Eye movement | EPFL Graph Search

Eye movement includes the voluntary or involuntary movement of the eyes. Eye movements are used by a number of organisms (e.g. primates, rodents, flies, birds, fish, cats, crabs, octopus) to fixate, inspect and track visual objects of interests. A special type of eye movement, rapid eye movement, occurs during REM sleep. The eyes are the visual organs of the human body, and move using a system of six muscles. The retina, a specialised type of tissue containing photoreceptors, senses light. These specialised cells convert light into electrochemical signals. These signals travel along the optic nerve fibers to the brain, where they are interpreted as vision in the visual cortex. Primates and many other vertebrates use three types of voluntary eye movement to track objects of interest: smooth pursuit, vergence shifts and saccades. These types of movements appear to be initiated by a small cortical region in the brain's frontal lobe. This is corroborated by removal of the frontal lobe.

Optische Augenlängenmessung für klinische Anwendungen

Beate Möller

Cataract is the leading cause of blindness worldwide. The clouding of the human eye lens is called "Grauer Star" in German. In the industrialized countries this process of increased blindness is corrected by the surgical removal of the clouded lens and its replacement by a plastic lens (intraocular lens, IOL). Cataract surgery is the most frequent surgery worldwide. In Europe more than two million operations are performed annually. For the selection of an optimum replacement the refractive power of the lens is calculated from the radius of curvature of the cornea, the length of the eye and the anterior chamber depth. Therefore these data have to be measured before every surgery. Until now the length of the eye was measured exclusively by ultrasound in the clinical praxis. For this procedure the sound transducer has to be placed directly or via a water funnel onto the cornea. It was the aim of the work presented in this thesis to lay the framework for introducing into the clinical praxis a contact free optical length measurement of the eye. The basic parameters for the measurement were determined. Starting from the state of the art new solutions were found, implemented in a laboratory device, and tested clinically. A two-beam interferometer with light of short coherence length that had been developed at the University Vienna was analyzed and compared with the one-beam technique. Neglecting practical limitations due to the optical arrangement and assuming shot-noise limited detection the one-beam technique allows the measurement of reflected light that is a factor of two lower compared to the two-beam technique. From the point of view of detection both techniques are therefore equivalent. The advantage of the two-beam technique is its independence from the eye movement. Until now the two-beam technique utilizes for detection only 1 over 400 of the light reflected from the retina and 1 over 2500 to 1 over 4000 of the light reflected from the cornea. This causes the dynamic range to drop by 26 dB. Furthermore the noise of the electronics (excess noise) comes into play. In this work a novel concept for the optical configuration in the two-beam interferometer is presented. By using a diffractive lens cornea and retina are imaged simultaneously onto the detector and the losses of the light from the retina are minimized. The cornea is imaged with an efficiency of 5.6 per cent. In order to minimize the requirements on the patient the measuring time was shortened to 1 s. Therefore the scan speed was increased by a factor of four compared to the state of the art. In the laboratory device it fluctuated by 20 per cent. To keep the detection narrow band it was necessary to implement a signal processing that is largely independent of the scan speed. By using a synchronous demodulation of two 90 degrees phase-shifted channels that are added as vector quantities a filter with self-adjusting center frequency was created. Its bandwidth is 4 kHz with signal frequencies between 30 kHz and 90 kHz. This concept was realized as a compact slit-lamp head and tested clinically. A shot-noise limited detection with a dynamic range of 105 dB was achieved (input power 200 µW). On the healthy eye the signal-to-noise ratio was limited by the digitization range of 8 bit. On the eye with cataract the measuring was not limited by the degree of clouding any more. The optical lengths were determined along the line of sight. They agreed with the post operative results. In some cases refractive errors of more than 2 dpt would have been avoidable by determining the IOL from the measurement of the optical length of the eye. This technique allows a comfortable, contact free optical eye length measurement that works correctly in 95 per cent of all eyes with cataract. Additionally smaller post operative refractive errors can be achieved as compared with traditional techniques. These results were the basis for the development of the IOL-Master – a novel device for the cataract treatment – at the Carl Zeiss Jena GmbH. With this device all biometric data can be measured in a non-contact fashion and intraocular lenses can be calculated and selected.

EPFL2001

Exploiting Recognition of Natural Visual Stimuli for Brain Computer Interfacing

Marija Uscumlic

Visual search is a good illustration about how the brain coordinates a variety of functions such as visual clues extraction from the scene, coordination of the eye-movements, accumu- lation of visual information and visual recognition. It has an important role in many daily life activities of people – the search of a familiar face in a crowd, a book in the library, or a building in a street. In all these scenarios, it represents a recognition of a specific target object in the presence of predominant task-irrelevant visual information. This thesis specifically addresses the visual search in context of brain-computer interaction (BCI) where the accent is on visually rich stimulation, approaching the real-world BCI applications. Different cognitive aspects of visual processing have been extensively studied using the Rapid Serial Visual Presentation (RSVP) protocol – a sequentially presentation of visual stimuli at the same location on a display. The main part of this thesis deals with the visual search under the RSVP protocol. In such a protocol, the electroencephalogram (EEG) reflects whether a subject recognize an image as a target. Our focus is on extracting and exploiting this informa- tion for the BCI application of image retrieval. We designed a framework for EEG-based image search which involves coupling of the EEG decoder with the computer vision technique for image retrieval. These two components of the system are seen as complementary – bringing in the semantic information and large scale computing, respectively. Our design is founded on an iterative coupling, which relaxes a performance requirement of the individual components. We report the results of the experimental evaluation, where natural color images were used as stimuli. This thesis also includes a comprehensive analysis of the EEG correlates of visual recogni- tion in the EEG-based image search task, addressing the effects of the less controllable stimuli to the EEG potentials’ attributes. To this end, our focus is on the spatio-temporal aspects of the EEG potentials, along with the modulation of the EEG-based cortical connectivity estimates. Finally, this thesis presents an exploratory research on active visual search in the context of BCI. An essential part of active searching of the visual scene represents eye movements. Hence, this type of task better reflects our natural behavior and as such extends the possibilities of BCI applications. We designed two experimental protocols to study the active visual search – guided and free-view visual search. We report the results of the combined study of eye tracking data and simultaneously recorded EEG.

EPFL2013