Dissociating perceptual and motor effects of prism adaptation in neglect

Investigating premotor reaching biases after prism adaptation

Prism adaptation (PA) is both a visuomotor learning task and potential treatment for spatial neglect after stroke. While PA's aftereffects can improve neglect symptoms, therapeutic benefits vary across individuals, possibly due to differences in neglect subtypes. Neglect symptoms can be described along an information processing pathway, yielding perceptual (input) and premotor (output) neglect subtypes. There is some evidence that PA mainly benefits persons with premotor neglect. We investigated whether PA modulates the premotor stage of information processing by examining whether PA could induce a premotor bias in healthy adults. We measured perceptual and premotor biases using a speeded reach task that compares the initiation time of leftward and rightward reaches to lateralized targets from different hand start positions. Using a randomized mixed experimental design, 30 right-handed healthy adults completed this speeded reach task before and after either left-shifting (n = 15) or right-shifting (n = 15) PA. As hypothesized, left-shifting PA speeded initiation time specifically for reaches in the rightward direction, regardless of target location (p = .02, ηp2 = .18), suggesting that PA induced a premotor bias in the direction of the prism aftereffect. These findings have implications for PA's underlying mechanisms, which can inform visuomotor learning theories and PA's use as a treatment for spatial neglect.

A scoping review and critique of the Input-Output subtyping dimension of spatial neglect

Spatial neglect is a common and debilitating disorder after stroke whereby individuals have difficulty reporting, orienting, and/or responding to the contralesional side of space. Given the heterogeneity of neglect symptom presentation, various neglect subtypes have been proposed to better characterize the disorder. This review focuses on the distinction between Input neglect (i.e., difficulty perceiving and/or attending to contralesional stimuli) and Output neglect (i.e., difficulty planning and/or executing movements toward contralesional stimuli). Conceptualizations of Input and Output neglect have varied considerably. We provide a novel summary of the terminology, measurement approaches, and neural correlates of these subtypes. A protocol detailing our systematic scoping review strategy is registered on the Open Science Framework (https://osf.io/bvtxf/). For feasibility and greater comparability across studies, we limited our inclusion criteria to tasks focused on visual stimuli and upper-limb movements. A total of 110 articles were included in the review. Subtyping tasks were categorized based on whether they mainly manipulated aspects of the input (i.e., congruence of visual input with motor output, presence of visual input) or the output (i.e., modality, goal, or direction of output) to produce an Input-Output subtype dissociation. We used our review results to identify four main critiques of this literature: 1) lack of consistency/clarity in conceptual models; 2) methodological issues of dissociating Input and Output subtypes; 3) a need for updated neural theories; and 4) barriers to clinical application. We discuss the lessons learned from this subtyping dimension that can be applied to future research on neglect subtype assessment and treatment.

Effects of prism adaptation and visual scanning training on perceptual and response bias in unilateral spatial neglect

In some patients with unilateral spatial neglect, symptoms reflect impaired lateralized spatial attention and representation (perceptual bias) whereas in others the inability to respond to stimuli located in contralesional space (response bias). Here, we investigated whether prismatic adaptation (PA) and visual scanning training (VST) differentially affect perceptual and response bias and whether rehabilitation outcome depends on the type of bias underlying symptoms. Two groups of neglect patients in the subacute phase were evaluated before, immediately after, and two weeks following 10 days of PA (n = 9) or VST (n = 9). Standard neuropsychological tests (i.e., Behavioural Inattentional Test, Diller cancellation test, and Line Bisection test) were administered to assess neglect symptoms, while the Landmark task was used to disentangle perceptual and response biases. Performance on the Landmark task revealed that PA was more effective in improving the perceptual bias, while VST mainly modulated the response bias. Neuropsychological tests performance suggested that VST is better suited to modulate neglect in patients with response bias, while PA may be effective in patients with both types of bias. These findings may offer novel insights into the efficacy of PA and VST in the rehabilitation of perceptual and response biases in patients with neglect.

Age-related changes to electroencephalographic markers of visuomotor error processing and learning in prism adaptation

Aging is associated with changes in cognitive function, including declines in learning, memory, and executive function. Prism adaptation (PA) is a useful paradigm to measure changes in explicit and implicit mechanisms of visuo-motor learning with age, but the neural correlates are not well understood. In the present study, we used PA to investigate visuo-motor learning and error processing in older adults. Twenty older adults (56-85 yrs) and 20 younger adults (18-33 yrs) underwent a goal-oriented reaching task while wearing prism goggles as continuous EEG was recorded to examine neural correlates of error detection. We examined behavioural measures of PA, as well as ERP components previously found associated with the early and late phases of adaptation to visual distortion caused by the prism goggles. Our results indicate important age-related behavioural and neurophysiological differences. Older adults reached more slowly than younger adults but showed the same accuracy throughout the prism exposure. Older adults also displayed larger aftereffects, indicating preserved visuomotor adaptation. EEG results indicated similar initial error processing in older and younger adults, as measured by the feedback error related negativity (FRN). As seen previously in young adults, the P3a and P3b declined over the prism exposure phase in both groups. Older adults displayed reduced P3a amplitude compared to the younger group in the early phase of adaptation, however, suggesting reduced attentional orienting. Finally, the older group exhibited a greater P3b amplitude compared to the younger group in the later phases of adaptation, potentially a marker of enhanced context updating underlying spatial realignment, leading to their larger aftereffect. Implications for age-related learning differences and clinical applications are discussed.

