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.”