In western countries, one of the main causes of permanent disability is stroke. When it occurs in the right hemisphere, it often leads to a hemi spatial neglect syndrome which is manifested as a discrepancy between the external environment and internal representations of it. One aspect of it is that patients can no longer attend to stimuli on their left hemispace. Thus far, different types of rehabilitation exist to rebalance their spatial attention, including Prism Adaptation (PA). This technique consists of wearing glasses with prismatic lenses shifting the visual field by ten degrees to the right side, thus creating a visuomotor disturbance during goal-directed tasks. This disruption results in a recalibration of spatial representations more towards the left, thereby improving hemineglect deficits.
The purpose of this study is to assess the benefit of using virtual reality (VR) based on PA as a rehabilitation tool. In order to do so, it is necessary to determine if the neural and behavioural modulations induced by this novel VR-PA paradigm are influencing perception and processing of real-life-like events that are shown through short-movies.
The healthy brain processes naturalistic stimuli with a balanced activity between both hemispheres. On the contrary, neglect patients usually suffer an unbalanced brain activity due to their right hemisphere affection which decreases the activation. Therefore VR-PA (virtual reality prism adaptation) can be a solution to compensate for that right hemisphere deficit: Knowing that we did the study on healthy subjects, we expected to find through the VR-PA a compensating modulation in favor of the right hemispheric processing following the rightward adaptation.
Participants were exposed to an immersive VR environment, while controlling a right virtual hand with a controller to reach dynamic and static targets at different spatial directions. During the training, a 20 rotational shift was introduced between the positions of the participant’s real hand and the perceived virtual one, in order to simulate the visual shift normally induced by PA and to generate a sensory-motor recalibration. After the subjects did the VR-PA without shift the first time, we did a functional MRI and repeated the action after they did the VR-PA with the 20 shift. During the fMRI, subjects were asked to freely watch naturalistic movie videos portraying everyday-life situations.
Results from fifteen neurologically intact participants showed that subjects exposed to the virtual visual shift did modulate the activity of the brain by increasing it in a visual area located in the right hemisphere (right parieto-occipital sulcus; POS) while freely watching realistic short videos. We found statistically significant increase of activity in the right visual cortex and consequently left visual field representation, between the pre- and post-session within each subject. Moreover, a link between the pointing straight-ahead modulation (which measures spatial biases following adaptation) and the POS activity modulation was discovered. This meaning that the behavioral visuo-motor adaptation aftereffect is correlated to a neural aftereffect.
Since VR training can increase right hemisphere visual cortex activity while subjects are watching a realistic movie, it could potentially be applied as new treatments for neglect patients. A promising solution to motivate patients with spatial deficits to stick to their rehabilitation protocol is the use of new therapeutic games using VR for example. Moreover, an essential effect of rehabilitation is improvement of the patient’s ability to perceive real-life audio-visual inputs. Our results represent a proof-of-concept for this technology, able to induce sensory-motor recalibration similar to that induced by prism adaptation that is effective even while visioning a naturalistic stimulus.