Prism adaptation combined with eye movement training for unilateral spatial neglect after stroke: Study protocol for a single-blind prospective, randomized controlled trial

Background

Unilateral spatial neglect (USN) is a complex neurological syndrome that often reduces rehabilitation outcomes, prolongs patients' hospital stays, and decreases their quality of life. However, the current therapies for USN have varying efficacy. We will explore a new treatment option that combines prism adaptation (PA) with eye movement training (EMT) for the treatment of USN after stroke.

Methods

We will conduct a single-blind, prospective, randomized controlled trial to assess the efficacy of the combined intervention (PA & EMT) on USN in an inpatient rehabilitation setting. The study aims to recruit 88 patients with USN after an ischemic or hemorrhagic stroke. Participants will be randomly assigned to the following four groups: (1) PA group (n = 22), (2) EMT group (n = 22), (3) PA and EMT group (n = 22), and (4) control group (n = 22). All groups will receive 10 sessions of interventions over 2 weeks, 5 times per week. Blinded assessors will conduct a baseline assessment, a post-intervention assessment, and a follow-up assessment (2 weeks post-intervention). The primary outcome measure will use the Behavioral Inattention Test-Conventional Subset (BIT-C) and Catherine Bergego Scale (CBS) to assess the levels of USN. Secondary outcome measures will assess the patient's ability to perform activities of daily living using the Modified Barthel Index (MBI). Patients who completed all treatment and assessment sessions will be included in the final analysis.

Discussion

This study will explore the effects of 10 sessions of combined interventions (PA & EMT) on USN and functional capacity. This study has the potential to identify a new, evidence-based treatment option and provide new ideas for the treatment of USN.

Ethics and dissemination

The study protocol has been approved by the Nanchong Central Hospital. Written informed consent will be obtained from all the participants. The results of this study will be disseminated to the public through scientific conferences and a peer-reviewed journal.

Trial registration

ChiCTR, ChiCTR2100049482. Registered on 2 August 2021, http://www.chictr.org.cn/showproj.aspx?proj=130823.

Using prism adaptation to alleviate perception of unilateral tinnitus: A case study

Tinnitus is described as an uncomfortable sound or noise heard by an individual in the absence of an external sound source. Treating this phantom perception remains difficult even if drug and nondrug therapies are used to alleviate symptoms. The present case study aimed to investigate whether prism adaptation could induce beneficial aftereffects in a tinnitus sufferer. A 75-year-old man, R. B., with chronic unilateral tinnitus in the left ear reported a self-estimation of parameters of his tinnitus-discomfort, pitch and loudness-and performed a manual line-bisection task to study the consequences of lateralized auditory disorder on spatial representation. Aftereffects of prism adaptation were assessed using a sensorimotor open-loop pointing task. In parallel, a control group completed the line-bisection task and the open-loop pointing task before and after lens exposure, under the same experimental condition as those of R. B. Throughout the pretests, the patient assessed his tinnitus at a constant medium pitch (around 3000 Hz), and he was biased toward the affected ear in both the sensorimotor task and the estimation of the subjective center in the manual line-bisection task. Although both optical deviations were effective, an exposure to prism adaptation to a rightward optical deviation (i.e., toward the unaffected ear) produced stronger aftereffects. In posttests, the tinnitus pitch decreased to 50 Hz and the subjective center was shifted toward the right side (i.e., unaffected ear side). Furthermore, the line-bisection task seemed to reflect the changes in the tinnitus perception, and spatial representation could be a new tool to assess tinnitus indirectly. Our findings suggest that prism adaptation may have benefits on unilateral tinnitus and open a new avenue for its treatment.

Choosing Sides: Impact of Prismatic Adaptation on the Lateralization of the Attentional System

Seminal studies revealed differences between the effect of adaptation to left- vs. right-deviating prisms (L-PA, R-PA) in normal subjects. Whereas L-PA leads to neglect-like shift in attention, demonstrated in numerous visuo-spatial and cognitive tasks, R-PA has only minor effects in specific aspects of a few tasks. The paucity of R-PA effects in normal subjects contrasts with the striking alleviation of neglect symptoms in patients with right hemispheric lesions. Current evidence from activation studies in normal subjects highlights the contribution of regions involved in visuo-motor control during prism exposure and a reorganization of spatial representations within the ventral attentional network (VAN) after the adaptation. The latter depends on the orientation of prisms used. R-PA leads to enhancement of the ipsilateral visual and auditory space within the left inferior parietal lobule (IPL), switching thus the dominance of VAN from the right to the left hemisphere. L-PA leads to enhancement of the ipsilateral space in right IPL, emphasizing thus the right hemispheric dominance of VAN. Similar reshaping has been demonstrated in patients. We propose here a model, which offers a parsimonious explanation of the effect of L-PA and R-PA both in normal subjects and in patients with hemispheric lesions. The model posits that prismatic adaptation induces instability in the synaptic organization of the visuo-motor system, which spreads to the VAN. The effect is lateralized, depending on the side of prism deviation. Successful pointing with prisms implies reaching into the space contralateral, and not ipsilateral, to the direction of prism deviation. Thus, in the hemisphere contralateral to prism deviation, reach-related neural activity decreases, leading to instability of the synaptic organization, which induces a reshuffling of spatial representations in IPL. Although reshuffled spatial representations in IPL may be functionally relevant, they are most likely less efficient than regular representations and may thus cause partial dysfunction. The former explains, e.g., the alleviation of neglect symptoms after R-PA in patients with right hemispheric lesions, the latter the occurrence of neglect-like symptoms in normal subjects after L-PA. Thus, opting for R- vs. L-PA means choosing the side of major IPL reshuffling, which leads to its partial dysfunction in normal subjects and to recruitment of alternative or enhanced spatial representations in patients with hemispheric lesions.

