BOOSTING THE SALIENCE OF A NEGLECTED HEMIFIELD: FROM SELECTIVE VISUAL INPUT TO SELECTIVE REINFORCEMENT

Hemispatial neglect is a common syndrome after stroke in which patients fail to report or respond or be aware of stimuli presented in a portion of space (Heilman & Valenstein, 1979; Kwon et al., 2012). Over the past 60 years more than 18 different rehabilitation techniques have been put forward to alleviate or reduce unilateral hemispatial neglect (Luauté et al., 2006; Ogourtsova et al., 2010), all of which have been shown to have a very limited efficacy. The thesis is carried out of two projects which study two different types of rehabilitation techniques for neglect deficits in neurological patients. Both these techniques do not require patients to learn new strategies and make behavioural adjustments. Instead, they employ a restitutive approach, with the aim to directly alter the cognitive impairment, by addressing the underlying neural circuits. The Project 1 studies Eye Patching, a technique that induces patients to turn attention towards the left hemispace, by completely eliminating afferent information from the ipsilesional hemispace and increasing the salience of the left portion of the visual field. Aim of this project is to review the literature on the effects of the Eye Patching in post-stroke hemispatial neglect in order to convey evidence-based practice recommendations to clinicians in stroke rehabilitation. The Project 2 studies a new protocol based on reward delivery that allows a selective reinforcement of locations within the left hemifield achieved through the a delivery of rewarding feedbacks on performance in tasks involving visual spatial attention, a type of manipulation that has never been used before within the rehabilitative context. Goal of this project is to test whether in neglect patients after stroke, the manipulation of rewards associated with the attentional selection of targets in a visuo-spatial attention task would help to reduce their spatial impairment. Observations (Mesulam, 1985) suggest that motivational factors may also influence spatial attention in neglect patients, such that left space omissions in a cancellation task can be reduced if patients search for highly valued objects (i.e., pictures of coins) and were promised monetary reward in turn for each found target (Malhotra et al., 2013). Studies on healthy subjects demonstrated that the deployment of visual selective attention can be biased through the administration of monetary rewards as feedbacks on performance (Della Libera & Chelazzi, 2006; Della Libera & Chelazzi, 2009; Della Libera et al., 2011; Chelazzi et al., 2013), so that objects that have led to higher rewards when attentionally selected in the past can attract attention more powerfully with respect to other, less rewarding, items. Moreover, very recent results suggest that in young healthy subjects similar effects can be found within the spatial domain, so that attention might be drawn more powerfully towards spatial locations that when selected in the past have led to more rewarding outcomes. The aim of this study is on one hand to examine whether similar effects can be found in older adults, on the other to test whether an attentional training with unbalanced rewarding feedbacks for targets found in the controlesional hemifield might alleviate neglect symptoms in brain damaged patients. Such finding would pave the way to the development of groundbreaking rehabilitative techniques, with restorative bases. As a pre-requisite to the development of the protocol to be delivered to neglect patients, I aimed at exploring whether the deployment of visuo-spatial attention could be affected by reward delivery in a population of healthy older subjects. I developed an experimental protocol made up of two similar visual search paradigms: one used as a test, the other as a training. During training, varying amounts of monetary rewards were delivered in turn for the correct detection of target stimuli appearing in different spatial locations within the visual display. In particular, higher rewards were more frequently associated to targets appearing in the left hemifield (usually associated with attentional disorders in neglect patients). The protocol comprised 2 test sessions at the beginning of the experimental protocol, three consecutive training sessions in the three subsequent days, a second test session 4 days after the end of training, and a follow-up test 1 week later. Eighteen healthy subjects (mean age=59.22) and five patients (mean age=66.33) with hemispatial neglect after right hemisphere stroke have been included in the main study. The main findings indicate that in all subjects, both healthy and brain damaged, task performance improved after training. Moreover, analyses of performance in the group of healthy subjects revealed that the attentional scanning of the visual display was significantly altered by training. The improvement was greater for the detection and identification of target stimuli shown on the left hemifield, that had been selectively reinforced, with respect to those shown on the right, confirming the role of reward contingencies in modulating attentional deployment (Della Libera & Chelazzi, 2009; Della Libera et al., 2011; Anderson et al., 2011a, 2011b; Chelazzi et al., under review). Interestingly, the findings revealed that, differently from what observed by Chelazzi and colleagues (under review), here the reward-related benefit was quite spread across all positions within the highly rewarded hemifield, and were not confined to the specific stimulus locations that were associated with the reward unbalances. The results on neglect patients concluded that patients with a too serious or too soft impairment do not show improvements in this specific training protocol, because it might be too difficult for the first and too simple for the latter. The results instead from the remaining patients, who are sufficiently preserved to successfully execute training and receive benefits are encouraging, and give hope that this protocol can be used at large as a new type of restitutive technique for neglect rehabilitation. On the basis of the results obtained, it is not possible yet to establish with certainty the effectiveness of either of these techniques, Eye Patching and the protocol based on the reward, because there is still an extreme variability of the effects, probably due to a number of reasons, including the location of the lesion, the time from stroke and interindividual differences. Both instruments, however, seem very promisings in the rehabilitation of neglect patients in all phases of stroke and in clinical practice in general. Future studies in larger patients population will be necessary to fully determine their efficacy, the degree to which their effects can generalize across different testing methods, as well as their durability in the long term.