopes centered at either A3 (220 Hz) and C6 (1,046 Hz). Pitch direction judgments (rising vs. falling) to spectrally consistent (A3–A3, C6–C6) and inconsistent (A3–C6, C6–A3) adaptor-target combinations were studied. Large significant contrastive aftereffects (0.08–0.21 change in fraction of pitch direction responses) were only found for the Shepard tones that were judged as higher in the control condition (judgments about the target sequences without adaptor sequences) for the consistent adaptor-target conditions (A3–A3, C6–C6). The experiments rule out explanations based on non-sensory decision making processes. Possible explanations in terms of perceptual aftereffects caused by adaptation in central auditory frequency-motion detectors are discussed.

Role of timbre and fundamental frequency in voice gender adaptation.

Prior adaptation to male (or female) voices causes androgynous voices to be perceived as more female (or male). Using a selective adaptation paradigm the authors investigate the relative impact of the vocal fold vibration rate (F0) and timbre (operationally in this paper as characteristics that differentiate two voices of the same F0 and loudness) on this basic voice gender aftereffect. TANDEM-STRAIGHT was used to morph between 10 pairs of male and female speakers uttering 2 different vowel-consonant-vowel sequences (20 continua). Adaptor stimuli had one parameter (either F0 or timbre) set at a clearly male or female level, while the other parameter was set at an androgynous level, as determined by an independent set of listeners. Compared to a control adaptation condition (in which both F0 and timbre were clearly male or female), aftereffects were clearly reduced in both F0 and timbre adaptation conditions. Critically, larger aftereffects were found after timbre adaptation (comprising androgynous F0) compared to F0 adaptation (comprising an androgynous timbre). Together these results suggest that timbre plays a larger role than F0 in voice gender adaptation. Finally, the authors found some evidence that individual differences among listeners reflect in part pre-experimental contact to male and female voices.

Prism adaptation in virtual and natural contexts: Evidence for a flexible adaptive process.

Prism exposure when aiming at a visual target in a virtual condition (e.g., when the hand is represented by a video representation) produces no or only small adaptations (after-effects), whereas prism exposure in a natural condition produces large after-effects. Some researchers suggested that this difference may arise from distinct adaptive processes, but other studies suggested a unique process. The present study reconciled these conflicting interpretations. Forty participants were divided into two groups: One group used visual feedback of their hand (natural context), and the other group used computer-generated representational feedback (virtual context). Visual feedback during adaptation was concurrent or terminal. All participants underwent laterally displacing prism perturbation. The results showed that the after-effects were twice as large in the "natural context" than in the "virtual context". No significant differences were observed between the concurrent and terminal feedback conditions. The after-effects generalized to untested targets and workspace. These results suggest that prism adaptation in virtual and natural contexts involves the same process. The smaller after-effects in the virtual context suggest that the depth of adaptation is a function of the degree of convergence between the proprioceptive and visual information that arises from the hand.

The role of the cross-sensory error signal in visuomotor adaptation

Reaching to targets with misaligned visual feedback of the hand leads to changes in proprioceptive estimates of hand position and reach aftereffects. In such tasks, subjects are able to make use of two error signals: the discrepancy between the desired and actual movement, known as the sensorimotor error signal, and the discrepancy between visual and proprioceptive estimates of hand position, which we refer to as the cross-sensory error signal. We have recently shown that mere exposure to a sensory discrepancy in the absence of goal-directed movement (i.e. no sensorimotor error signal) is sufficient to produce similar changes in felt hand position and reach aftereffects. Here, we sought to determine the extent that this cross-sensory error signal can contribute to proprioceptive recalibration and movement aftereffects by manipulating the magnitude of this signal in the absence of volitional aiming movements. Subjects pushed their hand out along a robot-generated linear path that was gradually rotated clockwise relative to the path of a cursor. On all trials, subjects viewed a cursor that headed directly towards a remembered target while their hand moved out synchronously. After exposure to a 30° rotated hand-cursor distortion, subjects recalibrated their sense of felt hand position and adapted their reaches. However, no additional increases in recalibration or aftereffects were observed following further increases in the cross-sensory error signal (e.g. up to 70°). This is in contrast to our previous study where subjects freely reached to targets with misaligned visual hand position feedback, hence experiencing both sensorimotor and cross-sensory errors, and the distortion magnitude systematically predicted increases in proprioceptive recalibration and reach aftereffects. Given these findings, we suggest that the cross-sensory error signal results in changes to felt hand position which drive partial reach aftereffects, while larger aftereffects that are produced after visuomotor adaptation (and that vary with the size of distortion) are related to the sensorimotor error signal.