Prism Adaptation and Optokinetic Stimulation Comparison in the Rehabilitation of Unilateral Spatial Neglect

Prism adaptation (PA) is one of the most effective treatments for the rehabilitation of unilateral spatial neglect. Optokinetic stimulation (OKS) has also been demonstrated to be effective in ameliorating symptoms of neglect. The aim of this study is to compare the effectiveness of these two methods in a group of neglect patients using a crossover design. A group of 13 post-acute brain-damaged patients with unilateral spatial neglect, who had never been rehabilitated, were treated using PA and OKS. Each treatment was applied for 10 sessions, twice a day, to all patients with both treatments in crossed order (i.e., PA followed by OKS or vice versa). Neuropsychological assessments were performed: before the first (T1), at the end of the first/beginning of the second (T2) and at the end of the second training sessions (T3), and two weeks after the end of treatment (T4). Both procedures produced a significant improvement in clinical tests at T2, independent of the type of training. The results suggest that either PA or OKS induces a significant amelioration of neglect in right brain-damaged patients, mainly in the first block of treatment. Since no differences between treatments were found, they could be applied in clinical practice, according to the requirements of the individual patient.

Effects of Prism Adaptation for Unilateral Spatial Neglect After Stroke: A Systematic Review and Meta-Analysis

OBJECTIVE

The aim of the study was to assess effects of prism adaptation on unilateral spatial neglect poststroke.

METHODS

Searching databases and referring to randomized controlled trial until December 30, 2019. There were two reviewers who searched some key words such as unilateral spatial neglect and prism adaptation, and only randomized controlled trial was included. There were two evaluators who screened literature and data regarding research design, treatment plan, etc. Assessment tool recommended in Cochrane Reviewers' Handbook 5.1.0 was adopted to assess the risks of bias of randomized controlled trial, and Revman 5.3 software was used for meta-analysis. Mean differences and 95% of confidence intervals were calculated, and heterogeneity was assessed through I2 statistics.

RESULTS

Eight randomized controlled trials were included, involving 244 patients of which 130 in experimental group and 114 in control group. According to meta-analysis results, prism adaptation group scored significantly higher than control group in improving Behavioral Inattention Test (mean difference = 8.99, 95% confidence interval = 0.93 to 17.06, P = 0.03) and Star Cancellation Test (mean difference = 3.04, 95% confidence interval = 0.19 to 5.88, P = 0.04). However, there was no significant statistical difference in Catherine Bergego Scale between groups (mean difference = -0.30, 95% confidence interval = -1.83 to 1.23, P = 0.70). The long-term follow-up visits (over 1 mo) appeared no statistical difference in Behavioral Inattention Test scores between the patients of two groups (mean difference = 8.93, 95% confidence interval = -1.98 to 19.84, P = 0.1).

CONCLUSIONS

Prism adaptation can improve neglect symptoms of patients with unilateral spatial neglect after stroke temporarily.

Exploring the time-course and the reference frames of adaptation to optical prisms and its aftereffects

Prism adaptation (PA) is used to investigate visuo-motor plasticity and to rehabilitate the syndrome of Unilateral Spatial Neglect (USN). After PA, participants show aftereffects (AEs), contralateral to the side of the optical displacement in several tasks. This study explored the features of these AEs, specifically the "egocentric" versus "allocentric, object-based", reference frames involved, and their time course. In three experiments, healthy participants adapted to prismatic lenses inducing a horizontal displacement of the visual field. In Experiment #1, participants adapted to rightward displacing prisms. Four tasks were used requiring repeated pointings towards the participant's subjective egocentric straight-ahead, with the availability of proprioceptive or visual-proprioceptive signals, and, in some conditions, of an external allocentric visual frame (i.e., a rectangular paper sheet). Experiment #2 explored the role of the position of the allocentric frame, with AEs being tested by straight-ahead and frame bisection tasks, requiring pointing toward the external visual frame, placed in different positions of the working space. An egocentric visual proprioceptive task was administered after prism removal and after the execution of the allocentric tasks, to assess the effectiveness of the PA, as indexed by the AEs, and their persistence up to the end of the administration of the allocentric tasks. Experiment #3 differed from #2 in that participants adapted to leftward displacing lenses. Consistent with evidence from USN patients, in Experiment #1, in the egocentric tasks, AEs lasting up to 30 min after PA were found. In Experiment #2, AEs in "allocentric" tasks did not occur, regardless of frame position. Experiment #3 showed AEs in both the "egocentric" and the "allocentric" tasks, with the latter being minor in size. These findings illustrate that the spatial reference systems modulated by PA in extra-personal space primarily operate in spatial "egocentric" reference frames, with a comparatively minor and direction-specific role of "allocentric" frames.

A Meta-Analysis of Line Bisection and Landmark Task Performance in Older Adults

Young adults exhibit a small asymmetry of visuospatial attention that favours the left side of space relative to the right (pseudoneglect). However, it remains unclear whether this leftward bias is maintained, eliminated or shifted rightward in older age. Here we present two meta-analyses that aimed to identify whether adults aged ≥50 years old display a group-level spatial attention bias, as indexed by the line bisection and the landmark tasks. A total of 69 datasets from 65 studies, involving 1654 participants, were analysed. In the meta-analysis of the line bisection task (n = 63), no bias was identified for studies where the mean age was ≥50, but there was a clear leftward bias in a subset where all individual participants were aged ≥50. There was no moderating effect of the participant’s age or sex, line length, line position, nor the presence of left or right cues. There was a small publication bias in favour of reporting rightward biases. Of note, biases were slightly more leftward in studies where participants had been recruited as part of a stand-alone older group, compared to studies where participants were recruited as controls for a clinical study. Similarly, no spatial bias was observed in the meta-analysis of the landmark task, although the number of studies included was small (n = 6). Overall, these results indicate that over 50s maintain a group-level leftward bias on the line bisection task, but more studies are needed to determine whether this bias can be modulated by stimulus- or state-dependent factors.

