Exploring the link between Input-Output subtypes of spatial neglect and prism adaptation mechanism

Abstract

Spatial neglect is a complex neurocognitive syndrome predicting greater chronic disability after stroke and other acquired brain injuries. The heterogeneity of neglect symptomatology poses challenges for assessment and treatment. These challenges are exemplified by research on prism adaptation (PA), a sensorimotor learning task that can reduce neglect symptoms but has shown inconsistent effects in clinical trials. Prism adaptation’s therapeutic effects may vary due to differences in neglect subtypes. The Input-Output neglect subtyping dimension describes neglect symptoms as arising at different stages of information processing, yielding symptoms at the input stage (perceptual or Input neglect), and/or the output stage (premotor or Output neglect). Some research suggests that PA mainly acts on Output neglect symptoms. However, variability in the conceptualization and measurement of Input-Output neglect makes it challenging to connect this subtyping concept to PA’s therapeutic mechanisms. This thesis aimed to: 1) explore the Input-Output neglect subtyping dimension, and 2) evaluate how PA impacts different stages of information processing by testing whether PA can induce temporary Output biases in healthy adults. Chapter 1 provides historical and theoretical foundations for the subsequent body of work. Chapter 2 presents a systematic scoping review summarizing the terminology, measurement tools, and neural theories/correlates of Input-Output neglect. Conclusions were drawn from review results in the areas of: 1) terminology and conceptual models; 2) methodological issues of dissociating Input and Output subtypes; 3) updating neural theories; and 4) connecting mechanisms underlying assessment and treatment. These scoping review conclusions informed the design of two behavioural experiments recruiting healthy adults. In the first experiment, left-shifting PA induced an Output bias on a speeded reach task in right-handers (Chapter 3), but not in left-handers (Chapter 3A). In the second experiment, PA did not impact Input-Output biases measured by a horizontal line judgment task (Landmark task; Chapter 4). Chapter 5 interprets these differential PA effects through the lens of the scoping review conclusions. Overall, this thesis concludes that future research on the link between Input-Output neglect and PA effects should recruit persons with neglect and consider the integration of Input and Output processing in addition to their dissociation.