Ecological visuomotor tasks during prism adaptation cause larger aftereffects than repeated pointings in healthy participants

Ecological visuomotor tasks during prism adaptation cause larger aftereffects than repeated pointings in healthy participants

The aftereffect of a spatial offset between Gabor patches depends on carrier orientations

This study explored the orientation connectivity in a contrast modulation processing mechanism, modeled as two filtering stages with nonlinear processing in between, by investigating how a negative aftereffect of a contrast-defined spatial offset is influenced by carrier orientations in the adapting stimulus. After adaptation to multiple, globally presented pairs of Gabor patches with a specific horizontal offset, subjects perceived a vertically aligned test pair of patches as offset in the orientation opposite to that of the adaptor. Although the orientations of the carrier gratings in the adaptor pairs were irrelevant to the task, the aftereffect magnitude depended on them. A large aftereffect was observed when the carrier orientations were parallel and/or perpendicular to the contrast-defined orientation, supporting the notion that second-stage filters receive strong inputs from first-stage filters with parallel and perpendicular orientation preferences. Furthermore, the aftereffect was also large when the carrier for only one patch was parallel or perpendicular, and no significant difference in the aftereffect magnitude was observed whether the adaptor pair contained one or two such patches. These results suggest that connectivity is not strictly selective to parallel and perpendicular relationships. Spatially heterogeneous connectivity might explain the observed effect.

Saccade adaptation improves in response to a gradually introduced stimulus perturbation

A major goal in the study of motor learning is to improve the extent to which subjects adapt their movements in response to errors. Recent attention has focused on the gradual-adaptation paradigm, in which an adaptive stimulus is introduced incrementally, rather than all at once as in conventional adaptation paradigms. However, there is disagreement – even among studies involving the same sensorimotor-learning task – as to the robustness of this approach. In particular, although all studies confirm that retention of learning is improved, not all agree that exposure to a gradual-adaptation paradigm can improve the extent of adaptation that takes place. Also, the paradigm has not previously been studied with saccadic eye movements, which are unique in that they typically lack online error feedback during each movement. To determine the effectiveness of gradual adaptation in this system, we compared saccadic adaptation performed with gradual and conventional adaptation paradigms. We find evidence consistent with more robust adaptation – in the sense of greater extent of adaptation as well as greater retention of learning (larger aftereffects) – in response to a gradual adaptation stimulus. The results suggest the need to develop alternative models of motor learning, as current error-based modeling efforts are unable to account for the increased extent of adaptation when subjects are only exposed to the full adaptive stimulus for a brief time.

Left Parietal Regions Are Critical for Adaptive Visuomotor Control

The question addressed in this study is whether parietal brain circuits involved in adaptation to novel visuomotor conditions are lateralized. This information is critical for characterizing the neural mechanisms mediating adaptive behavior in humans, as well as for assessing the effects of unilateral brain damage on function. Moreover, previous research has been controversial in this regard. We compared visuomotor adaptation in 10 patients with focal, unilateral, left or right parietal lesions and healthy control participants. All subjects reached to each of eight targets over three experimental sessions: a baseline session, where the visually displayed and actual hand motion were matched; an adaptation session, where the visual feedback deviated from the actual movement direction by 30°; and an after-effect session, where visual feedback was again matched to hand motion. Adaptation was primarily quantified as a change in initial movement direction throughout the adaptation session and the presence of after-effects when the rotation was removed. Patients with right parietal damage demonstrated normal adaptation and large after-effects, which was comparable to the performance of healthy controls. In contrast, patients with left parietal damage showed a clear deficit in adaptation and showed no after-effects. Thus, our results show that left but not right parietal regions are critical for visuomotor adaptation. These findings are discussed in the context that left parietal regions are critical for the modification of stored representations of the relationship between movement commands and limb and environmental state, as is thought to occur during visuomotor adaptation.