Improvement of phonemic fluency following leftward prism adaptation

Anatomo functional studies of prism adaptation (PA) have been shown to modulate a brain frontal-parieto-temporal network, increasing activation of this network in the hemisphere ipsilateral to the side of prism deviation. This effect raises the hypothesis that left prism adaptation, modulating frontal areas of the left hemisphere, could modify subjects’ performance on linguistic tasks that map on those areas. To test this hypothesis, 51 healthy subjects participated in experiments in which leftward or rightward prism adaptation were applied before the execution of a phonemic fluency task, i.e., a task with strict left hemispheric lateralization onto frontal areas. Results showed that leftward PA significantly increased the number of words produced whereas rightward PA did not significantly modulate phonemic fluency. The present findings document modulation of a language ability following prism adaptation. The results could have a huge clinical impact in neurological populations, opening new strategies of intervention for language and executive dysfunctions.

Effect of Prism Adaptation Therapy on the Activities of Daily Living and Awareness for Spatial Neglect: A Secondary Analysis of the Randomized, Controlled Trial

BACKGROUND

Rehabilitation for unilateral spatial neglect (USN) using prism adaptation (PA) is one of the most widely used methods, and the effectiveness of PA is well-evidenced. Although the effect of PA generalized various neglect symptoms, the effectiveness for some aspects of neglect is not fully proven. The Catherine Bergego Scale (CBS) was developed to identify problems with the activities of daily living (ADL) caused by USN. The CBS is composed of 10 observation assessments and a self-assessment questionnaire. To assess the self-awareness of USN, the anosognosia score is calculated as the difference between the observational scores and the self-assessment scores. To investigate how PA affects ADL and self-awareness in subacute USN patients during rehabilitation, we analyzed each item of the CBS and self-awareness from a randomized, controlled trial (RCT) that we previously conducted (Mizuno et al., 2011).

METHODS

A double-masked randomized, controlled trial was conducted to evaluate the effects of a 2-week PA therapy on USN in 8 hospitals in Japan. We compared each item of the CBS, anosognosia score, and absolute value of the anosognosia score between the prism group and the control group.

RESULTS

Two of ten items (gaze orientation and exploration of personal belongings) were significantly improved in the prism group compared with those in the control group. The absolute value of the anosognosia score was significantly improved by PA.

CONCLUSIONS

Improvement of oculomotor exploration by PA may generalize the behavioral level in a daily living environment. This study suggested that PA could accelerate the self-awareness of neglect during subacute rehabilitation.

A novel computerized assessment of manual spatial exploration in unilateral spatial neglect

Unilateral spatial neglect is a neuropsychological syndrome commonly observed after stroke and defined by the inability to attend or respond to contralesional stimuli. Typically, symptoms are assessed using clinical tests that rely upon visual/perceptual abilities. However, neglect may affect high-level representations controlling attention in other modalities as well. Here we developed a novel manual exploration test using a touch screen computer to quantify spatial search behaviour without visual input. Twelve chronic stroke patients with left neglect and 27 patients without neglect (based on clinical tests) completed our task. Four of the 12 "neglect" patients exhibited clear signs of neglect on our task as compared to "non-neglect" patients and healthy controls, and six other patients (from both groups) also demonstrated signs of neglect compared to healthy controls only. While some patients made asymmetrical responses on only one task, generally, patients with the strongest neglect performed poorly on multiple tasks. This suggests that representations associated with different modalities may be affected separately, but that severe forms of neglect are more likely related to damage in a common underlying representation. Our manual exploration task is easy to administer and can be added to standard neglect screenings to better measure symptom severity.

Frontal lesions predict response to prism adaptation treatment in spatial neglect: A randomised controlled study

Spatial neglect commonly follows right hemisphere stroke. It is defined as impaired contralesional stimulus detection, response, or action, causing functional disability. While prism adaptation treatment is highly promising to promote functional recovery of spatial neglect, not all individuals respond. Consistent with a primary effect of prism adaptation on spatial movements, we previously demonstrated that functional improvement after prism adaptation treatment is linked to frontal lobe lesions. However, that study was a treatment-only study with no randomised control group. The current study randomised individuals with spatial neglect to receive 10 days of prism adaptation treatment or to receive only standard care (control group). Replicating our earlier results, we found that the presence of frontal lesions moderated response to prism adaptation treatment: among prism-treated patients, only those with frontal lesions demonstrated functional improvements in their neglect symptoms. Conversely, among individuals in the standard care control group, the presence of frontal lesions did not modify recovery. These results suggest that further research is needed on how frontal lesions may predict response to prism adaptation treatment. Additionally, the results help elucidate the neural network involved in spatial movement and could be used to aid decisions about treatment.

