THE INITIATION OF BINOCULAR RIVALRY

Binocular rivalry refers to the perceptual alternation that occurs while viewing incompatible images, in which one monocular image is dominant and the other is suppressed. Rivalry has been closely studied but the neural site at which it is initiated is still controversial. The central claim of this thesis is that primary visual cortex is responsible for its initiation. This claim is supported by evidence from four experimental studies. The first study (described in Chapter 4) introduces the methodology for measuring visual sensitivity during dominance and suppression and compares several methods to see which yields the greatest difference between these two sensitivities. Suppression depth was measured by comparing the discrimination thresholds to a brief test stimulus delivered during dominance and suppression phases. The deepest suppression was achieved after a learning period, with the test stimulus presented for 100 ms and with post-test masking. The second study (Chapter 5) compares two hypotheses for the mechanism of binocular rivalry. Under eye suppression, visibility decreases when the tested eye is being suppressed, regardless of the test stimulus’s features. Feature suppression, however, predicts that reduction of visibility is caused by suppression of a stimulus feature, no matter which eye is suppressed. Eye suppression claims that monocular channels in the visual system alternate between dominance and suppression, while Feature suppression assumes that the features of stimuli inhibit each other perceptually in the high-level cortex. The experiment used a test stimulus similar in features to one, but not the other, rivalry-inducing stimulus. Test sensitivity was found to be lowered when the test stimulus was presented to the eye whose rivalry-inducing stimulus was suppressed. Sensitivity was not lowered when the test stimulus was presented to the other eye, even when the test shared features with the suppressed stimulus. The conclusion is that feature suppression is weak or does not exist without eye suppression, and that rivalry therefore originates in the primary visual cortex. If binocular rivalry is initiated in the primary visual cortex, stimuli producing no coherent activity in that area should produce no rivalry. In the third study (Chapter 6) this idea was tested with rotating arrays of short-lifetime dots. The dots with the shortest lifetime produced an image with no rotation signal, and an infinite lifetime produced rigid rotation. Subjects could discriminate the rotation direction with high accuracy at all but the shortest lifetime. When the two eyes were presented with opposite directions of rotation, there was binocular rivalry only at the longest lifetimes. Stimuli with short lifetimes produce a coherent motion signal, since their direction can be discriminated, but do not produce rivalry. A simple interpretation of this observation is that binocular rivalry is initiated at a level in the visual hierarchy below that which supports the motion signal. The model supported by the results of previous chapters requires that binocular rivalry suppression be small in the primary visual cortex, and builds up as signals progress along the visual pathway. This model predicts that for judgements dependent on activity in high visual cortex: 1. Binocular rivalry suppression should be deep; 2. Responses should be contrast invariant. The fourth and last study (chapter 7) confirmed these predictions by measuring suppression depth in two ways. First, two similar forms were briefly presented to one eye: the difference in shapes required for their discrimination was substantially greater during suppression than during dominance. Second, the two forms were made sufficiently different in shape to allow easy discrimination at high contrast, and the contrast of these forms was lowered to find the discrimination threshold. The results in the second experiment showed that contrast sensitivity did not differ between the suppression and dominance states. This invariance in contrast sensitivity is interpreted in terms of steep contrast-response functions in cortex beyond the primary visual area. The work in this thesis supports the idea that binocular rivalry is a process distributed along the visual pathway. More importantly, the results provide several lines of evidence that binocular rivalry is initiated in primary visual cortex

Automated coronary artery calcification scoring in non-gated chest CT: Agreement and reliability

Objective: To determine the agreement and reliability of fully automated coronary artery calcium (CAC) scoring in a lung cancer screening population. Materials and Methods: 1793 low-dose chest CT scans were analyzed (non-contrast-enhanced, non-gated). To establish the reference standard for CAC, first automated calcium scoring was performed using a preliminary version of a method employing coronary calcium atlas and machine learning approach. Thereafter, each scan was inspected by one of four trained raters. When needed, the raters corrected initially automaticity-identified results. In addition, an independent observer subsequently inspected manually corrected results and discarded scans with gross segmentation errors. Subsequently, fully automatic coronary calcium scoring was performed. Agatston score, CAC volume and number of calcifications were computed. Agreement was determined by calculating proportion of agreement and examining Bland-Altman plots. Reliability was determined by calculating linearly weighted kappa (κ) for Agatston strata and intraclass correlation coefficient (ICC) for continuous values. Results: 44 (2.5%) scans were excluded due to metal artifacts or gross segmentation errors. In the remaining 1749 scans, median Agatston score was 39.6 (P25-P75:0-345.9), median volume score was 60.4 mm3 (P25-P75:0-361.4) and median number of calcifications was 2 (P25-P75:0-4) for the automated scores. The k demonstrated very good reliability (0.85) for Agatston risk categories between the automated and reference scores. The Bland-Altman plots showed underestimation of calcium score values by automated quantification. Median difference was 2.5 (p25-p75:0.0-53.2) for Agatston score, 7.6 (p25-p75:0.0-94.4) for CAC volume and 1 (p25-p75:0-5) for number of calcifications. The ICC was very good for Agatston score (0.90), very good for calcium volume (0.88) and good for number of calcifications (0.64). Discussion: Fully automated coron