Adaptation to conflicting visual and physical heading directions during walking

We investigated the role of global optic flow for visual-motor adaptation of walking direction. In an immersive virtual environment, observers walked to a circular target lying on either a homogeneous ground plane (target-motion condition) or a textured ground plane (ground-flow condition). During adaptation trials, we changed the mapping from physical to visual space to create a conflict between physical and visual heading directions. On these trials, the visual heading specified by optic flow deviated from an observer's physical heading by ±10°. This conflict was not noticed by observers but caused them to walk along curved paths to the target. Over the course of 20 adaptation trials, observers adapted to partially compensate for the conflicts, resulting in straighter paths. When the conflicts were removed post-adaptation, observers showed aftereffects in the opposite direction. The amount of adaptation was similar for target-motion and ground-flow conditions (20-25%), with the ground-flow environment producing slightly faster adaptation and larger aftereffects. We conclude that the visual-motor system can rapidly recalibrate the mapping from physical to visual heading and that this adaptation does not strongly depend on full-field optic flow.

Sex-specific norms code face identity

Face identity aftereffects suggest that an average face, which is continuously updated by experience, functions as a norm for coding identity. Sex-contingent figural face aftereffects indicate that different norms are maintained for male and female faces but do not directly implicate them in coding identity. Here, we investigated whether sex-specific norms are used to code the identities of male and female faces or whether a generic, androgynous norm is used for all faces. We measured identity aftereffects for adapt-test pairs that were opposite relative to a sex-specific average and pairs that were opposite relative to an androgynous average. Identity aftereffects are generally larger for adapt-test pairs that lie opposite an average face, which functions as a norm for coding identity, than those that do not. Therefore, we reasoned that whichever average gives the larger aftereffect would be closer to the true psychological norm. Aftereffects were substantially and significantly larger for pairs that lie opposite a sex-specific than an androgynous average. This difference remained significant after correcting for differences in test trajectory length. These results indicate that, despite the common structure shared by all faces, identity is coded using sex-specific norms. We suggest that the use of category-specific norms may increase coding efficiency and help us discriminate thousands of faces despite their similarity as patterns.

Relation between reaction time and reach errors during visuomotor adaptation

Adaptation of reaching movements to visuomotor transformations is generally thought to involve implicit or procedural learning. However, there is evidence that explicit or cognitive processes can also play a role (Redding and Wallace, 2006 [31]). For example, the early phase of adaptation to a visuomotor rotation appears to involve spatial working memory processes linked to mental rotation (Anguera et al., 2010 [11]). Since it is known that cognitive processes like mental rotation lead to larger reaction times (Georgopoulos and Massey, 1987 [12]), here we explored the relation between reaction time (RT) and reach error reduction. Two groups of subjects adapted their reaching movements to a 60° visuomotor rotation either without RT constraints or with RT limited to 350ms. In the unconstrained group, we found that adaption rate varied widely across subjects and was strongly correlated with RT. Subjects who decreased hand direction error (DE) rapidly exhibited prolonged RTs whereas little RT cost was seen in subjects who decreased DE gradually. RTs were also correlated with after-effects seen when the visuomotor rotation was removed. Subjects with the longest RTs exhibited the smallest after-effects. In the RT constrained group, all subjects exhibited gradual DE adaptation and large after-effects, similar to the fast responders in the free group. These results suggest that adaptation to a visuomotor rotation can involve processes that produce faster error reductions without increasing after-effects, but at an expense of larger reaction times. Possible candidates are processes related to spatial working memory, and more specifically, to mental rotation.

Adaptation of egocentric distance perception under telestereoscopic viewing within reaching space

Telestereoscopic viewing provides a method to distort egocentric distance perception by artificially increasing the interpupillary distance. Adaptation to such a visual rearrangement is little understood. Two experiments were performed in order to dissociate the effects of a sustained increased vergence demand, from those of an active calibration of the vergence/distance mapping. Egocentric distances were assessed within reaching space through open-loop pointing to small targets in the dark. During the exposure condition of the first experiment, subjects were instructed to point to the targets without feedback, whereas in the second experiment, hand visual feedback was available, resulting in a modified relationship between vergence-specified distance and reach distance. The visual component of adaptation in the second experiment was assessed on the unexposed hand. In the post-tests of both experiments, subjects exhibited a constant distance overestimation across all targets, with a more than twice larger aftereffect in the second one. These findings suggest two different processes: (1) an alteration in the vergence effort following sustained increased vergence; (2) a calibration of the vergence/distance mapping uncovering the visual component of adaptation.

