The
eyes are a window into the workings of the brain. Many studies show that eye
movements are closely linked to cognitive processes such as attention, memory,
and decision-making. Eye movements not only reflect
certain aspects of brain function, providing information
about our thoughts and desires to others, but might also influence and actively
participate in them.
The eyes also provide a glimpse into the aging
and diseased brain. We now know that changes in eye movements accompany the
changes that occur in the brain as a result of both normal aging and
neurodegeneration. Eye movements may therefore prove to be important indicators
of these processes, and there is growing interest in using them as a marker for
early signs of conditions such as Alzheimer's and Parkinson's diseases. Though
the research is still in the early stages, it suggests that eye-tracking
technology might eventually be useful in diagnosing such diseases.
Scanning
the field
There
are four basic types of eye movements.
Vestibulo-ocular movements are reflexes that stabilize our eyes relative to the
outside world to compensate for head movements; vergence movements
simultaneously rotate the eyes in opposite directions to align the fovea (the
most sensitive part of the retina) of each eye to a near or distant object; and
smooth pursuit movements are slow tracking moves that keep a moving object on
the fovea.
The fourth type, called saccades, seems to tell
us the most about brain function and dysfunction. Saccades are rapid eye
movements that quickly change our point of fixation. They can be initiated both
voluntarily and reflexively, and range in size from the
'microsaccades' that occur during reading to the
larger, sweeping movements our eyes make when we look around a room. Each of us
makes more than 100,000 saccadic eye movements per day, including those that
accompany our dreams during the REM stage of sleep. [See: Decoding the
Patterns in Sleep]
Age affects some eye movements but not others,
with saccades in particular changing significantly, getting smaller, less
frequent, and harder to initiate with increasing age.
Consequently,
the patterns of eye movements associated with processes such as recognizing
faces change, too--and this could provide a way of measuring age-related
cognitive decline.
We know that young adults are better than older
ones at recognizing faces, and researchers in China recently showed that better performance
is associated with a particular pattern of eye movements.
Eye movement analysis revealed that younger people tend to focus on individual
facial features, whereas older people are more likely to adopt a “holistic”
strategy to process the face as a whole. Importantly, task performance was
closely related to the strategy used: Older participants performed better the
more they used the strategy most often employed by the younger ones.
This apparent link between eye movement
pattern, face recognition performance, and cognitive status could be very
useful in the clinic: If an over-reliance on the holistic strategy during a
face recognition task is indicative of accelerated age-related cognitive
decline, then it could help identify those at risk of developing full-blown
dementia.
Neurodegenerative
diseases
Eye
movements are controlled by a diverse network of brain stem and cerebral
cortical structures that are susceptible to degenerative processes; eye movement
abnormalities therefore accompany the pathological changes underlying many
neurodegenerative diseases. Researchers have paid
particular attention to Parkinson's disease, which is typically thought of as a
movement disorder but also has cognitive symptoms. [See: Motivational
Disorders in Brain Conditions]
Parkinson's involves degeneration in specific
parts of the basal ganglia, a
set of sub-cortical structures involved in motor control and motivation. The
basal ganglia also act to control eye movements, and so eye movement
measurements serve as useful indicators of basal ganglia function and
integrity.
People with Parkinson's typically exhibit
abnormally slow hand and limb movements; their eye movements slow down
accordingly, and saccades
become fragmented, less accurate, and take longer to execute.
Various inherited forms of Parkinson's each have a unique profile of eye
movement abnormalities, so studying eye movements in these conditions could
help clinicians differentiate between them and add to our understanding of how
they differ in terms of basal ganglia dysfunction.
Eye movements abnormalities are seen in many
other neurodegenerative diseases, including Alzheimer's disease, motor meuron disease,
and Huntington's, and less-common conditions such as the spino-cerebellar
ataxias—but in most cases, the exact nature and extent of the abnormalities in
each has yet to be fully determined. Advances in eye-tracking technology will
undoubtedly bolster this research—possibly aiding both early diagnosis and
monitoring disease progression.
Picking
up the slack?
An
intriguing new study shows that eye movements can help to compensate for
age-related cognitive decline, providing another potential way in which
examination of eye movements could be of benefit in the clinic.
Researchers at the Rotman Research Institute in
Toronto recorded participants' eye movements during a visuospatial memory task,
and found that people of different ages showed different eye movement patterns
when recalling the information. Specifically, older participants were more
likely to reinstate the same pattern of eye
movements they performed when they first looked at the
information, and this enhanced their recollection of it.
This apparent ability to use compensatory
eye-movements may decline in certain groups of people. “There's likely differences in the extent to which people may use
compensatory eye movement strategies depending on their cognitive health,” says
senior author Jennifer Ryan, a professor of cognitive neuroscience at the University of
Toronto, “but this still needs to be tested.”
“We have been
investigating whether we can devise eye movement-based tasks for use in the
community or in clinical settings to screen for different disorders, including mild
cognitive impairment, Alzheimer's disease,” she says. “I think eye-tracking
screening tools will become an increasing possibility as these technologies
continue to advance.”