Exploration of the functional consequences of fixational eye movements in the absence of a fovea.

A recent theory posits that ocular drifts of fixational eye movements serve to reformat the visual input of natural images, so that the power of the input image is equalized across a range of spatial frequencies. This "spectral whitening" effect is postulated to improve the processing of high-spatial-frequency information and requires normal fixational eye movements. Given that people with macular disease exhibit abnormal fixational eye movements, do they also exhibit spectral whitening? To answer this question, we computed the power spectral density of movies of natural images translated in space and time according to the fixational eye movements (thus simulating the retinal input) of a group of observers with long-standing bilateral macular disease. Just as for people with normal vision, the power of the retinal input at low spatial frequencies was lower than that based on the 1/f2 relationship, demonstrating spectral whitening. However, the amount of whitening was much less for observers with macular disease when compared with age-matched controls with normal vision. A mediation analysis showed that the eccentricity of the preferred retinal locus adopted by these observers and the characteristics of ocular drifts are important factors limiting the amount of whitening. Finally, we did not find a normal aging effect on spectral whitening. Although these findings alone cannot form a causal link between macular disease and spectral properties of eye movements, they suggest novel potential means of modifying the characteristics of fixational eye movements, which may in turn improve functional vision for people with macular disease

Suboptimal eye movements for seeing fine details.

Human eyes are never stable, even during attempts of maintaining gaze on a visual target. Considering transient response characteristics of retinal ganglion cells, a certain amount of motion of the eyes is required to efficiently encode information and to prevent neural adaptation. However, excessive motion of the eyes leads to insufficient exposure to the stimuli, which creates blur and reduces visual acuity. Normal miniature eye movements fall in between these extremes, but it is unclear if they are optimally tuned for seeing fine spatial details. We used a state-of-the-art retinal imaging technique with eye tracking to address this question. We sought to determine the optimal gain (stimulus/eye motion ratio) that corresponds to maximum performance in an orientation-discrimination task performed at the fovea. We found that miniature eye movements are tuned but may not be optimal for seeing fine spatial details

Motion Information via the Nonfixating Eye Can Drive Optokinetic Nystagmus in Strabismus

Purpose: Strabismic patients can perceptually suppress information from one eye to avoid double vision. However, evidence from prior studies shows that some parts of the visual field of the deviated eye are not suppressed. Our goal here was to investigate whether motion information available only to the deviated eye can be utilized by the oculomotor system to drive eye movements. Methods: Binocular eye movements were acquired in two exotropic monkeys in a dichoptic viewing task in which the fixating eye viewed a stationary spot and the deviated eye viewed a 10° × 10° stationary patch that contained a drifting grating stimulus moving at 10°/s to the right or left for 20 seconds. Spatial location and contrast of the grating were systematically varied in subsequent trials. For each trial, mean slow-phase velocity of the optokinetic nystagmus (OKN) elicited by grating motion was calculated. Results: We found that OKN responses can be elicited by a motion stimulus presented to the foveal region of the deviated eye. Optokinetic nystagmus magnitude varied depending on which eye was viewing the drifting grating and correlated well with fixation preference and fixation stability (indicators of amblyopia). The magnitude of OKN increased for increased relative contrast of the motion stimulus compared to the fixation spot. Conclusions: Our results show that motion information available only to the deviated eye can drive optokinetic eye movements. We conclude that the brain has access to visual information from portions of the deviated eye (including the fovea) in strabismus that it can use to drive eye movements

Comparison of Naso-temporal Asymmetry During Monocular Smooth Pursuit, Optokinetic Nystagmus, and Ocular Following Response in Strabismic Monkeys

Purpose: Under monocular viewing conditions, humans and monkeys with infantile strabismus exhibit asymmetric naso-temporal (N-T) responses to motion stimuli. The goal of this study was to compare and contrast these N-T asymmetries during 3 visually mediated eye tracking tasks-optokinetic nystagmus (OKN), smooth pursuit (SP) response, and ocular following responses (OFR). Methods: Two adult strabismic monkeys were tested under monocular viewing conditions during OKN, SP, or OFR stimulation. OKN stimulus was unidirectional motion of a 30°x30° random dot pattern at 20°, 40°, or 80°/s for 1 minute. OFR stimulus was brief (200 ms) unidirectional motion of a 38°x28°whitenoise at 20°, 40°, or 80°/s. SP stimulus consisted of foveal step-ramp target motion at 10°, 20°, or 40°/s. Results: Mean nasalward steady state gain (0.87±0.16) was larger than temporalward gain (0.67±0.19) during monocular OKN (P<0.001). In monocular OFR, the asymmetry is manifested as a difference in OFR velocity gain (nasalward: 0.33±0.19, temporalward: 0.22±0.12; P=0.007). During monocular SP, mean nasal gain (0.97±0.2) was larger than temporal gain (0.66±0.14; P<0.001) and the mean nasalward acceleration during pursuit initiation (156±61°/s2) was larger than temporalward acceleration (118±77°/s2; P=0.04). Comparison of N-T asymmetry ratio across the 3 conditions using ANOVA showed no significant difference. Conclusion: N-T asymmetries are identified in all 3 visual tracking paradigms in both monkeys with either eye viewing. Our data are consistent with the current hypothesis for the mechanism for N-T asymmetry that invokes an imbalance in cortical drive to brainstem circuits

