Institutionen för klinisk neurovetenskap / Department of Clinical Neuroscience
Abstract
Saccades are rapid eye movements that change the orientation of the eyes
onto objects of interest. In an every day situation saccadic eye
movements are used for e.g. reading, scanning objects and enviromnents.
The neuronal control of the saccadic eye movement is complex and involves
many different parts of the brain. The saccade is the fastest of eye
movements and its velocity is not subject to volitional control. Velocity
measurements of saccades are therefore interesting for the detection and
characterization of diseases that affect the oculomotor system of the
brain and the extra ocular muscles.
Different techniques have over the years been developed for tracking eye
movements. Among the methods, the magnetic scleral search coil (MSC)
system has stood out as a "gold standard" for recording fast eye
movements, as the saccade. The device is sensitive and recording noise is
low. It also records at a high frequency level, which is particularly
important when recording fast eye movements. However, one of the
drawbacks of the method is that it involves wearing, on the surface of
the eyes, rather uncomfortable silicone lenses in which a copper wire is
embedded. There have been suggestions that the coil may slide on the eye
surface and hence introduce an unwanted filtering effect. Moreover, the
weight of the coil might affect the oculomotor signal elicited in the
brainstem.
For conducting saccadic eye motility research and high velocity
eye-tracking in the clinics, a non invasive recording system would be of
great value. The infrared reflection (M) method is able to record the
eye's position and velocity at a high frequency level, but without any
contact with the eyes. The system works through emitting infrared light
from illuminators built into goggles against the three-dimensional
surface of the eyes. The reflected light is detected and processed in a
computerized system into eye movement signals. The recent developments of
the IR system have not been evaluated previously for detailed saccadic
eye movement velocity recordings. In the presented studies healthy
subjects performed saccadic tasks with both the MSC and IR methods. The
data were analysed regarding the amplitude-velocity relationship, also
called the main sequence. The results showed that the IR method generated
saccadic velocities that were significantly higher compared to the MSC
method. Intra individual as well as inter individual variability of the
main sequence was shown with both methods, but there was generally larger
variability with the IR recordings.
The reasons for the IR method generating higher eye velocity data
compared to the MSC method may be explained by drawbacks that are related
to both methods. First, there is an inherent risk of filtering in the MCS
recordings if the silicon contact lens slides on the eye surface and the
coil changes the oculomotor signal. Second, artefacts that are associated
to the IR system may induce inaccurately high peak velocity measures. A
risk that can not be left out of account is that the IR system does not
merely detect eye movements, but also concurrent changes in the
three-dimensional reflecting surface of the eyes and eyelids. It is also
sensitive to changes in lighting conditions, which was shown in our
experiments. Moreover, the inferior resolution and linearity of the IR
system may introduce falsely high velocity data as well as a generally
larger variability.
The IR system may be used for rapid eye velocity analysis and has some
advantages compared to the MSC system. However, the velocity data differ
from those measured with the coil system. Intra individual and inter
individual variability should be considered, especially in follow-up
studies. Improving the calibration procedure, paradigms and recording
conditions might increase the scope for using the method for reliable
saccadic eye movement recordings, both in scientific and clinical
settings