Multiple Motor Learning Strategies in Visuomotor Rotation

BackgroundWhen exposed to a continuous directional discrepancy between movements of a visible hand cursor and the actual hand (visuomotor rotation), subjects adapt their reaching movements so that the cursor is brought to the target. Abrupt removal of the discrepancy after training induces reaching error in the direction opposite to the original discrepancy, which is called an aftereffect. Previous studies have shown that training with gradually increasing visuomotor rotation results in a larger aftereffect than with a suddenly increasing one. Although the aftereffect difference implies a difference in the learning process, it is still unclear whether the learned visuomotor transformations are qualitatively different between the training conditions.Methodology/Principal FindingsWe examined the qualitative changes in the visuomotor transformation after the learning of the sudden and gradual visuomotor rotations. The learning of the sudden rotation led to a significant increase of the reaction time for arm movement initiation and then the reaching error decreased, indicating that the learning is associated with an increase of computational load in motor preparation (planning). In contrast, the learning of the gradual rotation did not change the reaction time but resulted in an increase of the gain of feedback control, suggesting that the online adjustment of the reaching contributes to the learning of the gradual rotation. When the online cursor feedback was eliminated during the learning of the gradual rotation, the reaction time increased, indicating that additional computations are involved in the learning of the gradual rotation.Conclusions/SignificanceThe results suggest that the change in the motor planning and online feedback adjustment of the movement are involved in the learning of the visuomotor rotation. The contributions of those computations to the learning are flexibly modulated according to the visual environment. Such multiple learning strategies would be required for reaching adaptation within a short training period.

Aftereffects for face attributes with different natural variability: Children are more adaptable than adolescents

Adults can be adapted to a particular facial distortion in which both eyes are shifted symmetrically (Robbins, R., McKone, E., & Edwards, M. (2007). Aftereffects for face attributes with different natural variability: Adapter position effects and neural models. Journal of Experimental Psychology: Human Perception and Performance, 33, 570–592), but they do not show as great adaptation to an asymmetrical eye distortion. We adapted children and adolescents to symmetrical and asymmetrical eye distortions and measured the aftereffects. Children (aged 6–12, mean age 9 years) showed larger aftereffects than adolescents (aged 13–18, mean age 15 years) and demonstrated aftereffects of a similar magnitude for both asymmetrical and symmetrical distortions. Adolescents only showed aftereffects for symmetrical distortions. We propose that children may have a more flexible face norm and neural responses that allow a broader range of adapted states compared to adolescents.

P.2.a.018 Lack of the large aftereffect of bright light on pupillary fluctuations in patients with non-seasonal depression

This study sought to observe the appearance of normal esophagus, measure and record the thickness of esophageal wall in order to offer reference for estimating esophageal wall abnormalities and delineating gross tumor target of esophageal carcinomas on CT images.From September 2006 to February 2007, 110 consecutive CT films from adult patients without esophageal diseases were collected and studied. On CT images the entire esophagus was divided into cervical, thoracic, retrocardiac and intraabdominal segments. The appearance of esophagus was described when the esophagus contracted or dilated. Thickness of esophageal wall and diameters of esophageal cavities were measured by hard-copy reading with a magnifying glass. Age, sex and the thickness of subcutaneous fat of each patient were recorded.It was observed that the esophagus presented both contracted and dilated status on CT images. In each segment there were certain portions of esophagus in complete contraction or dilatation. 47 images (42.7%) showed contracted esophagus in each segment available for measurement. The largest wall thickness when esophagus was in contraction and dilatation was 4.70 (95%CI: 4.44–4.95) mm and 2.11 (95%CI: 2.00–2.23) mm, respectively. When contracting, the intraabdominal esophagus was thicker than the cervical, thoracic and retrocardiac parts, and the average thickness was 5.68 (95%CI: 5.28–6.09) mm, 4.67 (95%CI: 4.36–4.86) mm, 4.56 (95%CI: 4.31–4.87) mm, and 4.05 (95%CI: 3.71–4.21) mm, respectively. When the esophagus was dilating, the average esophageal wall thickness was between 1.87 and 2.70 mm. The thickest part was cervical esophagus. Thickness of esophageal wall was larger in males than that of females (5.26 mm vs. 4.34 mm p < 0.001). Age and the thickness of subcutaneous fat had no significant impact on the thickness of esophageal wall (p-value was 0.056 and 0.173, respectively).The Observation of normal appearance and wall thickness of esophagus helps us to identify thickened esophageal wall on CT images using new CT scan technologies. Thus it is probably helpful in judging esophageal diseases and delineating gross tumor target of esophageal carcinomas in modern radiotherapy.