Blinks - a hidden gem in eye tracking research

As researchers increasingly use eye tracking to gain insights into human behavior and cognition, eye blink metrics emerge as a powerful yet often overlooked measurement in eye tracking experiments. Incorporating blink metrics into research studies offers an easy, non-invasive way to gauge cognitive and physiological states, from engagement and mental fatigue to dopaminergic system activity in the brain.

Blinks, the temporary closing and reopening of the eyelids, can be easily measured using eye trackers. Incorporating blink metrics into eye tracking studies provides an additional layer of information beyond what can be gleaned from saccade, fixation, and pupil measurements. This article explores why blink metrics deserve a place in scientific and consumer research, and highlights their value in advancing our understanding of human behavior and cognition. While there are various types of eye blinks, in this learn article, we refer specifically to spontaneous eye blinks when talking about eye blinks.

Markers of engagement and memory

Engagement—focused and sustained attention that involves engrossment in an activity—is critical for successful learning, skill development, and task execution. Blink rate fluctuates as a function of viewer engagement, with blinking rates reducing or even temporarily inhibiting when the viewer perceives a stimulus as engaging and subjectively important¹ ². Interestingly, blink rate can measure viewer engagement unfolding in as little as 60 seconds³. Changes in blink rate can also predict how well a person might remember something they engaged with or viewed. For instance, moments during a movie when viewers blinked less frequently corresponded to the moments they remembered best—not only immediately after the movie but even four weeks later⁴.

The link between blinks and engagement has significant implications across various fundamental and applied research fields. A viewer’s subjective perception of a stimulus’s importance reflected in reduced or inhibited blinks, allows researchers to use blink metrics as a non-intrusive, real-time measure of how captivating the task or stimulus is. In applied fields, it can inform the development of adaptive learning systems, improve user experience in digital media, and refine neuromarketing strategies by identifying engaging content. Blink metrics can complement eye tracking data and reduce reliance on self-reported measures. Additionally, the connection between blinking frequency and memory could help identify more captivating and memorable stimuli, leading to more effective long-term impacts. 

Assessing mental fatigue and vigilance

Mental fatigue, often associated with diminished vigilance, greater distractibility, and reduced attention due to prolonged mental effort, can be reliably detected by monitoring blink rate dynamics⁵. As mentioned in the previous section, a reduced blink rate occurs in response to heightened engagement and interest. However, depending on the context, a reduced blink rate and inhibition can occur when engaging in cognitively taxing activities. As time spent on a task progresses, a subsequent increase in blink rate typically signals the onset of mental fatigue⁶.

Detecting and managing mental fatigue promptly is essential in environments where continuous human attention is critical for safety and efficiency. Diminished alertness and vigilance due to mental fatigue can lead to serious risks in fields like air traffic control, military operations, healthcare monitoring, and long-haul driving. Monitoring blink rates during extended tasks provides valuable insights into when fatigue might impair performance or compromise safety. This information can guide the scheduling of rest breaks and the optimization of workflows to reduce mental exhaustion effectively.  

Detecting drowsiness

Closely related to the use of blink rate for indicating mental fatigue, eye blinks are also reliable indicators of drowsiness—a transitional state between awake and asleep⁷. While mental fatigue and drowsiness are interconnected, drowsiness is primarily linked to sleepiness and the body’s need for rest, whereas mental fatigue stems from cognitive exhaustion due to prolonged mental efforts, even without physical tiredness⁸. A decrease in the frequency of eye blinks or an increase in the duration of eye closure is a key indicator of driver drowsiness⁹ ¹⁰.

Drowsiness detection based on blink dynamics has significant implications for research and industry. In situations where maintaining alertness is critical, such as in high-risk occupations or during extended driving periods, monitoring blink metrics could serve as an early warning system for drowsiness. Blink dynamic measurements enable the development of interventions or tools to prevent drowsiness-related errors. For example, companies focused on driver safety technologies or workplace productivity solutions could integrate blink metrics into real-time feedback systems, helping individuals remain alert and safe. Additionally, eye tracking technology can detect microsleeps by identifying changes in blink patterns, enhancing safety protocols by signaling when timely intervention is necessary. 

Blinks are the gateway to the brain’s dopamine system  

Dopamine, a central nervous system neuromodulator, is essential for motivation, reward pursuit, and movement control¹¹. Although it supports numerous critical behavioral and cognitive functions, it is challenging to measure dopamine activity non-invasively in the human brain. Spontaneous blink rate, linked to brain dopamine levels, is a promising non-invasive biomarker of dopaminergic activity¹².  

Spontaneous blink rate is a valuable predictor of cognitive performance, showing an inverted U-shaped relationship: optimal performance correlates with medium blink rates in various cognitive domains such as inhibitory control, working memory, and creative thinking¹³ ¹⁴ ¹⁵ ¹⁶. This pattern mirrors the effects of dopamine levels on cognition, where both low and high levels can impair performance¹⁶. The predictive value of blink rate for optimal cognitive performance offers significant implications for personalized educational interventions, enhancing workplace productivity, and tailoring cognitive training programs. 

Variations in blink rates can reflect changes in dopamine levels in the brain, providing insights into conditions with disrupted physiological dopamine levels. A reduced blink rate is observed in hypo-dopaminergic conditions such as Parkinson’s disease, ADHD, and substance use disorders¹⁷ ¹⁸ ¹⁹ ²⁰, while an increased rate is documented in hyper-dopaminergic conditions like schizophrenia and autism ²¹ ²². As such, blink metrics can serve as non-invasive markers of underlying neural mechanisms and offer insights into diseases without costly neuroimaging. These accessible metrics could aid pharmaceutical companies in evaluating medication effects on dopamine pathways. With the rise of digital health technologies, integrating various biomarkers for early disease detection, eye tracking, and blink metrics offers meaningful, non-invasive inputs for continuous monitoring. 

Incorporating blinks in eye tracking experiments 

Adding blink measurement to research studies is relatively straightforward, as eye tracking devices have become increasingly accurate, affordable, and widely available. Tobii screen-based eye trackers detect blinks using eye openness data (eyelid tracking), unlike traditional methods based on data loss, which only indicate the presence, absence, or duration of blinks. Blinks detected from the eye openness signal allow to compute more advanced blink concepts that characterize blink dynamics such as amplitude, duration, and velocity.   

Tobii Pro Lab research software uses an independently developed and validated algorithm²³ to detect blinks. Tobii Pro Lab’s user-friendly interface makes various blink metrics easily accessible without requiring prior expertise in blink measurements. This new algorithm allows users to leverage blink metrics in ways previously unavailable in any commercial tool. 

For a deeper dive into this topic, watch our webinar
"Catch every blink: new blink detection method with eye tracking," where Dr. Marcus Nyström from Lund University presents the latest advancements in blink detection using eyelid tracking. 

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