Prism adaptation: a rehabilitation technique for spatial neglect

This doctoral thesis is divided in six chapters and presents the results of four experiments. It investigates the use of prism adaptation (PA) in the rehabilitation of spatial neglect, how PA may affect spatial cognition, and the specific mechanisms that are influenced by PA. Knowing more about the systems responsive to PA may help our understanding of which symptoms, and which patients, improve optimally after PA training. The Introduction explains concepts and backgrounds useful for the understanding of the experimental projects. First, the definition and characteristics of Unilateral Spatial Neglect are introduced, followed by a review of the distinction in the variety of spatial neglect that separates “perceptual” versus “premotor” neglect and the anatomical and functional dissociation between brain areas associated with these two subtypes of neglect. The Introduction summarizes paradigms that have been used to disentangle the perceptual and premotor components of spatial neglect and the patterns of perceptual and premotor biases reported in neglect patients. A description of different methods that have previously been used in the rehabilitation of spatial neglect is also provided. Among them, the PA technique is described in detail, including the background in which PA has been used, measures to assess the presence of adaptation and aftereffects following the adaptation procedure, processes that have been proposed to be involved in the sensori-motor transformation that occurs during PA, the beneficial effects reported after PA paradigms in neglect patients, and the effects of PA in healthy individuals. Finally, the Introduction gives an overview of the cerebral circuits that appear to be involved in PA based on data derived from brain imaging studies, studies with brain-damaged patients, and studies with primates. The next four chapters report the results of four experiments. These chapters are divided in two main parts. The first section focuses on the feasibility of using ecological visuo-motor activities, based on diverse and engaging visuo-motor tasks, during adaptation to prism. In Experiment 1, 10 neglect patients were submitted to both a standard pointing adaptation training (Frassinetti et al., 2002) and a training involving diverse ecological visuo-motor tasks (Ecological procedure). The effect of the two treatments was compared in a large assessment including a variety of neuropsychological tests as well as functional scales. In Experiment 2, the presence of adaptation and aftereffects was assessed during the ecological procedure, and these measures were compared with those obtained during the traditional pointing task. In Experiment 3, we used a modified version of the paradigm of Schwartz et al. (1997) and Na et al., (1998) to decouple perceptual-attention “where” and motor-intention “aiming” components in visuo-motor tasks (line bisection). The effects of PA on where and aiming components were tested in a large group of neurologically healthy individuals. In Experiment 4, the same effects were tested in a group of five neglect patients. Lastly, the General Discussion summarizes and integrates the results of the four experiments, highlighting the implications for the rehabilitation of spatial neglect. Results from these experiments show that PA training associated with varied visuomotor activities is an effective tool to ameliorate some aspects of spatial neglect as well as functional disabilities, being as effective as the more established pointing task. In the four experiments, measures of adaptation and aftereffects were obtained using three different adaptation procedures: pointing, ecological, and line bisection tasks. It is argued that these measures, especially the aftereffect measures, may be important for establishing the effectiveness of adaptation procedures in neglect rehabilitation. It appears that the three adaptation procedures (pointing, ecological, line bisection) can all induce error correction during the exposure phase. However, the ecological and the pointing procedure seem to create strong and prolonged aftereffects, with the ecological task even better in inducing aftereffects than the pointing task. By contrast, the line bisection task appears to induce weaker aftereffects, suggesting that its use may not be optimal in prism paradigms. Reasons for such differences are explored, focusing on the motivations for the increased aftereffects during the ecological procedure. Indeed, exposure to prism decreased the aiming bias after PA in both studies. In the group of healthy individuals, the initial left aiming bias was reduced after exposure to left-shifting prisms (Experiment 3). In a similar way, in the group of neglect patients the initial right aiming bias improved after exposure to right-shifting prisms (Experiment 4). In addition, in the healthy participants no changes in the aiming bias were found after exposure to right-shifting prisms and control goggles, indicating that the effect of left-shifting prisms was not due to increased familiarity with the task (Experiment 3). These results are interpreted and integrated in light of recent findings. Experiments 1 and 4 also showed the cortical areas associated with neglect in our patients and the responsiveness to the PA paradigm. Results confirmed that patients with right-sided brain lesions in the frontal-parietal cortical and subcortical areas are still able to adapt to the lateral shift induced by prism, and further suggested that adaption to prism and improvement in neglect symptoms can occur even in the presence of an occipital lesion. Finally, the Discussion addresses the question of how to differentiate the effect of the experimental manipulation from spontaneous recovery while testing the efficacy of new rehabilitation methods in brain-damaged individuals. Therefore, evidence for the specificity of our intervention is provided. It is argued that spontaneous recovery cannot fully account for the present findings of improvement in neglect symptoms after PA treatment (Experiment 1 and 4). It is also suggested that performing studies employing neurologically healthy subjects can help in providing evidence for the effect of a treatment on cognitive function. In particular, testing healthy subjects to better understand the functioning of PA in neglect patients is facilitated by the fact that healthy individuals show biases in spatial cognition that mirror the biases in neglect patients.