Studies investigating human information processing have provided evidence that in some cases, movements can be prepared in advance. Although evidence for motor preparation has been shown at cortical and spinal levels, motor preparation at a subcortical level is not well described. One line of inquiry has involved the use of a startling acoustic stimulus (115-124 dB) that can act as an early trigger for pre-programmed actions in reaction time (RT) tasks. In light of this new research paradigm, the startle reflex may be used as a tool to investigate motor preparation. Here, six experiments were conducted that work towards the goals of understanding the mechanism of RT shortening due to startle, and motor preparation at a subcortical level. The first section (2 experiments) of this dissertation provides evidence that when a motor action can be prepared in advance, it is pre-programmed and stored subcortically awaiting the normal cortical “go” signal. A startle appears to activate structures directly that are involved with the voluntary response channel leading to early triggering of the pre-programmed response, and dramatically reduced RT. In the current dissertation we investigated alternative mechanisms to explain startle RT facilitation, including the stimulus intensity effect, and a fast transcortical route, with results supporting the original subcortical storage hypothesis. The second section (4 experiments) presents data which together provide insight into motor programming processes, and the circumstances under which a response is pre-programmed. For example, when the possibility of not having to make the response existed, a known response was not pre-programmed. Similarly, no pre-programming occurred when certainty existed regarding when to respond. However, while a previous experiment showed that having to make a choice between several response alternatives precluded pre-programming, this dissertation shows that if possible response alternatives are not in conflict with one another, multiple responses can be prepared in parallel. Finally, the complexity of a response such as one involving multiple sequenced sub-components may limit the ability to pre-program in a simple RT task. Taken together, these results suggest that pre-programming is dependent on the task characteristics and appears to involve implementation of strategies to increase programming efficiency.

Studies investigating human information processing have provided evidence that in some cases, movements can be prepared in advance. Although evidence for motor preparation has been shown at cortical and spinal levels, motor preparation at a subcortical level is not well described. One line of inquiry has involved the use of a startling acoustic stimulus (115-124 dB) that can act as an early trigger for pre-programmed actions in reaction time (RT) tasks. In light of this new research paradigm, the startle reflex may be used as a tool to investigate motor preparation. Here, six experiments were conducted that work towards the goals of understanding the mechanism of RT shortening due to startle, and motor preparation at a subcortical level. The first section (2 experiments) of this dissertation provides evidence that when a motor action can be prepared in advance, it is pre-programmed and stored subcortically awaiting the normal cortical “go” signal. A startle appears to activate structures directly that are involved with the voluntary response channel leading to early triggering of the pre-programmed response, and dramatically reduced RT. In the current dissertation we investigated alternative mechanisms to explain startle RT facilitation, including the stimulus intensity effect, and a fast transcortical route, with results supporting the original subcortical storage hypothesis. The second section (4 experiments) presents data which together provide insight into motor programming processes, and the circumstances under which a response is pre-programmed. For example, when the possibility of not having to make the response existed, a known response was not pre-programmed. Similarly, no pre-programming occurred when certainty existed regarding when to respond. However, while a previous experiment showed that having to make a choice between several response alternatives precluded pre-programming, this dissertation shows that if possible response alternatives are not in conflict with one another, multiple responses can be prepared in parallel. Finally, the complexity of a response such as one involving multiple sequenced sub-components may limit the ability to pre-program in a simple RT task. Taken together, these results suggest that pre-programming is dependent on the task characteristics and appears to involve implementation of strategies to increase programming efficiency.

The human hand has evolved to be remarkably good at skillfully manipulating objects. This manipulation requires knowledge of the dynamic properties of an object, which is represented in the central nervous system (CNS) by what has been referred to as an internal model. Internal models are neural representations of the predicted behaviour of objects or limbs with a known state in response to a given motor command. Our ability to successfully manipulate a wide variety of objects suggests that the CNS maintains multiple internal models of familiar object dynamics. People are able to both recruit these models for use when an object is grasped and to rapidly switch to another model when the object is exchanged. The purpose of this study was to investigate how internal models of objects are accessed and used for action. In experiment 1, subjects learned to move a cursor to a target by manipulating a robotic arm with complex dynamics. We used event-related fMRI to measure the neural activity associated with grasping the robot handle in preparation for movement. In comparison to control tasks, subjects showed significant neural activation in the ipsilateral cerebellum and the contralateral primary motor and supplementary motor areas, suggesting the likely involvement of these areas in recruitment of internal models. In experiment 2, we used a precision lifting task to investigate how the internal representation of weight asymmetry transfers across changes in hand, hand orientation and object orientation. Subjects demonstrated positive transfer in all cases when the hand was rotated, indicating that internal models of objects can be adapted to accommodate changes in hand orientation. When the object was rotated, positive transfer was seen only when the hand also rotated, suggesting that this change in hand orientation facilitated mental rotation of the object. Overall, these results support the idea that people maintain an internal representation of object dynamics but can not always link this model to the configuration of the object in space. / Thesis (Master, Neuroscience Studies) -- Queen's University, 2007-08-29 10:57:23.511

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