Clinical investigations of fixation stability and reading eye movements

Reading and writing plays a fundamental role in our culture. Compared with, e.g. speech, written language has an immense impact as it offers the possibility to share information over unprecedented distance in time and space. By observing how the eyes move while reading we can obtain knowledge of the recognition process. To enable reading, not only moving the eyes are required, but also a good quality of fixation is essential. This thesis presents findings from four eye movement studies performed with the Scanning Laser Ophthalmoscope (SLO) and Tobii Eye-tracker.

The aims were to 1) investigate the fixational pattern in healthy subjects using the microperimetry technique obtained with the SLO in order to learn more about normal fixational pattern. This is important since changes in fixational pattern, due to pathology, can occur prior to detectable changes in the macula or visual pathway; 2) use the SLO to map solar induced scotomas and evaluate the fixational pattern after such injury; 3) evaluate if the Tobii system could be used to evaluate reading performance despite its relatively low resolution and sampling frequency; and 4) evaluate if patients with neovascular AMD, treated with Lucentis, gain a better reading ability by using the Tobii Eye tracker.

The results from the SLO studies demonstrated that the fixational pattern in healthy subjects measured with the SLO had a mean centre of gravity located at a mean absolute distance of 0.27º from the fixation point (FP) and a directional predominance of the fixational pattern more frequently distributed vertically than horizontally. This means that computerized fixation control when performing microperimetry with the SLO provides information about the fixation pattern, which cannot be obtained with standard clinical perimetry techniques. The studies further showed that SLO microperimetry session with fixational mapping are useful in patients with unstable and/or extrafoveal fixation, which is a common situation in patients with a foveal lesion/injury.

The Tobii Eye tracker studies showed that the Tobii system, despite its relative low resolution and sampling frequency, is suitable for evaluation of reading performance in clinical settings. Furthermore, that texts used for studying reading performance must be chosen carefully since even texts of similar linguistic difficulty, due to the nature of the text content, can yield the differences found in reading performance. When investigating AMD patients, the findings outlined that Lucentis treatment gave a significant better visual acuity, not necessarily connected with improved reading eye movements and reading ability. However, reading should be tested in order to fully understand a patient's complaints; however, as a direct measure of visual improvement after, for example Lucentis treatment, other tests should also be used.

List of scientific papers

I. Källmark FP, Ygge J (2008). Fixation pattern in healthy subjects during microperimetry with the scanning laser ophthalmoscope. Med Sci Monit. 14(6): CR311-315
https://pubmed.ncbi.nlm.nih.gov/18509274

II. Källmark FP, Ygge J (2005). Photo-induced foveal injury after viewing a solar eclipse. Acta Ophthalmol Scand. 83(5): 586-9
https://pubmed.ncbi.nlm.nih.gov/16187997

III. Källmark F, Öqvist-Seimyr G, Brautaset R (2008). Evaluation of reading performance in clinical settings using the Tobii 1750 eye tracker. [Submitted]

IV. Källmark F, Öqvist-Seimyr G, Kvanta A, Brautaset R (2009). Evaluation of reading eye movements before and after treatments with ranibizumab (Lucentis). [Manuscript]