International Symposium on Fixational Eye Movemements and Visual Stabilization

Themes and Aims

In Sweden and at Karolinska Institutet we have a long and proud tradition of vision research. The Sigvard and Marianne Bernadotte’s Research Laboratory for Pediatric Ophthalmology is the country’s most advanced laboratory for research on children’s eyes and vision. At the lab, we are ophthalmologists, opticians, orthoptists, and engineers. Most of the research is clinical and involves eye tracking. The aim with our studies is to increase knowledge and invent new methods that can be used to learn about development aspects of visual functions and eye movements in both healthy and diseased subjects. In recent years, we have recognized that fixational eye movements and visual stabilization is a promising and important research area that we wish to keep a focus on in the future.

Visual Stabilisation

A common proverb is that “the eye is the mirror of the soul”. The saying makes a lot of sense given that the eye and its movements actually reflect the activity of the brain. The light sensitive neural tissue that covers the back of the eye, the retina, can even be said to be a specialized part of the brain itself. Over 90 percent of the information that reaches the brain does so via the optical nerve. Since the eyes are quite easy to observe and their movements may tell us about how the brain works, they are the object of an intensive research interest. Eye movement studies have informed us on the physiological properties of vision, they have enabled us to diagnose differences between diseased and healthy, and they may tell us much more about the complicated processes by which we interpret the world around us.

Keeping the gaze still is central for our ability to assimilate detailed visual impressions. It is only in a very small area of the retina, the fovea, that we have a sufficient concentration of the type of light sensitive cells that enable us to see with high acuity and discern colours. In order to forward a sharp image via the optic nerve, the incoming light must be focused in the fovea and remain stabilized there for a brief moment. Around the eye there are six muscles whose task it is to both stabilize the gaze, during what we call fixations, and move it around in very fast movements called saccades. The muscles also enable us to follow moving targets and keep the gaze on a target while we ourselves are moving. In order for us to perceive depth, the eyes must synchronize these movements in tandem.

Eye movements during fixations were noted already in the first eye movement studies performed at the turn of the 18th century. Today, researchers agree on three types of fixational eye movements: tremor, drifts, and microsaccades. The size of these movements should make them visible to us, but we are unaware of them. If they are counteracted, our visual perception fades to nothing due to neural adaptation. This results in a paradox – to make out details in the world around us we must keep the gaze as stable as possible, but if we kept it perfectly stable, the world would fade from view. Since fixational eye movements are involuntary and closely related to how the brain makes our environment visible, they are interesting to study as we may learn more about the very foundation of visual awareness.

Fixational Eye Movements

Tremor, the smallest of the movements, can be described as a regular and high frequent trembling of the eye. Exactly which functions tremor fulfils in the visual system is poorly understood. The movement is so small that it fits within the radius of a single cell. One hypothesis is that the trembling is a result of the electrical activity in the brain stem. If nothing else, this says something about the sensitivity of the system. In collaboration with the Royal Institutet of Technology and Mälardalen University, we are currently developing the first Swedish system for tremor measurement. Our aim is to have the prototype finished in time for the symposium so that we will be able to discuss findings with those invited guests that have experience of working with these kinds of measurements.

Drifts occur simultaneously with tremor and can be described as a slow sliding movement across a dozen or so cells. Apart from being easier to measure, they have generated more interest since it is obvious the movement stimulates different cells. If drifts are inhibited by locking the stimuli to a specific position on the retina, the neurons will adapt to the light and the image fades away. The cells require shifts in the light to fire signals which can be forwarded through the optic nerve. How drifts affect visual performance, how they are affected by different stimuli, and if there are individual differences between persons are some of the questions that remain unanswered. To the symposium we have invited several speakers that have both complimentary and contradictory views on the role of drifts.

Microsaccades are distinct and directional movements. They span approximately the same distances as drifts but are performed very quickly. The function of microsaccades is probably to correct for the drift and keep the gaze focused in the fovea. The movements seem to enable the visual system to prevent neural adaptation without straying to far from the centre. Microsaccades share many properties with voluntary saccades, but we still do not know how they work binocularly or if known deficits in saccadic system also show up in the microsaccades. To the symposium we have invited many of those who made the groundbreaking discoveries regarding tremor, drifts and microsaccades. We look forward to rewarding discussions about their role in the visual system.

Aims of the Symposium

The aim of the symposium is to bring together the international expertise on fixational eye movements and visual stabilization. Apart from covering fixational eye movements and visual stabilization in general, the symposium will focus on developmental and clinical aspects. If we were able to detect deficits in children’s fixational eye movements, it might be possible to devise treatments that could make use of the child’s neuroplasticity in order to enhance their vision. The clinical aspects concern how we may utilize new measurement techniques and research findings in order to treat patients that experience problems with visual stabilization. Most of the invited speakers have extensive experience of using eye tracking in clinical environments and several have a special interest in children and infants.

The symposium will be part of Karolinska Institute’s bicentennial celebration as well as the celebration of the 20th anniversary of the Sigvard and Marianne Bernadotte Research Foundation for Children’s Eye care. We strive for the symposium to become a scientifically stimulating environment for lively discussions and future collaborations!