About MacSphere

MacSphere is McMaster University's Institutional Repository (IR). The purpose of an IR is to bring together all of a University's research under one umbrella, with an aim to preserve and provide access to that research. The research and scholarly output included in MacSphere has been selected and deposited by the individual university departments and centres on campus.

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Recent Submissions

Effects of Density, Nitrogen and Photoperiod on Morphology and Biomass Allocation in Alliaria Petiolata
(2002-12) Sanchez, Maria Isabel M.
Traits associated with carbon acquisition may exhibit plastic responses to different density, nitrogen and photoperiod treatments. In this experiment, petiole elongation and shifts in biomass allocation were examined in Alliaria petiolata (also known as garlic mustard) grown in high and low densities, given moderate and high nitrogen fertilizer treatments, and subjected to either short day or long day photoperiod treatments. The results indicate that rosette plants exhibit density-dependent elongation and an increased shift to above ground plant parts in high density. Fibrous root biomass was reduced in high density and we suggest that this observed reduction is not a cost of elongation but is itself adaptive. Although garlic mustard is considered a nitrophile, our results yield few significant nitrogen effects. Photoperiod treatments affected plant size, elongation and biomass allocation patterns, but depend on what density plants were grown. Low density plants responded more conservatively to short day photoperiod, producing shorter petioles and allocating less resources to above ground parts, suggesting that the difference in microclimate between low and high density stands may play an important role in these plants’ response to spatial and temporal cues.
Vortex Shedding and Galloping of Parabolic Bluff Bodies
(2004-03) Jeejeebhoy, Cyrus
The present study was prompted by a desire to further understand a serious, resilient instability found by Veljkovic (2001) in his study of the flow induced vibration of a parabolic bluff body. Veljkovic (2001) found that when the center of mass was moved forward of the elastic axis by 95 mm parabolic bluff bodies began to experience what appeared to be galloping. Quantitative data seemed to suggest the body was first experiencing vortex shedding which triggered galloping. The purpose of this study was to improve our understanding of the flow excitation of open parabolic bluff bodies, and how the separation distance between the center of mass and the elastic axis affect the critical velocity. To this end an experiment was designed to verify the excitation mechanism, and secondly to study the relationship between the change in critical velocity with changes in separation distances between elastic axis and center of mass. An experiment was designed to be performed in the water tunnel with two different separation distances between elastic axis and centre of mass: 250 mm and 95 mm. The most efficient way to study the excitation mechanism was to use flow visualization in a water tunnel. The method of flow visualization selected for this experiment was seeding the flow with aluminum particles. The flow velocity of the water was increased at incremental steps with the amplitude and frequency of vibration measured at each step. This experiment was carried out over a Reynolds number range of 1.6xl04 to 9.97x104 which is similar to Veljkovic’s (2001) range and in the range where the wake is in transition to turbulence. The Strouhal number for this experiment was 0.12, which is similar to Veljokovic’s (2001) 0.13. By using a water tunnel the flow visualization capabilities were enhanced, though the damping was significantly increased. The 95 mm experiment appeared to suffer from high damping and flow velocities sufficiently low that no obvious vortex shedding excitation was observed. The 250 mm experiment appeared to mimic Veljkovic’s (2001) 95 mm test and seemed to be excited by vortex shedding and subsequently galloping. From seeding the flow, the aluminium tracers clearly showed that the excitation mechanism for the 250 mm, and therefore Veljkovic’s (2001) 95 mm experiment, was vortex shedding followed by galloping. Flow reattachment to the after body clearly distinguishes galloping from vortex shedding.
Towards a neurocomputational and dynamical understanding of rumination
(2026) Singh, Selena; Becker, Suzanna
Ruminative brooding, the maladaptive repetitive dwelling on abstract, self-referential thoughts, is a transdiagnostic feature of many psychiatric disorders, yet its neurocognitive mechanisms remain poorly understood. Brooding is both repetitive and perseverative, implicating deficits in cognitive control and maladaptive emotion regulation. We investigated these properties using experimental psychology, computational modeling, network neuroscience, and dynamical systems approaches. First, we reported brooding-associated inhibitory control deficits that were exacerbated by emotional cues, as reflected by performance on Stroop interference tasks. Numerically fitting parallel distributed processing models to participant Stroop data revealed a set of brooding-associated parameters that reflected network sensitivity to changing task demands and activity persistence of emotional processes, but not cognitive control processes. Next, we analyzed relationships between self-regulatory processes and brooding, and their corresponding EEG functional connectivity patterns. We found evidence of internal resistance to emotions and thoughts in brooding. Furthermore, functional connectivity patterns indicated that brooding and emotion dysregulation co-varied with aberrant top-down modulation from prefrontal regions to emotion and interoceptive systems, potentially reflecting compensatory regulation of internal emotional states. Finally, we proposed that brooding is an emergent property of an attractor state within the brain’s default mode network (DMN). Here, we hypothesised that brooding-related attractor dynamics would produce stable, recurrent neural dynamics that resist perturbation. Supporting this, DMN regions demonstrated persistent activity when switching from a cued rumination to a working memory task, consistent with resistance of ruminative thought to perturbation. In addition, nonlinear analyses of the recurrence properties of EEG demonstrated a positive association between brooding and neural recurrence, further supporting this attractor state hypothesis. Together, these studies advance a mechanistic understanding of brooding from neurocomputational, network, and dynamical perspectives.
RISK OF CANCER AND OCULAR DISEASES FOLLOWING CT SCANS
(2026) Emami, Pirouz
Computed tomography (CT) is an essential tool in modern medicine, enabling quick and accurate diagnosis of various conditions. Since CT relies on ionizing radiation (IR) to produce internal images, its widespread use has led to concerns regarding patient safety, particularly in children who are more sensitive. In this thesis, the potential association between low-dose IR exposure from CT scans and the development of certain ocular conditions and cancers among residents of Ontario, Canada was studied. Data on CT scans were obtained from the Ontario Health Insurance Plan (OHIP) database. We first estimated organ doses from various CT procedures based on sex, age, and scan year, using CT dose index (CTDI) values from national surveys and dose coefficients from the National Cancer Institute dosimetry system for CT (NCICT). These dose estimates were then used to assess disease risk. All analyses were conducted using multivariate Cox regression, including demographic factors and relevant predisposing medical conditions as covariates. Appropriate lag periods were applied to account for disease latency. Using OHIP data from 1994 to 2023, we evaluated the risk of cataract and glaucoma in relation to radiation dose to the eye lens following head CT scans. Analyses based on cataract surgery, glaucoma diagnosis, or glaucoma treatment, lagged by 3, 5, and 7 years, showed no increased risk due to radiation exposure. Similarly, using data from 1992 to 2019, we studied the risk of leukemia, lymphoma, and brain tumors in individuals who had received CT scans at or before age 19. Analyses lagged by 2, 5, and 10 years revealed some elevated risks for the primary cancer types but also highlighted no association or even reduced risk for some known radiogenic cancer sub-types. Taken together, the results did not support a clear association between cancer risk and CT exposure in this population. Overall, this thesis provides valuable insight into the potential risks associated with CT scans, supporting clinicians in making more informed decisions when ordering these procedures.
Travels in Africa and the Middle East
(POLYDYNAMICS INC, 2026-01-09) John Vlachopoulos
Historical travelogue in South Africa, Algeria, Morocco, United Arab Emirates, Israel and Turkey.

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