Eye dominance in binocular viewing conditions.

Different degrees of eye dominance may need to be considered when comparing monocular and binocular performance or estimating binocular summation effects. The purpose of this study was to explore eye dominance under binocular viewing conditions and observe gradual differences in preference. Two binocular eye-dominance tests were evaluated and compared to the hole-in-the-card sighting test. The first experiment was based on a binocular sighting test. The second originated from the variable-angle mirror test, utilizing physiological diplopia to determine which eye dominates the binocular percept. The participants were 32 healthy subject with normal sight. For both experiments there were plausible effects of different degrees of eye dominance affecting how the subjects positioned or perceived the scene. The outcomes were supported by a statistically significant correlation with an interocular difference in sensitivity to degraded visibility. A statistically significant correlation was found between the hole-in-the-card test and the variable-angle mirror test (r = 0.527, p < 0.01). The experiments confirm the plausible occurrence of forms of eye dominance under binocular viewing conditions. The correlation with the hole-in-the-card test was weak to moderate, due to factors that appear to be related to testing and viewing conditions. Interocular differences in sensitivity to blur appear to be a useful factor in further grading of eye dominance.

Slow fluctuations in eye position and resting-state functional magnetic resonance imaging brain activity during visual fixation.

The neuronal circuitry that supports voluntary changes in eye position in tasks that require attention-driven oculo-motor control is well known. However, less is known about the neuronal basis for eye control during visual fixation. This, together with the fact that visual fixation is one of the most commonly used baseline conditions in resting-state functional magnetic resonance imaging (fMRI) studies, prompted us to conduct a study in which we employed resting-state fMRI and concurrent recordings of eye gaze to investigate the relationship between spontaneous changes in eye position during passive visual fixation and intrinsic brain activity. As a control experiment, we recorded fMRI brain activity related to cued voluntary vertical and horizontal changes in eye position in a block-related task-evoked fMRI experiment. Our results for the voluntarily performed changes in eye position elicited brain activity in the bilateral occipitotemporal cortex, supplementary motor cortex and frontal eye fields. In contrast, we show that slow fluctuations in eye position during passive visual fixation are linked to intrinsic brain activity, foremost in midline cortical brain regions located in the posteromedial parietal cortex and the medial prefrontal cortex, brain regions that act as core cortical hubs in the brain's default mode network. Our results suggest that subconscious and sustained changes in behavior are tied to intrinsic brain activity on a moment-by-moment basis.

The effect of contrast on monocular versus binocular reading performance.

The binocular advantage in reading performance is typically small. On the other hand research shows binocular reading to be remarkably robust to degraded stimulus properties. We hypothesized that this robustness may stem from an increasing binocular contribution. The main objective was to compare monocular and binocular performance at different stimulus contrasts and assess the level of binocular superiority. A secondary objective was to assess any asymmetry in performance related to ocular dominance. In a balanced repeated measures experiment 18 subjects read texts at three levels of contrast monocularly and binocularly while their eye movements were recorded. The binocular advantage increased with reduced contrast producing a 7% slower monocular reading at 40% contrast, 9% slower at 20% contrast, and 21% slower at 10% contrast. A statistically significant interaction effect was found in fixation duration displaying a more adverse effect in the monocular condition at lowest contrast. No significant effects of ocular dominance were observed. The outcome suggests that binocularity contributes increasingly to reading performance as stimulus contrast decreases. The strongest difference between monocular and binocular performance was due to fixation duration. The findings may pose a clinical point that it may be necessary to consider tests at different contrast levels when estimating reading performance.

Reading Alterations in Parkinson's Disease Indicate Worse Cognitive Status.

