The rapid movements our eyes make every day could help scientists better understand a rare and complex brain disease.
Researchers from UQ’s Queensland Brain Institute (QBI) have used high-speed eye tracking to challenge a long-held belief about progressive supranuclear palsy, or PSP.
PSP is a degenerative brain condition that can affect balance, movement, behaviour, speech, thinking and eye control. People may develop stiffness in the neck and body, symptoms resembling Parkinson’s disease, and early, frequent falls.
Others first experience language difficulties known as primary progressive aphasia.
QBI’s Professor Peter Nestor said some patients initially attend a cognitive clinic because of changes in behaviour or because their speech and language are deteriorating, only to develop clearer signs of PSP one or two years later.
“One of the defining features of PSP is this eye movement abnormality,” Professor Nestor said.
Doctors have traditionally believed that PSP affects vertical eye movements (looking up and down), more severely than horizontal movements (looking from side to side).
The researchers originally hoped that measuring vertical movements might help them identify PSP before its other symptoms became obvious. However, their findings suggested that the accepted explanation was not as straightforward as previously thought.
The study examined rapid eye movements called saccades. These movements happen whenever we quickly shift our gaze, such as looking from a computer screen to someone’s face.
Participants looked at targets appearing vertically and horizontally on a screen while an eye-tracking machine recorded the position of their eyes 1,000 times each second.
This allowed the researchers to measure not only how fast the eyes moved, but also how smooth and direct each movement was.
Lead author on the paper, researcher Dr Duy Duan (John) Nguyen, created three-dimensional plots showing the paths taken by the eyes. In people with PSP, movements could appear curved, interrupted or “wobbly”.
“The horizontal is very smooth and well aligned, while the vertical is curved,” Dr Nguyen said of eye movements in the healthy population.
“In PSP, they both declined. The horizontal also appeared to be wobbly.”
The team developed new methods for measuring this wobbliness, including how often an eye changed direction and the angle of each change.
The results showed that PSP did not damage only vertical eye movements. Instead, movements in both directions became slower and less stable.
Vertical difficulties may simply be noticed first because healthy people are naturally less skilled at looking rapidly up and down than from side to side. When PSP affects the entire eye-movement system, vertical movements cross the “noticeable” threshold earlier.
The discovery could also change where scientists look for the origins of the disease.
Professor Nestor compared it to finding that a car’s front light is not working.
“If one only looks at the front light, one could believe there was a problem specific to that light bulb,” he said.
“But if one were to step back and notice that none of the other lights were working either, then that puts the problem in a different part of the electrical system.
“This is essentially what was happening in PSP; because the vertical movement abnormality was easier to spot, people focused just on that.
“By finding that it is a general problem of saccadic eye movements, it means the degeneration must be in a different part of the brain altogether from what had been previously thought.
“If we can work out which bits of the brain are most vulnerable to degeneration, we can look at what makes those cells die.”
Eye tracking may eventually provide researchers with an objective way to measure whether future PSP treatments are working, even when patients have very different symptoms.
“This is much more about discovering what is going wrong in this disease,” Professor Nestor said.
This paper was published in Annals of Clinical and Translational Neurology.