Effects of Prism Adaptation on Reference Systems for Extrapersonal Space in Neglect Patients

Up to now, rehabilitation of unilateral spatial neglect has focused on egocentric forms of neglect, whereas less is known about the possibility to improve allocentric deficits. The present study aimed to examine the efficacy of prism adaptation (PA) training on patients with different forms of neglect: egocentric, allocentric, or mixed. Twenty-eight patients were assessed with specific neglect tests before (T0) and after (T1) 10 sessions of PA training. Performance in the Apples Cancellation test was used to identify patients with egocentric (n = 6), allocentric (n = 5), or mixed (n = 17) forms of neglect. In the overall group of patients, PA training produced significant improvements in performance across different neglect tests. In terms of the egocentric–allocentric distinction, the training was effective in reducing omissions in the left part of space in the Apples Cancellation test both for patients with egocentric neglect and mixed neglect. By contrast, errors of commissions (marking the inability to detect the left part of the target stimulus, i.e., allocentric neglect) remained unchanged after PA in patients with allocentric neglect and actually increased marginally in patients with mixed neglect. The PA training is effective in improving egocentric neglect, while it is ineffective on the allocentric form of the disturbance. Notably, the allocentric component of neglect is frequently impaired, although this is most often in conjunction with the egocentric impairment, yielding the mixed form of neglect. This stresses the importance of developing exercises tuned to improving allocentric neglect.

Regression of left hyperschematia after prism adaptation: A single case study

Prism adaptation (PA) is a promising treatment in the rehabilitation of post-stroke cognitive disorders such as unilateral spatial neglect or constructional deficits. Right brain damage can bring about another representational spatial disorder, termed «hyperschematia», and defined by a left-sided disproportionate expansion of drawings by copy and from memory, and by an overestimation of left lateral extent when a leftward movement is required. This case study aimed at evaluating the effect of PA induced by prismatic lenses creating a shift to the left on hyperschematia signs. A 63-year-old woman with left hyperschematia, consecutive to a right fronto-temporo-parietal hematoma, was exposed to a leftward optical deviation produced by prismatic lenses. An anatomical MRI studied topography of the brain lesion; the patient's lesion was then mapped onto tractography reconstructions of white matter pathways. Results showed that PA significantly reduced the left-sided expansion of drawing by copy and from memory, and the overestimation of left lateral extent, immediately after prism removal and 4 days later, indicating a persistent long lasting cognitive effect. MRI showed a right hemisphere disconnection of the posterior and long segments of the arcuate fasciculus, and of the inferior longitudinal and fronto-occipital fasciculi. Overall, these findings suggest that: i) PA is effective also in hyperschematia by re-orientating spatial attention towards the right side of space, with a relative rightward PA-induced unbalance, and re-setting the spatial representation to the left side of space, contralateral to the side of the lesion; ii) the left misrepresentation of lateral extent may be related to a disconnection between visual coordinates and attentional networks to the frontal lobe.

Effect of prism adaptation on neglect hemianesthesia

Prism adaptation (PA) has proven to be effective in alleviating many signs of unilateral spatial neglect (USN). Generally, the principal improvement after PA treatment was found to be in the high-level cognitive function. Nevertheless, some evidence has also been found for it in somatosensory function. We have aimed to test the influence of PA on neglect hemianesthesia, a condition in which the high-level neglect-related deficit mimics hemianesthesia. Twenty-one USN patients were enrolled in the study. Each patient performed two sessions of PA, one with neutral glasses and one with prism glasses using a cross-over design. Sensitivity on the upper limb was tested using two methods. The first task was the sensibility subtest which was derived from the standard clinical examination. The second was the perceptual and motor electro-cutaneous threshold on the forearms using an electro-cutaneous stimulator. Four neuropsychological tests were used to diagnose USN and to check improvement: Star cancellation, Line bisection, Sentence reading and the Comb & Razor test. Comparing prism with sham conditions, our results show significant improvements in double extinction and in the electro-cutaneous perceptual threshold only for the contralesional hand. No improvement was found for the ipsilesional hand, for the motor threshold, and for neutral glasses. Significant improvement was found in personal neglect. Replication of the task in a subgroup of patients confirmed the primary results. The improvements in somatosensory perception together with the amelioration of personal neglect suggest that PA also has a specific effect on the neglect hemianesthesia.

Neuroanatomical and behavioural factors associated with the effectiveness of two weekly sessions of prism adaptation in the treatment of unilateral neglect

Among the different interventions to alleviate the symptoms of unilateral neglect, prism adaptation (PA) appears especially promising. To elucidate the contribution of some neuroanatomical and behavioural factors to PA's effectiveness, we conducted a study combining neuropsychological and lesion mapping methods on a group of 19 neglect patients who underwent two sessions of PA during one week and assessed their improvement relative to the baseline until the following week (7-8 days later). Correlation analyses revealed a significant positive relationship between the magnitude of the proprioceptive after-effect and the improvement at the follow-up session in two perceptual tasks requiring motor responses. Conversely, no correlation was found between the proprioceptive after-effect and the improvement in a perceptual task with no motor involvement. This finding suggests that patients' potential to show a prism-related improvement in motor-related tasks might be indicated by the strength of their proprioceptive response (proprioceptive after-effect). As for the neuroanatomical basis of this relationship, subtraction analyses suggested that patients' improvement in perceptual tasks with high motor involvement might be facilitated by the integrity of temporo-parietal areas and the damage of frontal and subcortical areas.

Prismatic Adaptation Modulates Oscillatory EEG Correlates of Motor Preparation but Not Visual Attention in Healthy Participants

Prismatic adaption (PA) has been proposed as a tool to induce neural plasticity and is used to help neglect rehabilitation. It leads to a recalibration of visuomotor coordination during pointing as well as to aftereffects on a number of sensorimotor and attention tasks, but whether these effects originate at a motor or attentional level remains a matter of debate. Our aim was to further characterize PA aftereffects by using an approach that allows distinguishing between effects on attentional and motor processes. We recorded EEG in healthy human participants (9 females and 7 males) while performing a new double step, anticipatory attention/motor preparation paradigm before and after adaptation to rightward-shifting prisms, with neutral lenses as a control. We then examined PA aftereffects through changes in known oscillatory EEG signatures of spatial attention orienting and motor preparation in the alpha and beta frequency bands. Our results were twofold. First, we found PA to rightward-shifting prisms to selectively affect EEG signatures of motor but not attentional processes. More specifically, PA modulated preparatory motor EEG activity over central electrodes in the right hemisphere, contralateral to the PA-induced, compensatory leftward shift in pointing movements. No effects were found on EEG signatures of spatial attention orienting over occipitoparietal sites. Second, we found the PA effect on preparatory motor EEG activity to dominate in the beta frequency band. We conclude that changes to intentional visuomotor, rather than attentional visuospatial, processes underlie the PA aftereffect of rightward-deviating prisms in healthy participants.

