Imagine a world where people can go into healthcare clinics, watch some video clips, and subsequently receive a diagnosis for neurodegenerative diseases, such as Parkinson’s disease (PD). As a matter of fact, research in eye tracking is aiming to do just that. The reason why eye movement behaviours might be so telling of underlying neurodegenerative diseases is because certain eye behaviours are controlled by the brain structures that are directly affected by the disease. When these brain structures are degenerated or damaged as a result of the disease, the control they have over eye movements can also be disrupted. Tracking eye movements has the potential to inform us about these underlying abnormalities. With regards to PD, eye tracking has the potential to help diagnose PD and even track disease progression.
In recent decades, countless studies have shown that people with PD display distinct behavioural patterns in different types of eye movements. One type of eye movement is called saccades, which are reflexive eye movements to a distant stimulus. Saccades can be further categorized into prosaccades, where subjects allow their eyes to follow their reflex, and antisaccades, where subjects inhibit their reflex and try to look in the opposite direction of the stimulus. Studies have shown that people with PD tend to make faster prosaccades and slower antisaccades (Chan et al., 2005; Antoniades et al., 2012; Cameron et al., 2012). This suggests that people with PD have reduced inhibition control when it comes to reflexive eye movements, which might manifest from pathologies well characterized in PD - in the basal ganglia (Chan et al., 2005; Cameron et al., 2012).
The basal ganglia also projects onto structures that control eye blinks and pupil modulation, such as pupil dilation and constriction in response to differences in illumination. Abnormal activities in the basal ganglia of PD patients might cause abnormal blinking behaviours and impaired pupil modulation. As a matter of fact, studies have shown that people with PD have slower spontaneous and voluntary eye blinks (Kimura et al., 2017; Maremmani et al., 2019) and weaker pupil modulation (Wang et al., 2016).
So where do we sit with this seemingly promising diagnostic tool in the PD world? In the US, a company called RightEye has developed an eye-tracking device aimed to identify PD in its earlier stages through eye tremors (Anon, 2020). In 2019, their device was rewarded with a breakthrough device designation by the FDA which allows them to fast track the development and commercialization of their devices. In our own community at Queen’s, we have an eye tracking lab lead by Douglas Munoz, the principal investigator. One of their research interests is PD and how people with PD display distinctive eye movement behaviours.
We have yet to see eye tracking implemented in clinical settings. While many studies have demonstrated distinct patterns of oculomotor behaviours in PD groups compared to age-matched controls, we have yet to identify a specific set of measurable ocular parameters that would distinguish between a person with PD and a person without. That’s not to say we should give up. For example, the RightEye device seems promising! The implications of having an accurate and precise PD diagnostic tool such as eye tracking would be profound. The additional advantages of eye tracking are that it is non-invasive and could be quite cost effective. So, while the current pandemic imposes greater limitations on research, we should still remain hopeful in the emerging utility of eye tracking as a diagnostic tool for PD.
Anon (2020) Eye-tracking device for early Parkinson’s detection. US Dep Veterans Aff.
Antoniades CA, Hu M, Kennard C (2012) IMPAIRED ANTISACCADES IN PARKINSON'S DISEASE. J Neurol Neurosurg &amp; Psychiatry 83:A10 LP-A10 Available at: http://jnnp.bmj.com/content/83/Suppl_2/A10.3.abstract.
Cameron IGM, Pari G, Alahyane N, Brien DC, Coe BC, Stroman PW, Munoz DP (2012) Impaired executive function signals in motor brain regions in Parkinson’s disease. Neuroimage 60:1156–1170 Available at: http://dx.doi.org/10.1016/j.neuroimage.2012.01.057.
Chan F, Armstrong IT, Pari G, Riopelle RJ, Munoz DP (2005) Deficits in saccadic eye-movement control in Parkinson’s disease. Neuropsychologia 43:784–796.
Kimura N, Watanabe A, Suzuki K, Toyoda H, Hakamata N, Fukuoka H, Washimi Y, Arahata Y, Takeda A, Kondo M, Mizuno T, Kinoshita S (2017) Measurement of spontaneous blinks in patients with Parkinson’s disease using a new high-speed blink analysis system. J Neurol Sci 380:200–204 Available at: http://www.sciencedirect.com/science/article/pii/S0022510X17304690.
Maremmani C, Monastero R, Orlandi G, Salvadori S, Pieroni A, Baschi R, Pecori A, Dolciotti C, Berchina G, Rovini E, Cuddemi F, Cavallo F (2019) Objective assessment of blinking and facial expressions in Parkinson’s disease using a vertical electro-oculogram and facial surface electromyography. Physiol Meas 40:65005.
Wang C-A, McInnis H, Brien DC, Pari G, Munoz DP (2016) Disruption of pupil size modulation correlates with voluntary motor preparation deficits in Parkinson’s disease. Neuropsychologia 80:176–184 Available at: http://www.sciencedirect.com/science/article/pii/S0028393215302347.