9,734 research outputs found
Letter from Ed Hamilton to Carl Hayden
No full textLetter from Ed Hamilton to Carl Hayden requesting maintenance for the road between Grand Canyon Village and Rowe's Well
Integrating evidence from neuroimaging and neuropsychological studies of obsessive-compulsive disorder: the orbitofronto-striatal model revisited
No full textObsessive-compulsive disorder (OCD) is a common, heritable and disabling neuropsychiatric disorder. Theoretical models suggest that OCD is underpinned by functional and structural abnormalities in orbitofronto-striatal circuits. Evidence from cognitive and neuroimaging studies (functional and structural magnetic resonance imaging (MRI) and positron emission tomography (PET)) have generally been taken to be supportive of these theoretical models; however, results from these studies have not been entirely congruent with each other. With the advent of whole brain-based structural imaging techniques, such as voxel-based morphometry and multivoxel analyses, we consider it timely to assess neuroimaging findings to date, and to examine their compatibility with cognitive studies and orbitofronto-striatal models. As part of this assessment, we performed a quantitative, voxel-level meta-analysis of functional MRI findings, which revealed consistent abnormalities in orbitofronto-striatal and other additional areas in OCD. This review also considers the evidence for involvement of other brain areas outside orbitofronto-striatal regions in OCD, the limitations of current imaging techniques, and how future developments in imaging may aid our understanding of OCD.</p
Letter from Carl Hayden to Ed Hamilton
No full textLetter from Carl Hayden to Ed Hamilton explaining that although there are currently no funds to improve the road between Grand Canyon Village and Rowe's Well, it will be taken into consideration when funding does become available
Letter from Carl Hayden to Ed Hamilton
No full textLetter from Carl Hayden to Ed Hamilton informing him that Superintendent Tillotson will submit a report on the conditions of the road between Grand Canyon Village and Rowe's Well
Letter from Carl Hayden to Ed Hamilton
No full textLetter from Carl Hayden to Ed Hamilton expressing regret that he cannot do more to secure funds for the road between Grand Canyon Village and Rowe's Well
Letter from Ed Hamilton to Carl Hayden
No full textLetter from Ed Hamilton to Carl Hayden outlining his opposition to Superintendent Tillotson's report on the road between Grand Canyon Village and Rowe's Well
Brain network analysis : separating cost from topology using cost-integration
No full textA statistically principled way of conducting brain network analysis is still lacking. Comparison of different populations of
brain networks is hard because topology is inherently dependent on wiring cost, where cost is defined as the number of
edges in an unweighted graph. In this paper, we evaluate the benefits and limitations associated with using cost-integrated
topological metrics. Our focus is on comparing populations of weighted undirected graphs that differ in mean association
weight, using global efficiency. Our key result shows that integrating over cost is equivalent to controlling for any
monotonic transformation of the weight set of a weighted graph. That is, when integrating over cost, we eliminate the
differences in topology that may be due to a monotonic transformation of the weight set. Our result holds for any
unweighted topological measure, and for any choice of distribution over cost levels. Cost-integration is therefore helpful in
disentangling differences in cost from differences in topology. By contrast, we show that the use of the weighted version of
a topological metric is generally not a valid approach to this problem. Indeed, we prove that, under weak conditions, the
use of the weighted version of global efficiency is equivalent to simply comparing weighted costs. Thus, we recommend the
reporting of (i) differences in weighted costs and (ii) differences in cost-integrated topological measures with respect to
different distributions over the cost domain. We demonstrate the application of these techniques in a re-analysis of an fMRI
working memory task. We also provide a Monte Carlo method for approximating cost-integrated topological measures.
Finally, we discuss the limitations of integrating topology over cost, which may pose problems when some weights are zero,
when multiplicities exist in the ranks of the weights, and when one expects subtle cost-dependent topological differences,
which could be masked by cost-integration
Letter from Ed Hamilton to Carl Hayden Regarding Powder Magazine at Grand Canyon
No full textLetter from Ed Hamilton to Carl Hayden regarding the storage of dynamite in Shoski Canyon
Broadband criticality of human brain network synchronization
No full textSelf-organized criticality is an attractive model for human brain dynamics, but there has been little direct evidence for its existence in large-scale systems measured by neuroimaging. In general, critical systems are associated with fractal or power law scaling, long-range correlations in space and time, and rapid reconfiguration in response to external inputs. Here, we consider two measures of phase synchronization: the phase-lock interval, or duration of coupling between a pair of (neurophysiological) processes, and the lability of global synchronization of a (brain functional) network. Using computational simulations of two mechanistically distinct systems displaying complex dynamics, the Ising model and the Kuramoto model, we show that both synchronization metrics have power law probability distributions specifically when these systems are in a critical state. We then demonstrate power law scaling of both pairwise and global synchronization metrics in functional MRI and magnetoencephalographic data recorded from normal volunteers under resting conditions. These results strongly suggest that human brain functional systems exist in an endogenous state of dynamical criticality, characterized by a greater than random probability of both prolonged periods of phase-locking and occurrence of large rapid changes in the state of global synchronization, analogous to the neuronal “avalanches” previously described in cellular systems. Moreover, evidence for critical dynamics was identified consistently in neurophysiological systems operating at frequency intervals ranging from 0.05–0.11 to 62.5–125 Hz, confirming that criticality is a property of human brain functional network organization at all frequency intervals in the brain's physiological bandwidth
White matter abnormalities in patients with obsessive-compulsive disorder and their first-degree relatives
No full textObjective: obsessive-compulsive disorder (OCD) is a common, heritable neuropsychiatric disorder, hypothetically underpinned by dysconnectivity of large-scale brain systems. The extent of white matter abnormalities in OCD is unknown, and the genetic basis of this disorder is poorly understood. The authors used diffusion tensor imaging, a magnetic resonance imaging technique, for examining white matter abnormalities in brain structure through quantification of water diffusion, to confirm whether white matter abnormalities exist in OCD. They also explored whether such abnormalities occur in healthy first-degree relatives of patients, indicating they may be endophenotypes representing increased genetic risk for OCD. Method: the authors used diffusion tensor imaging to measure fractional anisotropy of white matter in 30 patients with OCD, 30 unaffected first-degree relatives, and 30 matched healthy comparison subjects. Regions of significantly abnormal fractional anisotropy in patients in relation to healthy comparison subjects were identified by permutation tests. The authors assessed whether these abnormalities were also evident in the first-degree relatives. A secondary region-of-interest analysis was undertaken to assess the extent of replication between our data and previous relevant literature.Results: patients with OCD demonstrated significantly reduced fractional anisotropy in a large region of right inferior parietal white matter and significantly increased fractional anisotropy in a right medial frontal region. Relatives also exhibited significant abnormalities of fractional anisotropy in these regions. Conclusions: these findings indicate that OCD is associated with white matter abnormalities in parietal and frontal regions. Similar abnormalities in unaffected first-degree relatives suggest these may be white matter endophenotypes for OCD.</p
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