The use of functional neuroimaging to study reorganisation of the motor system during task performance following altered corticospinal excitability caused by repetitive transcranial magnetic stimulation

Repetitive transcranial magnetic stimulation (rTMS) in motor areas has been shown to induce transient and reversible changes in the corticospinal excitability of healthy individuals' brains. Furthermore, a number of studies have shown that this reorganisation can occur not only at the site of stimulation but also in other areas which might be anatomically or functionally connected to it. These effects are thought to depend on various parameters such as the protocol of stimulation, the site of stimulation and the behavioural paradigm chosen to study the effects. The aim of this thesis was to use functional neuroimaging in order to explore how the effects of long-lasting rTMS protocols (30-60min stimulation) on the motor system depend on specific conditioning parameters such as the frequency, the site and the pattern of stimulation. We examined how the pattern of activity in the brain reorganises depending on the anatomical and effective connectivity of the stimulated area, and on the behavioural task. Initial studies presented in this thesis used Positron Emission Tomography to compare the effects of high (5Hz) versus low (1Hz) frequency of rTMS on activity and motor network connectivity of the primary motor cortex (M1) during performance of a simple finger movement task or at rest. I found that task-related activity of motor area 4p within M1 and its connectivity with non-primary motor areas, such as the ipsilesional dorsal premotor (PMd) cortex could be modulated bidirectionally with low or high frequency rTMS over M1: compared to sham stimulation, 5Hz rTMS reduced task-related activity and network connectivity of that area. The opposite was true for 1Hz rTMS. A further study using the same task examined whether such reorganisation would be observed with 5Hz rTMS over the PMd. The effect of 5Hz rTMS on this site of stimulation revealed a different pattern of regional cerebral blood flow than rTMS over M1. However, a similar reorganisation in task-related activity and network connectivity was observed, particularly within the PMd area caudal to the stimulated site, which increased in activity after 5Hz rTMS. These two studies demonstrated that rTMS leads to widespread activity changes. However, changes in task-related activity and motor network connectivity occur in motor areas adjacent to the stimulated site. Given the lack of any behavioural effects in these studies, it can be hypothesised that these changes occur as compensatory mechanisms to the "virtual lesion" caused by rTMS. A functional magnetic resonance imaging (fMRI) study of the effects of rTMS over the lateral prefrontal cortex showed task-related changes in activity during performance of a cued choice reaction time task. Targeting the dorsolateral prefrontal cortex (DLPFC) with 5Hz rTMS led to task-related decreases in activity in the adjacent ventrolateral prefrontal cortex. This is reminiscent of the task-related decreases in activity of area 4p observed following 5Hz rTMS over M1 (described above). The side of lateral prefrontal conditioning affected behavioural performance in a further study which distinguished motor from spatial attention in the same behavioural paradigm. Left DLPFC stimulation led to a more prominent switch cost in the motor attention version of this task, confirming a left-lateralised dominance for switching motor responses, described in previous studies. These results provide new evidence that reorganisation in the brain observed following rTMS conditioning is very similar to reorganisation observed following lesions in patients. This reorganisation seems to depend on the task that is required to be performed and determines the pattern of activity obtained in functional neuroimaging studies. In addition, whether this will lead to a behavioural effect depends on the role of that particular area during task performance which is a function of its effective connectivity. These factors seem to determine whether the brain can compensate for the "virtual lesion" induced by rTMS

Multiple-inlet BIPV/T Modeling: Wind Effects and Fan Induced Suction

AbstractBuilding Integrated Photovoltaic/Thermal (BIPV/T) collectors take up the role of energy and heat production, while acting as a rain-screen cladding. A multiple inlet BIPV/T system counters the effect of high temperature stratification on the PV layer, by enhancing the convection inside the air channel with the introduction of more than one openings for the intake of fresh air that break up the surface boundary layer. To investigate the uniformity of heat extraction from the PV panels, the fluid mechanics of the system are studied separately from the thermal effects. A numerical flow distribution model, which incorporates wind effects, is introduced for the optimal design of multiple inlet systems so as to have flow rates through each inlet that maximize the heat extracted from the PV panels

A direct effect of perception on action when grasping a cup

Affordances represent features of an object that trigger specific actions. Here we tested whether the presence and orientation of a handle on a cup could bias grasping movements towards it in conditions where subjects were explicitly told to ignore the handle. We quantified the grip aperture profile of twelve healthy participants instructed to grasp a cup from its body while it either had no handle, a handle pointing towards, or away from the grasping hand (3 'move' conditions, with large grip aperture). To ensure the smaller grip aperture afforded by the handle was implicitly processed, we interspersed trials in which participants had to grasp the cup from its handle or a handle not attached to a cup with a small grip aperture. We found that grip aperture was smaller in the presence of a handle in the 'move' conditions, independently of its orientation. Our finding, of an effect of the handle during the execution of a grasp action, extends previous evidence of such an influence measured during motor preparation using simple reaction times. It suggests that the specific action elicited by an object's attribute can affect movement performance in a sustained manner throughout movement execution

Top-down control is not lost in the attentional blink: evidence from intact endogenous cuing.

The attentional blink (AB) refers to the finding that performance on the second of two targets (T1 and T2) is impaired when the targets are presented at a target onset asynchrony (TOA) of less than 500 ms. One account of the AB assumes that the processing load of T1 leads to a loss of top-down control over stimulus selection. The present study tested this account by examining whether an endogenous spatial cue that indicates the location of a following T2 can facilitate T2 report even when the cue and T2 occur within the time window of the AB. Results from three experiments showed that endogenous cuing had a significant effect on T2 report, both during and outside of the AB; this cuing effect was modulated by both the cue-target onset asynchrony and by cue validity, while it was invariant to the AB. These results suggest that top-down control over target selection is not lost during the AB. © 2007 Springer-Verlag

A neural circuit model of decision uncertainty and change-of-mind

Decision-making is often accompanied by a degree of confidence on whether a choice is correct. Decision uncertainty, or lack in confidence, may lead to change-of-mind. Studies have identified the behavioural characteristics associated with decision confidence or change-of-mind, and their neural correlates. Although several theoretical accounts have been proposed, there is no neural model that can compute decision uncertainty and explain its effects on change-of-mind. We propose a neuronal circuit model that computes decision uncertainty while accounting for a variety of behavioural and neural data of decision confidence and change-of-mind, including testable model predictions. Our theoretical analysis suggests that change-of-mind occurs due to the presence of a transient uncertainty-induced choice-neutral stable steady state and noisy fluctuation within the neuronal network. Our distributed network model indicates that the neural basis of change-of-mind is more distinctively identified in motor-based neurons. Overall, our model provides a framework that unifies decision confidence and change-of-mind