SIGNIFICANCE STATEMENT Prismatic adaptation (PA) has been proposed as a tool to induce neural plasticity in both healthy participants and patients, due to its aftereffect impacting on a number of visuospatial and visuomotor functions. However, the neural mechanisms underlying PA aftereffects are poorly understood as only little neuroimaging evidence is available. Here, we examined, for the first time, the origin of PA aftereffects studying oscillatory brain activity. Our results show a selective modulation of preparatory motor activity following PA in healthy participants but no effect on attention-related activity. This provides novel insight into the PA aftereffect in the healthy brain and may help to inform interventions in neglect patients.

Supramodal effect of rightward prismatic adaptation on spatial representations within the ventral attentional system

Rightward prismatic adaptation (R-PA) was shown to alleviate not only visuo-spatial but also auditory symptoms in neglect. The neural mechanisms underlying the effect of R-PA have been previously investigated in visual tasks, demonstrating a shift of hemispheric dominance for visuo-spatial attention from the right to the left hemisphere both in normal subjects and in patients. We have investigated whether the same neural mechanisms underlie the supramodal effect of R-PA on auditory attention. Normal subjects underwent a brief session of R-PA, which was preceded and followed by an fMRI evaluation during which subjects detected targets within the left, central and right space in the auditory or visual modality. R-PA-related changes in activation patterns were found bilaterally in the inferior parietal lobule. In either modality, the representation of the left, central and right space increased in the left IPL, whereas the representation of the right space decreased in the right IPL. Thus, a brief exposure to R-PA modulated the representation of the auditory and visual space within the ventral attentional system. This shift in hemispheric dominance for auditory spatial attention offers a parsimonious explanation for the previously reported effects of R-PA on auditory symptoms in neglect.

Valence activates motor fluency simulation and biases perceptual judgment

The concept of motor fluency, defined as the positive marking associated with the easy realisation of a movement, is used to explain the various compatibility effects observed between emotional valence and lateral space. In this work, we propose that these effects arise from the motor fluency simulation induced by emotionally positive stimuli. In a perceptual line bisection task (Landmark task) we primed each trial with an emotionally positive word, negative word, neutral word or no word before asking participants to verbally indicate the side of the vertical mark on the horizontal line (Experiment 1) or to indicate the longest side of the line (Experiment 2). After positive words and for bisected lines, participants' responses were biased towards their dominant side for both right- and left-handers and similarly under the two different instructions. As movements of the dominant hand or in the dominant hemispace have been described as the most fluent lateral actions, this result supports our hypothesis that positive stimuli induce a mental simulation of fluent lateral movements. Furthermore, the replication of the effect under opposite instructions between the two experiments is in line with an explanation in terms of a bias in response selection rather than variations in perceptual content.

Prismatic Adaptation Induces Plastic Changes onto Spatial and Temporal Domains in Near and Far Space

A large literature has documented interactions between space and time suggesting that the two experiential domains may share a common format in a generalized magnitude system (ATOM theory). To further explore this hypothesis, here we measured the extent to which time and space are sensitive to the same sensorimotor plasticity processes, as induced by classical prismatic adaptation procedures (PA). We also exanimated whether spatial-attention shifts on time and space processing, produced through PA, extend to stimuli presented beyond the immediate near space. Results indicated that PA affected both temporal and spatial representations not only in the near space (i.e., the region within which the adaptation occurred), but also in the far space. In addition, both rightward and leftward PA directions caused opposite and symmetrical modulations on time processing, whereas only leftward PA biased space processing rightward. We discuss these findings within the ATOM framework and models that account for PA effects on space and time processing. We propose that the differential and asymmetrical effects following PA may suggest that temporal and spatial representations are not perfectly aligned.

Beyond the Sensorimotor Plasticity: Cognitive Expansion of Prism Adaptation in Healthy Individuals

Sensorimotor plasticity allows us to maintain an efficient motor behavior in reaction to environmental changes. One of the classical models for the study of sensorimotor plasticity is prism adaptation. It consists of pointing to visual targets while wearing prismatic lenses that shift the visual field laterally. The conditions of the development of the plasticity and the sensorimotor after-effects have been extensively studied for more than a century. However, the interest taken in this phenomenon was considerably increased since the demonstration of neglect rehabilitation following prism adaptation by Rossetti et al. (1998). Mirror effects, i.e., simulation of neglect in healthy individuals, were observed for the first time by Colent et al. (2000). The present review focuses on the expansion of prism adaptation to cognitive functions in healthy individuals during the last 15 years. Cognitive after-effects have been shown in numerous tasks even in those that are not intrinsically spatial in nature. Altogether, these results suggest the existence of a strong link between low-level sensorimotor plasticity and high-level cognitive functions and raise important questions about the mechanisms involved in producing unexpected cognitive effects following prism adaptation. Implications for the functional mechanisms and neuroanatomical network of prism adaptation are discussed to explain how sensorimotor plasticity may affect cognitive processes.

