Remodelling the attentional system after left hemispheric stroke: Effect of leftward prismatic adaptation

Right Unilateral Spatial Neglect Improves with Intrinsic Motivation

Background. In the acute phase of stroke, it is well known that the incidence and severity of unilateral spatial neglect (USN) are more significant in the right hemisphere injuries. Still, the detection of USN in left hemisphere injuries has been increasing in recent years. This trend is because behavioral assessments have prevented the exclusion of patients who are difficult to assess for USN or apathy using conventional paper-and-pencil tests (e.g., aphasia). Right USN and post-stroke apathy share many common lesions. Therefore, clinical symptoms may overlap, but little validation considers this. Case Study. A man (62 years old) determined to have the right USN and apathy was treated for six weeks in 3 terms. In the first term (weeks 1 to 2), the patient was treated for the right USN by conventional therapy. In the second term (3–4 weeks), treatment for right USN and apathy by goal-directed therapy based on affinity behavior was implemented. In the third term (5–6 weeks), goal-directed therapy based on affinity behavior was discontinued, and treatment was returned to conventional therapy only. In the second term (goal-directed therapy based on affinity behavior), the improvement in patients' apathy (clinical assessment for spontaneity) was more significant than the effect size in the third term (conventional therapy). There were no significant differences in USN (catherine bergego scale) and intrinsic motivation (pittsburgh rehabilitation participation scale). However, the effect size in the second term tended to be larger than in the third term (conventional therapy). Clinical Rehabilitation Impact. This report aims to demonstrate the limitations of current treatment for cases determined to have both right USN and apathy. Second, to assess the extent to which this new intervention can complement the limitations of current treatment.

Does hand modulate the reshaping of the attentional system during rightward prism adaptation? An fMRI study

Adaptation to right-deviating prisms (R-PA), that is, learning to point with the right hand to targets perceived through prisms, has been shown to change spatial topography within the inferior parietal lobule (IPL) by increasing responses to left, central, and right targets on the left hemisphere and decreasing responses to right and central targets on the right hemisphere. As pointed out previously, this corresponds to a switch of the dominance of the ventral attentional network from the right to the left hemisphere. Since the encoding of hand movements in pointing paradigms is side-dependent, the choice of right vs. left hand for pointing during R-PA may influence the visuomotor adaptation process and hence the reshaping of the attentional system. We have tested this hypothesis in normal subjects by comparing activation patterns to visual targets in left, central, and right fields elicited before and after adaptation to rightward-deviating prisms using the right hand (RWRH) with those in two control groups. The first control group underwent adaptation to rightward-deviating prisms using the left hand, whereas the second control group underwent adaptation to leftward-deviating prisms using the right hand. The present study confirmed the previously described enhancement of left and central visual field representation within left IPL following R-PA. It further showed that the use of right vs. left hand during adaptation modulates this enhancement in some but not all parts of the left IPL. Interestingly, in some clusters identified in this study, L-PA with right hand mimics partially the effect of R-PA by enhancing activation elicited by left stimuli in the left IPL and by decreasing activation elicited by right stimuli in the right IPL. Thus, the use of right vs. left hand modulates the R-PA-induced reshaping of the ventral attentional system. Whether the choice of hand during R-PA affects also the reshaping of the dorsal attentional system remains to be determined as well as possible clinical applications of this approach. Depending on the patients' conditions, using the right or the left hand during PA might potentiate the beneficial effects of this intervention.

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.

Neural Mechanisms of Prism Adaptation in Healthy Adults and Individuals with Spatial Neglect after Unilateral Stroke: A Review of fMRI Studies

Functional disability due to spatial neglect hinders recovery in up to 30% of stroke survivors. Prism adaptation treatment (PAT) may alleviate the disabling consequences of spatial neglect, but we do not yet know why some individuals show much better outcomes following PAT than others. The goal of this scoping review and meta-analysis was to investigate the neural mechanisms underlying prism adaptation (PA). We conducted both quantitative and qualitative analyses across fMRI studies investigating brain activity before, during, and after PA, in healthy individuals and patients with right or left brain damage (RBD or LBD) due to stroke. In healthy adults, PA was linked with activity in posterior parietal and cerebellar clusters, reduced bilateral parieto-frontal connectivity, and increased fronto-limbic and sensorimotor network connectivity. In contrast, RBD individuals with spatial neglect relied on different circuits, including an activity cluster in the intact left occipital cortex. This finding is consistent with a shift in hemispheric dominance in spatial processing to the left hemisphere. However, more studies are needed to clarify the contribution of lesion location and load on the circuits involved in PA after unilateral brain damage. Future studies are also needed to clarify the relationship of decreasing resting state functional connectivity (rsFC) to visuomotor function.

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.

The role of the right posterior parietal cortex in prism adaptation and its aftereffects

Adaptation to optical prisms (Prismatic Adaptation, PA) displacing the visual scene laterally, on one side of visual space, is both a procedure for investigating visuo-motor plasticity and a powerful tool for the rehabilitation of Unilateral Spatial Neglect (USN). Two processes are involved in PA: i) recalibration (the reduction of the error of manual pointings toward the direction of the prism-induced displacement of the visual scene); ii) the successive realignment after prisms' removal, indexed by the Aftereffects (AEs, in egocentric straight-ahead pointing tasks, the deviation in a direction opposite to the visual displacement previously induced by prisms). This study investigated the role of the posterior parietal cortex (PPC) of the right hemisphere in PA and AEs, by means of low frequency repetitive Transcranial Magnetic Stimulation (rTMS). Proprioceptive and Visuo-proprioceptive egocentric straight-ahead pointing tasks were used to assess the presence and magnitude of AEs. The primary right visual cortex (V1) was also stimulated, to assess the selectivity of the PPC effects on the two processes of PA (recalibration and realignment) in comparison with a cortical region involved in visual processing. Results showed a slower adaptation to prisms when rTMS was delivered before PA, regardless of target site (right PPC or V1). AEs were reduced only by PPC rTMS applied before or after PA, as compared to a sham stimulation. These findings suggest a functional and neural dissociation between realignment and recalibration. Indeed, PA interference was induced by rTMS to both the PPC and V1, indicating that recalibration is supported by a parieto-occipital network. Conversely, AEs were disrupted only by rTMS delivered to the PPC, thus unveiling a relevant role of this region in the development and maintenance of the realignment.