Stress free configuration of the human eye

Numerical simulations of eye globes often rely on topographies that have been measured in vivo using devices such as the Pentacam or OCT. The topographies, which represent the form of the already stressed eye under the existing intraocular pressure, introduce approximations in the analysis. The accuracy of the simulations could be improved if either the stress state of the eye under the effect of intraocular pressure is determined, or the stress-free form of the eye estimated prior to conducting the analysis. This study reviews earlier attempts to address this problem and assesses the performance of an iterative technique proposed by Pandolfi and Holzapfel [1], which is both simple to implement and promises high accuracy in estimating the eye's stress-free form. A parametric study has been conducted and demonstrated reliance of the error level on the level of flexibility of the eye model, especially in the cornea region. However, in all cases considered 3-4 analysis iterations were sufficient to produce a stress-free form with average errors in node location <10(-6)mm and a maximal error <10(-4)mm. This error level, which is similar to what has been achieved with other methods and orders of magnitude lower than the accuracy of current clinical topography systems, justifies the use of the technique as a pre-processing step in ocular numerical simulations.

What was I thinking? Eye-tracking experiments underscore the bias that architecture exerts on nuclear grading in prostate cancer

We previously reported that nuclear grade assignment of prostate carcinomas is subject to a cognitive bias induced by the tumor architecture. Here, we asked whether this bias is mediated by the non-conscious selection of nuclei that "match the expectation" induced by the inadvertent glance at the tumor architecture. 20 pathologists were asked to grade nuclei in high power fields of 20 prostate carcinomas displayed on a computer screen. Unknown to the pathologists, each carcinoma was shown twice, once before a background of a low grade, tubule-rich carcinoma and once before the background of a high grade, solid carcinoma. Eye tracking allowed to identify which nuclei the pathologists fixated during the 8 second projection period. For all 20 pathologists, nuclear grade assignment was significantly biased by tumor architecture. Pathologists tended to fixate on bigger, darker, and more irregular nuclei when those were projected before kigh grade, solid carcinomas than before low grade, tubule-rich carcinomas (and vice versa). However, the morphometric differences of the selected nuclei accounted for only 11% of the architecture-induced bias, suggesting that it can only to a small part be explained by the unconscious fixation on nuclei that "match the expectation". In conclusion, selection of « matching nuclei » represents an unconscious effort to vindicate the gravitation of nuclear grades towards the tumor architecture.

Statistical modeling of the eye for multimodal treatment planning for external beam radiation therapy of intraocular tumors

Ocular anatomy and radiation-associated toxicities provide unique challenges for external beam radiation therapy. For treatment planning, precise modeling of organs at risk and tumor volume are crucial. Development of a precise eye model and automatic adaptation of this model to patients' anatomy remain problematic because of organ shape variability. This work introduces the application of a 3-dimensional (3D) statistical shape model as a novel method for precise eye modeling for external beam radiation therapy of intraocular tumors.

One-Hertz transcranial magnetic stimulation over the frontal eye field induces lasting inhibition of saccade triggering

The aim of the study was to examine the effect of low-frequency repetitive transcranial magnetic stimulation on saccade triggering. In five participants, a train of 600 pulses with a frequency of 1 Hz was applied over the right frontal eye field and--as control condition--over the vertex. After repetitive transcranial magnetic stimulation application, oculomotor performance was evaluated with an overlap paradigm. The results show that the repetitive transcranial magnetic stimulation effect was specific for frontal eye field stimulation. Saccade latencies were found to be increased bilaterally for several minutes after the stimulation, and the time course of recovery was different for the ipsilateral and contralateral sides. The results are discussed in the light of possible local and remote repetitive transcranial magnetic stimulation effects on the oculomotor network.

Role of calcium-independent phospholipase A2 in cortex striatum thalamus cortex circuitry-enzyme inhibition causes vacuous chewing movements in rats

RATIONALE: High levels of calcium independent phospholipase A2 (iPLA2) are present in certain regions of the brain, including the cerebral cortex, striatum, and cerebellum (Ong et al. 2005). OBJECTIVES: The present study was carried out to elucidate a possible role of the enzyme in the motor system. METHODS: The selective iPLA2 inhibitor bromoenol lactone (BEL), the nonselective PLA2 inhibitor methyl arachidonyl fluorophosphonate (MAFP), and an antisense oligonucleotide were used to interfere with iPLA2 activity in various components of the motor system. Control animals received injections of carrier (phosphate buffered saline, PBS) at the same locations. The number of vacuous chewing movements (VCM) was counted from 1 to 14 days after injection. RESULTS: Rats that received BEL and high-dose MAFP injections in the striatum, thalamus, and motor cortex, but not the cerebellum, showed significant increase in VCM, compared to those injected with PBS at these locations. BEL-induced VCM were blocked by intramuscular injections of the anticholinergic drug, benztropine. Increased VCM was also observed after intrastriatal injection of antisense oligonucleotide to iPLA2. The latter caused a decrease in striatal iPLA2 levels, confirming a role of decreased enzyme activity in the appearance of VCM. CONCLUSIONS: These results suggest an important role for iPLA2 in the cortex-striatum-thalamus-cortex circuitry. It is postulated that VCM induced by iPLA2 inhibition may be a model of human parkinsonian tremor.

Time course of blood oxygenation level-dependent signal response after theta burst transcranial magnetic stimulation of the frontal eye field

The aim of the current study was to examine the effect of theta burst repetitive transcranial magnetic stimulation (rTMS) on the blood oxygenation level-dependent (BOLD) activation during repeated functional magnetic resonance imaging (fMRI) measurements. Theta burst rTMS was applied over the right frontal eye field in seven healthy subjects. Subsequently, repeated fMRI measurements were performed during a saccade-fixation task (block design) 5, 20, 35, and 60 min after stimulation. We found that theta burst rTMS induced a strong and long-lasting decrease of the BOLD signal response of the stimulated frontal eye field at 20 and 35 min. Furthermore, less pronounced alterations of the BOLD signal response with different dynamics were found for remote oculomotor areas such as the left frontal eye field, the pre-supplementary eye field, the supplementary eye field, and both parietal eye fields. Recovery of the BOLD signal changes in the anterior remote areas started earlier than in the posterior remote areas. These results show that a) the major inhibitory impact of theta burst rTMS occurs directly in the stimulated area itself, and that b) a lower effect on remote, oculomotor areas can be induced.