A cost effeciency approach to universal access for public transport for disabled people

Purpose To determine the intervendor variability of Agatston scoring determined with state-of-the-art computed tomographic (CT) systems from the four major vendors in an ex vivo setup and to simulate the subsequent effects on cardiovascular risk reclassification in a large population-based cohort. Materials and Methods Research ethics board approval was not necessary because cadaveric hearts from individuals who donated their bodies to science were used. Agatston scores obtained with CT scanners from four different vendors were compared. Fifteen ex vivo human hearts were placed in a phantom resembling an average human adult. Hearts were scanned at equal radiation dose settings for the systems of all four vendors. Agatston scores were quantified semiautomatically with software used clinically. The ex vivo Agatston scores were used to simulate the effects of different CT scanners on reclassification of 432 individuals aged 55 years or older from a population-based study who were at intermediate cardiovascular risk based on Framingham risk scores. The Friedman test was used to evaluate overall differences, and post hoc analyses were performed by using the Wilcoxon signed-rank test with Bonferroni correction. Results Agatston scores differed substantially when CT scanners from different vendors were used, with median Agatston scores ranging from 332 (interquartile range, 114-1135) to 469 (interquartile range, 183-1381; P < .05). Simulation showed that these differences resulted in a change in cardiovascular risk classification in 0.5\%-6.5\% of individuals at intermediate risk when a CT scanner from a different vendor was used. Conclusion Among individuals at intermediate cardiovascular risk, state-of the-art CT scanners made by different vendors produced substantially different Agatston scores, which can result in reclassification of patients to the high- or low-risk categories in up to 6.5\% of cases. © RSNA, 2014

Inter-observer and inter-examination variability of manual vertebral bone attenuation measurements on computed tomography

Objective: To determine inter-observer and inter-examination variability of manual attenuation measurements of the vertebrae in low-dose unenhanced chest computed tomography (CT). Methods: Three hundred and sixty-seven lung cancer screening trial participants who underwent baseline and repeat unenhanced low-dose CT after 3 months because of an indeterminate lung nodule were included. The CT attenuation value of the first lumbar vertebrae (L1) was measured in all CTs by one observer to obtain inter-examination reliability. Six observers performed measurements in 100 randomly selected CTs to determine agreement with limits of agreement and Bland-Altman plots and reliability with intraclass correlation coefficients (ICCs). Reclassification analyses were performed using a threshold of 110 HU to define osteoporosis. Results: Inter-examination reliability was excellent with an ICC of 0.92 (p < 0.001). Inter-examination limits of agreement ranged from -26 to 28 HU with a mean difference of 1 ± 14 HU. Inter-observer reliability ICCs ranged from 0.70 to 0.91. Inter-examination variability led to 11.2 % reclassification of participants and inter-observer variability led to 22.1 % reclassification. Conclusions: Vertebral attenuation values can be manually quantified with good to excellent inter-examination and inter-observer reliability on unenhanced low-dose chest CT. This information is valuable for early detection of osteoporosis on low-dose chest CT. Key Points: • Vertebral attenuation values can be manually quantified on low-dose unenhanced CT reliably.• Vertebral attenuation measurements may be helpful in detecting subclinical low bone density.• This could become of importance in the detection of osteoporosis

Automatic coronary artery calcium scoring on radiotherapy planning CT Scans of breast cancer patients: Reproducibility and association with traditional cardiovascular risk factors