Using brain potentials to understand prism adaptation: the error-related negativity and the P300

Prism adaptation (PA) is both a perceptual-motor learning task as well as a promising rehabilitation tool for visuo-spatial neglect (VSN)—a spatial attention disorder often experienced after stroke resulting in slowed and/or inaccurate motor responses to contralesional targets. During PA, individuals are exposed to prism-induced shifts of the visual-field while performing a visuo-guided reaching task. After adaptation, with goggles removed, visuomotor responding is shifted to the opposite direction of that initially induced by the prisms. This visuomotor aftereffect has been used to study visuomotor learning and adaptation and has been applied clinically to reduce VSN severity by improving motor responding to stimuli in contralesional (usually left-sided) space. In order to optimize PA's use for VSN patients, it is important to elucidate the neural and cognitive processes that alter visuomotor function during PA. In the present study, healthy young adults underwent PA while event-related potentials (ERPs) were recorded at the termination of each reach (screen-touch), then binned according to accuracy (hit vs. miss) and phase of exposure block (early, middle, late). Results show that two ERP components were evoked by screen-touch: an error-related negativity (ERN), and a P300. The ERN was consistently evoked on miss trials during adaptation, while the P300 amplitude was largest during the early phase of adaptation for both hit and miss trials. This study provides evidence of two neural signals sensitive to visual feedback during PA that may sub-serve changes in visuomotor responding. Prior ERP research suggests that the ERN reflects an error processing system in medial-frontal cortex, while the P300 is suggested to reflect a system for context updating and learning. Future research is needed to elucidate the role of these ERP components in improving visuomotor responses among individuals with VSN.

Integrity of medial temporal structures may predict better improvement of spatial neglect with prism adaptation treatment

Prism adaptation treatment (PAT) is a promising rehabilitative method for functional recovery in persons with spatial neglect. Previous research suggests that PAT improves motor-intentional "aiming" deficits that frequently occur with frontal lesions. To test whether presence of frontal lesions predicted better improvement of spatial neglect after PAT, the current study evaluated neglect-specific improvement in functional activities (assessment with the Catherine Bergego Scale) over time in 21 right-brain-damaged stroke survivors with left-sided spatial neglect. The results demonstrated that neglect patients' functional activities improved after two weeks of PAT and continued improving for four weeks. Such functional improvement did not occur equally in all of the participants: Neglect patients with lesions involving the frontal cortex (n = 13) experienced significantly better functional improvement than did those without frontal lesions (n = 8). More importantly, voxel-based lesion-behavior mapping (VLBM) revealed that in comparison to the group of patients without frontal lesions, the frontal-lesioned neglect patients had intact regions in the medial temporal areas, the superior temporal areas, and the inferior longitudinal fasciculus. The medial cortical and subcortical areas in the temporal lobe were especially distinguished in the "frontal lesion" group. The findings suggest that the integrity of medial temporal structures may play an important role in supporting functional improvement after PAT.

Prism adaptation and spatial neglect: the need for dose-finding studies

Spatial neglect is a devastating disorder in 50–70% of right-brain stroke survivors, who have problems attending to, or making movements towards, left-sided stimuli, and experience a high risk of chronic dependence. Prism adaptation is a promising treatment for neglect that involves brief, daily visuo-motor training sessions while wearing optical prisms. Its benefits extend to functional behaviors such as dressing, with effects lasting 6 months or longer. Because one to two sessions of prism adaptation induce adaptive changes in both spatial-motor behavior (Fortis et al., 2011) and brain function (Saj et al., 2013), it is possible stroke patients may benefit from treatment periods shorter than the standard, intensive protocol of ten sessions over two weeks—a protocol that is impractical for either US inpatient or outpatient rehabilitation. Demonstrating the effectiveness of a lower dose will maximize the availability of neglect treatment. We present preliminary data suggesting that four to six sessions of prism treatment may induce a large treatment effect, maintained three to four weeks post-treatment. We call for a systematic, randomized clinical trial to establish the minimal effective dose suitable for stroke intervention.

Ipsilesional neglect: behavioral and anatomical correlates

[Correction Notice: An Erratum for this article was reported in Vol 29(2) of Neuropsychology (see record 2014-42242-001). The funding source information was missing from the author note, and A. M. Barrett's institutional affiliation was incorrect. The funding source information and Barrett's correct institutional affiliation are provided in the erratum.]

OBJECTIVE

The sparse existing research on ipsilesional neglect supports an association of this disorder with damage to the right frontal and subcortical brain networks. It is believed that dysfunction in these networks may result in primarily "aiming" motor-intentional spatial errors. The purpose of this study was to confirm whether frontal-subcortical circuits are indeed commonly affected in ipsilesional neglect and to determine the relative presence of "aiming" motor-intentional versus "where" perceptual-attentional spatial errors in these individuals.

METHODS

We identified 12 participants with ipsilesional neglect based on a computerized line bisection task and used the line bisection data to quantify participants' perceptual-attentional and motor-intentional errors. We were able to discriminate between these 2 biases using the algebraic solutions for 2 separate equations, one for "aiming" and one for "where" biases. Lesion mapping was conducted for all participants using MRIcron software; lesion checklist and overlap analysis were created from these images.

RESULTS

A greater percentage of participants with ipsilesional neglect had frontal/subcortical damage (83%) compared with the expected percentage (27%) observed in published patient samples with contralesional neglect. We observed the greatest area of lesion overlap in frontal lobe white matter pathways. Nevertheless, participants with ipsilesional neglect made primarily "where" rather than "aiming" spatial errors.

