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Title

Μελέτη οφθαλμικών κινήσεων προσήλωσης

Author

Βρεττός, Ιωάννης

Thesis advisor

Πλαίνης, Σωτήρης

Reviewer

Παλλήκαρης, Ιωάννης
Παλλήκαρης, Αριστοφάνης

Abstract

PURPOSE: It is known that the static visual field responsible for detecting and perceiving visual stimuli of interest depends on the photoreceptor / ganglion cell distribution on the retina. However the ability of the eyes to move continuously and coordinately expands the way we understand our environment. Eye movements are distinguished in those responsible for the detection of points of interest and those responsible for fixating them. The aim of the present study was to investigate fixational eye movements while attenuating the optical parameters of a fixation spot. Specifically, the effect of contrast and target size in the precision and stability of fixation was studied. Furthermore stability of the fixational system were studied under monocular and binocular conditions. METHODOLOGY: Eye position was recorded using a video based eye tracker (Eyelink II, SR Research) with high spatial resolution (< 0.01ο) and high sampling rate (500 Hz). Nine volunteers were measured, aged 25 35 years, with the best spectacle-corrected vision for distance. The stimulus was a circle of constant luminance in its central half region increasing cosinusoidally in periphery. We used three different target sizes (1, 0.5 and 0.2 deg diameter). The target contrast changed between 100, 40, 10, 4 and 1% and the recording duration was 15 sec. Binocular and monocular recordings were performed, which allowed recording of both eyes. A system calibration was performed prior each recording so that the output results could refer to the exact position of the pupils centre on the display screen. The fixation stability was analysed with the calculation of the Bivariate Ellipse Contour Areas which contained 68% of fixation points on their scatter plots. The fixation ability was expressed in terms of visual areas ( in arcmins2) RESULTS: The fixation duration was found to influence significantly its stability as the elliptic areas of fixational points. Statistical significant differences were found between the first 5 sec and the first 10 sec and between first 5 sec and 15 sec of the recording. The effect of target size was found significant on the size of the elliptic areas. As the target increased in size the elliptic areas increased as well. The ellipse area when the diameter of the target was 1 deg was found smaller than the target area but this reversed in the case of the two smaller targets. Target contrast was also found to affect the fixation stability for values, with this effect being more pronounced for contrast levels < 10% while no statistically-significant difference was found above this. Finally a reduced ability for fixation was found when viewing the target monocularly : elliptic areas were always larger than those of the binocular recordings at all conditions with the effect being more important at lower contrasts (3-4 times increase in fixation area under monocular conditions). When studying the effect of eye dominance (binocular recordings) a reduced ability for fixation was found for the non dominant eye in comparison to the dominant eye. At the corresponding monocular recordings eye dominance did not play a role in fixational ability : the occluded eye showed in all cases higher fixational instability. CONCLUSIONS : It is possible that the increasing area of fixation in the time domain is attributable to retinal fatigue and eye dryness. Considering the effect of target size on fixation, it seems that decreasing size can increase fixational ability up to a level that no further improvement can be made. The effect of target contrast is obvious for values below 10% and this may be due to the sensitivity for this stimulus (~2%C). The target is well detectable for values above 10% and maybe this is the reason why no significant differences were observed. The fixation area was significantly decreased, in comparison to binocular fixation, when fixating monocularly, especially for contrast values below 10%. This could partly be due to the fact that the sensitivity of monocular vision is reduced in comparison with the binocular vision (detection threshold ~4%). However, the 4 times difference reveals that the increased instability when fixating monocularly may arise from the activation of different group of cortical neurons under the two conditions.

Language

Greek

Subject

Eye Movements

Fixation, Ocular

Visual Perception

Οπτική αντίληψη

Οφθαλμού κινήσεις

Προσήλωση, Οφθαλμική

Issue date

2006-03-22

Date available