Background: Reading difficulties are commonly reported in Parkinson's disease (PD). So far, only a few studies have assessed reading in PD, most of them confirming a different pattern in patients compared with healthy populations. Impaired oculomotor control is an early feature of PD. Cognitive deficits, on the other hand, may appear early, but they are most prominent at later stages. Although these two factors are thought to be responsible for the alterations in reading performance, it is unclear how each factor contributes to them. Objectives: To evaluate eye movements during reading in PD and healthy controls (HCs). Methods: Data from 42 HCs (36% men) and 48 patients with PD (67% men) at Hoehn and Yahr stages ≤3 were analyzed. PD participants were further divided into 2 groups based on their Montreal Cognitive Assessment (MoCA) score using a cutoff of ≥26. Eye movements were recorded with Tobii Pro Spectrum, a screen-based eye tracker with a sampling rate of 1200 Hz. Results: PD participants performed fewer fixations per second (P = 0.033), with a longer mean (P = 0.037) and standard deviation fixation duration (P = 0.033) than HC, and further analysis showed that only patients with a lower MoCA score performed worse than HCs. Reading parameters were weakly associated with MoCA scores, irrespective of age and education. Conclusion: Changes in the reading pattern of PD patients are probably attributed to cognitive rather than pure oculomotor alterations.

Fixation Duration and Pupil Size as Diagnostic Tools in Parkinson's Disease.

BACKGROUND: Visual and oculomotor problems are very common in Parkinson's disease (PD) and by using eye-tracking such problems could be characterized in more detail. However, eye-tracking is not part of the routine clinical investigation of parkinsonism. OBJECTIVE: To evaluate gaze stability and pupil size in stable light conditions, as well as eye movements during sustained fixation in a population of PD patients and healthy controls (HC). METHODS: In total, 50 PD patients (66% males) with unilateral to mild-to-moderate disease (Hoehn & Yahr 1-3, Schwab and England 70-90%) and 43 HC (37% males) were included in the study. Eye movements were recorded with Tobii Pro Spectrum, a screen-based eye tracker with a sampling rate of 1200 Hz. Logistic regression analysis was applied to investigate the strength of association of eye-movement measures with diagnosis. RESULTS: Median pupil size (OR 0.811; 95% CI 0.666-0.987; p = 0.037) and longest fixation period (OR 0.798; 95% CI 0.691-0.921; p = 0.002), were the eye-movement parameters that were independently associated with diagnosis, after adjustment for sex (OR 4.35; 95% CI 1.516-12.483; p = 0.006) and visuospatial/executive score in Montreal Cognitive Assessment (OR 0.422; 95% CI 0.233-0.764; p = 0.004). The area under the ROC curve was determined to 0.817; 95% (CI) 0.732-0.901. CONCLUSION: Eye-tracking based measurements of gaze fixation and pupil reaction may be useful biomarkers of PD diagnosis. However, larger studies of eye-tracking parameters integrated into the screening of patients with suspected PD are necessary, to further investigate and confirm their diagnostic value.

Monocular and binocular reading performance in subjects with normal binocular vision.

BACKGROUND: It is well known that problems with binocular vision can cause issues for reading; less known is to what extent binocular vision improves reading performance. The purpose of this study was to explore the role of binocularity by directly comparing monocular and binocular reading in subjects with typical reading skills and normal binocular vision. A secondary purpose was to assess any asymmetry in monocular performance and its association with the sighting dominant eye. METHODS: In a balanced repeated measures experiment, 18 subjects read paragraphs of text under monocular and binocular conditions. All subjects went through an optometric examination before inclusion. Reading speed and eye movements were recorded with an eye tracker. RESULTS: The mean difference in reading speed (2.1 per cent) between monocular (dominant and non-dominant eye averaged) and binocular reading speed was not significant. A significant difference in reading speed was found between binocular and the non-dominant eye, as determined by the far sighting test (p = 0.03). Monocular reading showed significantly increased (8.9 per cent) fixation duration (p < 0.01) and longer regressive saccades by 0.43 character spaces (p < 0.01). Reading with the non-dominant eye, as determined by the near sighting test, showed increased progressive saccade length by 0.2 characters compared to the dominant eye (p = 0.03). No other significant differences between dominant and non-dominant eyes were found. The agreement between the faster reading eye and ocular dominance was 44 to 56 per cent depending on whether dominance was determined at near or far. CONCLUSION: The outcomes suggest that in subjects with normal binocular vision, there is no marked enhancement in reading performance by binocular vision when reading paragraphs of text. Furthermore, the monocular reading performance appears to be close to equal and any small differences in performance appear not to be strongly associated with ocular dominance.