CONCLUSION

Our data confirm previous research suggesting that ipsilesional neglect may result from lesions to the right frontal-subcortical networks. Furthermore, in our group, ipsilesional neglect was also strongly associated with primarily "where" perceptual-attentional bias, and less so with "aiming" motor-intentional spatial bias.

Prism adaptation power on spatial cognition: adaptation to different optical deviations in healthy individuals

The main objective of the present study was to determine the minimal optical deviation responsible for cognitive after-effects in healthy individuals and to explore whether there was a relationship between the degree of optical deviation and cognitive after-effects. Therefore different leftward optical deviations (8°, 10° and 15°) were used in three different groups of healthy participants. Sensorimotor after-effects (evaluating the visuo-manual realignment) were assessed using an open-loop pointing task and cognitive after-effects (evaluating changes in spatial representation) were assessed using manual and perceptual (landmark) line bisection tasks. Results revealed that exposure to 8°, 10° and 15° optical shifts produced sensorimotor after-effects. In contrast, the occurrence of cognitive after-effects depended on the optical deviation. Adaptation to an 8° leftward optical deviation did not produce cognitive after-effects. Adaptation to a 10° leftward optical deviation was responsible for after-effects in the manual line bisection task only. Adaptation to a 15° leftward optical deviation produced after-effects in both the manual and perceptual line bisection tasks. All cognitive after-effects were rightward and were similar to mild, neglect-like manifestations. Both sensorimotor and cognitive after-effects were correlated with the degree of optical deviation. Our results are of methodological and theoretical interest to those interested in sensorimotor plasticity and spatial cognition.

Modelling the differential effects of prisms on perception and action in neglect

Damage to the right parietal cortex often leads to a syndrome known as unilateral neglect in which the patient fails to attend or respond to stimuli in left space. Recent work attempting to rehabilitate the disorder has made use of rightward-shifting prisms that displace visual input further rightward. After a brief period of adaptation to prisms, many of the symptoms of neglect show improvements that can last for hours or longer, depending on the adaptation procedure. Recent work has shown, however, that differential effects of prisms can be observed on actions (which are typically improved) and perceptual biases (which often remain unchanged). Here, we present a computational model capable of explaining some basic symptoms of neglect (line bisection behaviour), the effects of prism adaptation in both healthy controls and neglect patients and the observed dissociation between action and perception following prisms. The results of our simulations support recent contentions that prisms primarily influence behaviours normally thought to be controlled by the dorsal stream.

Implications of CI therapy for visual deficit training

We address here the question of whether the techniques of Constraint Induced (CI) therapy, a family of treatments that has been employed in the rehabilitation of movement and language after brain damage might apply to the rehabilitation of such visual deficits as unilateral spatial neglect and visual field deficits. CI therapy has been used successfully for the upper and lower extremities after chronic stroke, cerebral palsy (CP), multiple sclerosis (MS), other central nervous system (CNS) degenerative conditions, resection of motor areas of the brain, focal hand dystonia, and aphasia. Treatments making use of similar methods have proven efficacious for amblyopia. The CI therapy approach consists of four major components: intensive training, training by shaping, a "transfer package" to facilitate the transfer of gains from the treatment setting to everyday activities, and strong discouragement of compensatory strategies. CI therapy is said to be effective because it overcomes learned nonuse, a learned inhibition of movement that follows injury to the CNS. In addition, CI therapy produces substantial increases in the gray matter of motor areas on both sides of the brain. We propose here that these mechanisms are examples of more general processes: learned nonuse being considered parallel to sensory nonuse following damage to sensory areas of the brain, with both having in common diminished neural connections (DNCs) in the nervous system as an underlying mechanism. CI therapy would achieve its therapeutic effect by strengthening the DNCs. Use-dependent cortical reorganization is considered to be an example of the more general neuroplastic mechanism of brain structure repurposing. If the mechanisms involved in these broader categories are involved in each of the deficits being considered, then it may be the principles underlying efficacious treatment in each case may be similar. The lessons learned during CI therapy research might then prove useful for the treatment of visual deficits.

Prism adaptation in the healthy brain: the shift in line bisection judgments is long lasting and fluctuates

Rightward prism adaptation has been shown to ameliorate visuospatial biases in right brain-damaged patients with neglect, and a single session of prism adaptation can lead to improvements that last up to several hours. Leftward prism adaptation in neurologically healthy individuals induces neglect-like biases in visuospatial tasks. The duration of these effects in healthy individuals, typically assumed to be ephemeral, has never been investigated. Here we assessed the time-course of the adaptation-induced modifications in a classical perceptual line bisection task that was repeatedly administered for approximately 40min after a single session of adaptation to either a leftward or rightward prismatic deviation. Consistent with previous reports, only adaptation to leftward-deviating prisms induced a visuospatial shift on perceptual line bisection judgments. The typical pattern of pseudoneglect was counteracted by a rightward shift in midline judgments, which became significant between 5 and 10 min after adaptation, fluctuated between being significant or not several times in the 40 min following adaptation, and was present as late as 35 min. In contrast, the sensorimotor aftereffect was present immediately after adaptation to both rightward and leftward deviating prisms, decayed initially then remained stable until 40 min. These results demonstrate that both the sensorimotor and visuospatial effects last for at least 35 min, but that the visuospatial shift needs time to fully develop and fluctuates. By showing that the effects of prism adaptation in the undamaged brain are not ephemeral, these findings reveal the presence of another, so-far neglected dimension in the domain of the cognitive effects induced by prism adaptation, namely time. The prolonged duration of the induced visuospatial shift, previously considered to be a feature of prism adaptation unique to brain-damaged subjects, also applies to the normal brain.