Objectives Coronary artery calcium (CAC) is a strong and independent predictor of cardiovascular disease (CVD) risk. This study assesses reproducibility of automatic CAC scoring on radiotherapy planning computed tomography (CT) scans of breast cancer patients, and examines its association with traditional cardiovascular risk factors. Methods This study included 561 breast cancer patients undergoing radiotherapy between 2013 and 2015. CAC was automatically scored with an algorithm using supervised pattern recognition, expressed as Agatston scores and categorized into five categories (0, 1-10, 11-100, 101-400, >400). Reproducibility between automatic and manual expert scoring was assessed in 79 patients with automatically determined CAC above zero and 84 randomly selected patients without automatically determined CAC. Interscan reproducibility of automatic scoring was assessed in 294 patients having received two scans (82% on the same day). Association between CAC and CVD risk factors was assessed in 36 patients with CAC scores >100, 72 randomly selected patients with scores 1-100, and 72 randomly selected patients without CAC. Reliability was assessed with linearly weighted kappa and agreement with proportional agreement. Results 134 out of 561 (24%) patients had a CAC score above zero. Reliability of CVD risk categorization between automatic and manual scoring was 0.80 (95% Confidence Interval (CI): 0.74-0.87), and slightly higher for scans with breath-hold. Agreement was 0.79 (95% CI: 0.72-0.85). Interscan reliability was 0.61 (95% CI: 0.50-0.72) with an agreement of 0.84 (95% CI: 0.80-0.89). Ten out of 36 (27.8%) patients with CAC scores above 100 did not have other cardiovascular risk factors. Conclusions Automatic CAC scoring on radiotherapy planning CT scans is a reliable method to assess CVD risk based on Agatston scores. One in four breast cancer patients planned for radiotherapy have elevated CAC score. One in three patients with high CAC scores don't have other CVD risk factors and wouldn't have been identified as high risk

Prognostic value of heart valve calcifications for cardiovascular events in a lung cancer screening population

To assess the prognostic value of aortic valve and mitral valve/annulus calcifications for cardiovascular events in heavily smoking men without a history of cardiovascular disease. Heavily smoking men without a cardiovascular disease history who underwent non-contrast-enhanced low-radiation-dose chest CT for lung cancer screening were included. Non-imaging predictors (age, smoking status and pack-years) were collected and imaging-predictors (calcium volume of the coronary arteries, aorta, aortic valve and mitral valve/annulus) were obtained. The outcome was the occurrence of cardiovascular events. Multivariable Cox proportional-hazards regression was used to calculate hazard-ratios (HRs) with 95 % confidence interval (CI). Subsequently, concordance-statistics were calculated. In total 3111 individuals were included, of whom 186 (6.0 %) developed a cardiovascular event during a follow-up of 2.9 (Q1–Q3, 2.7–3.3) years. If aortic (n = 657) or mitral (n = 85) annulus/valve calcifications were present, cardiovascular event incidence increased to 9.0 % (n = 59) or 12.9 % (n = 11), respectively. HRs of aortic and mitral valve/annulus calcium volume for cardiovascular events were 1.46 (95 % CI, 1.09–1.84) and 2.74 (95 % CI, 0.92–4.56) per 500 mm3. The c-statistic of a basic model including age, pack-years, current smoking status, coronary and aorta calcium volume was 0.68 (95 % CI, 0.63–0.72), which did not change after adding heart valve calcium volume. Aortic valve calcifications are predictors of future cardiovascular events. However, there was no added prognostic value beyond age, number of pack-years, current smoking status, coronary and aorta calcium volume for short term cardiovascular events

CT Imaging of Atherosclerotic Cardiovascular Disease

The topic of this thesis is Computed Tomography (CT) imaging of atherosclerotic cardiovascular disease. The first part focusing on the technical developments and the diagnostic performance of CT. We provided a systematic review of the literature which lists the merits and shortcomings of the administration of nitroglycerin in coronary CT angiography; discussed the effect of iterative reconstruction on CT assessment of plaque composition; performed a meta-analysis with the aim to determine the diagnostic accuracy of myocardial perfusion imaging compared with invasive coronary angiography combined with fractional flow reserve for the diagnosis of hemodynamically significant coronary artery disease; and evaluated the use of low radiation dose scan protocol for acquiring ventricular function with CT compared to magnetic resonance imaging. In the second part the prognostic value of pulmonary CT measurements and coronary calcium score was evaluated in a lung cancer screening population. In the last part, the use of coronary CT angiography in clinical practice and current risk model for statin eligibility was assessed. We evaluated the use of coronary CT angiography for workup of suspected acute coronary syndrome in a patient presenting at the emergency department; finally we incorporated the prognostic value of non-obstructive coronary artery disease into a risk calculator to determine sex and ethnicity based thresholds for reclassification of statin eligibility

