Walking around in dynamically changing environments require the integration of three of our sensory systems: visual, vestibular, and kinesethic. Vision is the only modality of these three sensory systems that provides information at a distance for proactively controlling locomotion (Gibson, 1958). The visual system provides information about self-motion, about body position and body segments relative to one another and the environment, and environmental information at a distance (Patla, 1998). Gibson (1979) developed the idea that everyday behaviour is controlled by perception-action coupling between an action and some specific information picked up from the optic flow that is generated by that action. Such that visual perception guides the action required to navigate safely through an environment and the action in turn alters perception. The objective of my thesis was to determine how well perception and action are coupled when approaching and walking through moving doors with dynamically changing apertures. My first two studies were grouped together and here I found that as the level of threat increased, the parameters of control changed and not the controlling mechanism. The two dominant action control parameters observed were a change in approach velocity and a change in posture (i. e. shoulder rotation). These findings add to previous work done in this area using a similar set-up in virtual reality, where after much practice participants increased success rate by decreasing velocity prior to crossing the doors. In my third study I found that visual fixation patterns and action parameters were similar when the location of the aperture was predictable and when it was not. Previous work from other researchers has shown that vision and a subsequent action are tightly coupled with a latency of about 1second. I have found that vision only tightly couples action when a specific action is required and the threat of a collision increases. My findings also point in the same direction as previous work that has shown that individuals look where they are going. My last study was designed to determine if we go where we are looking. Here I found that action does follow vision but is only loosely correlated. The most important and common finding from all the studies is that at 2 seconds prior to crossing the moving doors (any type of movement) vision seems to have the most profound effect on action. At this time variability in action is significantly lower than at prior times. I believe that my findings will help to understand how individuals use vision to modify actions in order to avoid colliding with other people or other moving objects within the environment. And this knowledge will help elderly individuals to be better able to cope with walking in cluttered environments and avoid contacting other objects.

Walking around in dynamically changing environments require the integration of three of our sensory systems: visual, vestibular, and kinesethic. Vision is the only modality of these three sensory systems that provides information at a distance for proactively controlling locomotion (Gibson, 1958). The visual system provides information about self-motion, about body position and body segments relative to one another and the environment, and environmental information at a distance (Patla, 1998). Gibson (1979) developed the idea that everyday behaviour is controlled by perception-action coupling between an action and some specific information picked up from the optic flow that is generated by that action. Such that visual perception guides the action required to navigate safely through an environment and the action in turn alters perception. The objective of my thesis was to determine how well perception and action are coupled when approaching and walking through moving doors with dynamically changing apertures. My first two studies were grouped together and here I found that as the level of threat increased, the parameters of control changed and not the controlling mechanism. The two dominant action control parameters observed were a change in approach velocity and a change in posture (i. e. shoulder rotation). These findings add to previous work done in this area using a similar set-up in virtual reality, where after much practice participants increased success rate by decreasing velocity prior to crossing the doors. In my third study I found that visual fixation patterns and action parameters were similar when the location of the aperture was predictable and when it was not. Previous work from other researchers has shown that vision and a subsequent action are tightly coupled with a latency of about 1second. I have found that vision only tightly couples action when a specific action is required and the threat of a collision increases. My findings also point in the same direction as previous work that has shown that individuals look where they are going. My last study was designed to determine if we go where we are looking. Here I found that action does follow vision but is only loosely correlated. The most important and common finding from all the studies is that at 2 seconds prior to crossing the moving doors (any type of movement) vision seems to have the most profound effect on action. At this time variability in action is significantly lower than at prior times. I believe that my findings will help to understand how individuals use vision to modify actions in order to avoid colliding with other people or other moving objects within the environment. And this knowledge will help elderly individuals to be better able to cope with walking in cluttered environments and avoid contacting other objects.

Mental health problems are the leading causes of disability in Canada. Nearly 70% of mental health problems have their onset during childhood or adolescence. Thus, identifying modifiable determinants of mental health problems in children and adolescents can inform future interventions intended to prevent them in this age group. Until recently, research has examined relationships of movement behaviours, including sleep, sedentary behaviour and physical activity mainly with physical health indicators (e.g., adiposity, cardiovascular disease risk factors, etc.). The few studies that have examined the relationships between movement behaviours and mental health indicators have considered the former individually and in isolation of each other, ignoring the intrinsic and empirical interactions between these behaviours. Adjusting for all these behaviours in a traditional regression model that assumes independence between variables has been shown to produce flawed and inconsistent findings. The purpose of this doctoral dissertation is to examine how the combinations of physical activity, sedentary behaviour, and sleep duration are associated with depression and other mental health outcomes in children and adolescents, through a series of five research studies (one systematic review, 3 cross-sectional studies, and one longitudinal study). Empirical studies used data from 3 large and diverse samples of children and adolescents from Canada and the United States. Conventional regression models and structural equation modelling, and novel analytical techniques, including compositional data analysis were used to analyze the data. The systematic review confirmed the paucity of existing research in this area and identified important research gaps to be filled. Collectively, the results from cross-sectional studies showed that meeting all three recommendations was associated with lower odds of depressive symptoms and other mental health outcomes. However, this association appeared to be mainly driven by meeting the sleep duration recommendation, and to a lesser extent the screen time + sleep duration recommendations. There was a dose-response gradient from meeting none of the recommendations up to meeting two recommendations. Results from the longitudinal study using compositional data analysis provided further evidence suggesting that increasing sleep duration relative to the remaining behaviours (i.e. screen time and physical activity) was associated with lower depressive symptoms among all age/sex subgroups. Results further indicated that predicted changes in depressive symptoms were strongest and most beneficial when removing screen time while adding sleep duration. Finally, results from both cross-sectional and longitudinal analyses suggest that age and sex moderate the association between movement behaviour recommendations (individual or combined) and mental health indicators, depending on the type of movement behaviour and the type of mental health indicators. The findings from this body of work have shed new light on the association between movement behaviours and mental health indicators in children and adolescents by demonstrating that meeting all three movement behaviour recommendations is associated with better mental health, and that sleep duration and screen time were more strongly associated with mental health compared with physical activity in our studies.