Spatial Patterns of Fixation-Switch Behavior in Strabismic Monkeys

Purpose: Patients with strabismus perceptually suppress information from one eye to avoid double vision. Mechanisms of visual suppression likely lead to fixation-switch behavior wherein the subject acquires targets with a specific eye depending on target location in space. The purpose of this study was to investigate spatial patterns of fixation-switch behavior in strabismic monkeys. Methods: Eye movements were acquired in three exotropic and one esotropic monkey in a binocular viewing saccade task. Spatial patterns of fixation were analyzed by calculating incidence of using either eye to fixate targets presented at various gaze locations. Results: Broadly, spatial fixation patterns and fixation-switch behavior followed expectations if a portion of the temporal retina was suppressed in exotropia and a portion of the nasal retina was suppressed in esotropia. Fixation-switch occurred for horizontal target locations that were approximately greater than halfway between the lines of sight of the foveating and strabismic eyes. Surprisingly, the border between right eye and left eye fixation zones was not sharply defined and there was a significant extent (>10°) over which the monkeys could acquire a target with either eye. Conclusions: We propose that spatial fixation patterns in strabismus can be accounted for in a decision framework wherein the oculomotor system has access to retinal error information from each eye and the brain chooses between them to prepare a saccade. For target locations approximately midway between the two foveae, strength of retinal error representations from each eye is almost equal, leading to trial-to-trial variability in choice of fixating eye

Motion-based nearest vector metric for reference frame selection in the perception of motion

We investigated how the visual system selects a reference frame for the perception of motion. Two concentric arcs underwent circular motion around the center of the display, where observers fixated. The outer (target) arc&apos;s angular velocity profile was modulated by a sine wave midflight whereas the inner (reference) arc moved at a constant angular speed. The task was to report whether the target reversed its direction of motion at any point during its motion. We investigated the effects of spatial and figural factors by systematically varying the radial and angular distances between the arcs, and their relative sizes. We found that the effectiveness of the reference frame decreases with increasing radial-and angular-distance measures. Drastic changes in the relative sizes of the arcs did not influence motion reversal thresholds, suggesting no influence of stimulus form on perceived motion. We also investigated the effect of common velocity by introducing velocity fluctuations to the reference arc as well. We found no effect of whether or not a reference frame has a constant motion. We examined several form-and motion-based metrics, which could potentially unify our findings. We found that a motion-based nearest vector metric can fully account for all the data reported here. These findings suggest that the selection of reference frames for motion processing does not result from a winner-take-all process, but instead, can be explained by a field whose strength decreases with the distance between the nearest motion vectors regardless of the form of the moving objects

Predictive model of biliocystic communication in liver hydatid cysts using classification and regression tree analysis

<p>Abstract</p> <p>Background</p> <p>Incidence of liver hydatid cyst (LHC) rupture ranged 15%-40% of all cases and most of them concern the bile duct tree. Patients with biliocystic communication (BCC) had specific clinic and therapeutic aspect. The purpose of this study was to determine witch patients with LHC may develop BCC using classification and regression tree (CART) analysis</p> <p>Methods</p> <p>A retrospective study of 672 patients with liver hydatid cyst treated at the surgery department "A" at Ibn Sina University Hospital, Rabat Morocco. Four-teen risk factors for BCC occurrence were entered into CART analysis to build an algorithm that can predict at the best way the occurrence of BCC.</p> <p>Results</p> <p><b>I</b>ncidence of BCC was 24.5%. Subgroups with high risk were patients with jaundice and thick pericyst risk at 73.2% and patients with thick pericyst, with no jaundice 36.5 years and younger with no past history of LHC risk at 40.5%. Our developed CART model has sensitivity at 39.6%, specificity at 93.3%, positive predictive value at 65.6%, a negative predictive value at 82.6% and accuracy of good classification at 80.1%. Discriminating ability of the model was good 82%.</p> <p>Conclusion</p> <p>we developed a simple classification tool to identify LHC patients with high risk BCC during a routine clinic visit (only on clinical history and examination followed by an ultrasonography). Predictive factors were based on pericyst aspect, jaundice, age, past history of liver hydatidosis and morphological Gharbi cyst aspect. We think that this classification can be useful with efficacy to direct patients at appropriated medical struct's.</p

Exploratory study of ultraviolet B (UVB) radiation and age of onset of bipolar disorder

Background: Sunlight contains ultraviolet B (UVB) radiation that triggers the production of vitamin D by skin. Vitamin D has widespread effects on brain function in both developing and adult brains. However, many people live at latitudes (about &gt; 40 N or S) that do not receive enough UVB in winter to produce vitamin D. This exploratory study investigated the association between the age of onset of bipolar I disorder and the threshold for UVB sufficient for vitamin D production in a large global sample. Methods: Data for 6972 patients with bipolar I disorder were obtained at 75 collection sites in 41 countries in both hemispheres. The best model to assess the relation between the threshold for UVB sufficient for vitamin D production and age of onset included 1 or more months below the threshold, family history of mood disorders, and birth cohort. All coefficients estimated at P ≤ 0.001. Results: The 6972 patients had an onset in 582 locations in 70 countries, with a mean age of onset of 25.6 years. Of the onset locations, 34.0% had at least 1 month below the threshold for UVB sufficient for vitamin D production. The age of onset at locations with 1 or more months of less than or equal to the threshold for UVB was 1.66 years younger. Conclusion: UVB and vitamin D may have an important influence on the development of bipolar disorder. Study limitations included a lack of data on patient vitamin D levels, lifestyles, or supplement use. More study of the impacts of UVB and vitamin D in bipolar disorder is needed to evaluate this supposition.</p