Sex Differences in Coronary Artery and Thoracic Aorta Calcification and Their Association With Cardiovascular Mortality in Heavy Smokers

OBJECTIVES: The aim of this study was to investigate sex differences in the prevalence, extent, and association of coronary artery calcium (CAC) and thoracic aorta calcium (TAC) scores with cardiovascular mortality in a population eligible for lung screening. BACKGROUND: CAC and TAC scores derived from chest computed tomography (CT) might be useful biomarkers for individualized cardiovascular disease prevention and could be especially relevant in high-risk populations such as heavy smokers. Therefore, it is important to know the prevalence of arterial calcifications in male and female heavy smokers, and if there are differences in the predictive value calcifications carry. METHODS: We performed a nested case-control study with 5,718 participants of the CT arm of the NLST (National Lung Screening Trial). Prevalence and extent of CAC and TAC were resampled to the full cohort to provide unbiased estimates of the typical calcium burden of male and female heavy smokers. Weighted Cox proportional hazards regression was used to assess differences in the association of CAC and TAC scores with all-cause and cardiovascular mortality. RESULTS: CAC was substantially more common and more severe in men (prevalence: 81% vs. 60%; median volume: 104 mm³ vs.12 mm³). Women had CAC comparable to that of men who were 10 years younger. TAC was equally common in men and women, with a tendency to be more pronounced in women (prevalence: 92% vs. 93%; median volume: 388 mm³ vs. 404 mm³). Both types of calcification were associated with increased cardiovascular and all-cause mortality. TAC scores improved the prediction of coronary heart disease mortality over CAC in men, but not in women. In both sexes, TAC, but not CAC, was associated with cardiovascular mortality other than coronary heart disease. CONCLUSIONS: CAC develops later in women, whereas TAC develops equally in both sexes. CAC is strongly associated with coronary heart disease, whereas TAC is especially associated with extracardiac vascular mortality in either sex

Supplementary Material for: Clinical and Imaging Predictors of Recurrent Ischemic Stroke: A Systematic Review and Meta-Analysis

<b><i>Background:</i></b> Predictors of recurrent ischemic stroke are less well known in patients with a recent ischemic stroke than in patients with transient ischemic attack (TIA). We identified clinical and radiological factors for predicting recurrent ischemic stroke in patients with recent ischemic stroke. <b><i>Methods:</i></b> A systematic search in PubMed, Embase, Cochrane Library, and CINAHL was performed with the terms “ischemic stroke,” “predictors/determinants,” and “recurrence.” Quality assessment of the articles was performed and the level of evidence was graded for the articles included for the meta-analysis. Pooled risk ratios (RR) and heterogeneity (<i>I</i>²) were calculated using inverse variance random effects models. <b><i>Results:</i></b> Ten articles with high-quality results were identified for meta-analysis. Past medical history of stroke or TIA was a predictor of recurrent ischemic stroke (pooled RR 2.5, 95% CI 2.1–3.1). Small vessel strokes were associated with a lower risk of recurrence than large vessel strokes (pooled RR 0.3, 95% CI 0.1–0.7). Patients with stroke of an undetermined cause had a lower risk of recurrence than patients with large artery atherosclerosis (pooled RR 0.5, 95% CI 0.2–1.1). We found no studies using CT or ultrasound for the prediction of recurrent ischemic stroke. The following MRI findings were predictors of recurrent ischemic stroke: multiple lesions (pooled RR 1.7, 95% CI 1.5–2.0), multiple stage lesions (pooled RR 4.1, 95% CI 3.1–5.5), multiple territory lesions (pooled RR 2.9, 95% CI 2.0–4.2), chronic infarcts (pooled RR 1.5, 95% CI 1.2–1.9), and isolated cortical lesions (pooled RR 2.2, 95% CI 1.5–3.2). <b><i>Conclusions:</i></b> In patients with a recent ischemic stroke, a history of stroke or TIA and the subtype large artery atherosclerosis are associated with an increased risk of recurrent ischemic stroke. Predictors evaluated with MRI include multiple ischemic changes and isolated cortical lesions. Predictors of recurrent ischemic stroke concerning CT or ultrasound have not been published