Eyes, Window to the Soul - and to Dopamine Levels?

The ancient proverb "the eyes are the window to the soul" may in some ways be validated by cognitive neuroscience. Pupil diameter is gaining currency as an index of mental effort ("cognitive workload") as well as arousal. In the most compelling finding from this literature, pupil diameter has been observed to increase with each successive item maintained in memory, up until each subject's working memory capacity - and then to contract incrementally as each item is reported back to the experimenter. Some recent work suggests that spontaneous eye blink rate - how quickly the eyes blink in normal, everyday situations - may also be an index of prefrontal or executive processes.

Blinking is a behavior that can be triggered voluntarily, reflexively (to protect from foreign objects) or "spontaneously." Spontaneous eye-blink rates can vary substantially between individuals (in adults, children, and nonhuman primates), and can be distinguished from reflexive and voluntary blinks both in terms of their shorter duration and possibly smaller amplitude. Spontaneous eye blink rate is measured over the course of several minutes (generally ranging from 1 to 5), and analyzed in terms of mean blink rate or inter-blink interval, the maximum inter-blink interval, and the modal eyeblink frequency. One unintersting determinant of spontaneous eye blink rate is the relative moisture or "tear film" of the eye.

However, a far more interesting determinant of spontaneous eye blink rate appears to be levels of cortical dopamine. If dopamine receptor 1 activity is enhanced (with D1 agonists), eye blink rate increases, and if it is blocked, eye blink rate decreases. Likewise, monkeys treated with MPTP (known to cause Parkinsonian type symptoms, a disorder associated with decreased dopaminergic activity) show reductions in eye blink rate, which is remediated by administration of D1 agonists (as are the Parkinsonian symptoms). This relationship is also robust in human populations, where people with schizophrenia show elevated blink rates and those with Parkinson's show the opposite trend. Some research implicates the rostral ventromedial caudate nucleus in the dopaminergic modulation of eye blink rate; however, eye blink rate may be modulated only by D1 and not D2 activity.

One hypothesized function of cortical dopamine is the "gating" of representations into prefrontal cortex. In other words, new items become updated into working memory by increases in dopamine activity, where they are maintained until another phasic increase in dopamine. In this way, dopamine levels are thought to maintain a balance between flexibility (being able to switch attention to new items) and stability (being able to ignore distracting items). This is sometimes called the flexibility/stability dilemma. There is also evidence that this "balance" of dopamine is arrived at through reward conditioning, such that stimuli associated with rewards tend to evoke larger increases in dopamine and thus are more likely to be attended.

One study of cognitive flexibility/stability supports the above claims for dopamine function as well as its relationship of eye-blink rate. Subjects with high spontaneous eye blink rates were less affected than those with low blink rates by switching to a task that required attention to a new aspect of the display. However, they were more affected than those with low blink rates by switching to a task that required attention to a previously ignored aspect of the display. In this way, eye-blink rate does appear to be a behaviorally-relevant marker of the relative balance between flexibility and stability in human adults, and by extension dopaminergic function. (Interestingly, the DRD4 and COMT genetic polymorphisms associated with differences in dopaminergic function were not correlated with these "switch costs," although they did interact with eye blink rate. In this case, blink rate is more sensitive than genetic analysis!)

Similarly, a study of retarded elderly adults shows that blink rate is reduced among those with highly stereotyped behavior (i.e., a lack of flexibility), suggesting that similar mechanisms govern spontaneous blinking across the lifespan. Studies with human children show elevated eye-blink rate among unmedicated children with ADHD, schizophrenia, epilepsy, as well as those with autism.

Relatively little work has been done with eye blink rate in infants, but it does not appear sensitive to feeding method, time of day, time since feeding, and body weight, changes in heart rate or body movement. In contrast, it is affected by social interaction or the presence of novel or moving stimuli. Individual variation among infants may be even greater than that among adults, mostly because blink rate among infants is surprisingly low. According to some reports, 2 month olds blink less than one time per minute, which steadily increases to an average of 14-17 blinks per minute by age 20.

Other studies have shown that blink rate is affected by cognitive workload (speaking, memorizing, and mental arithmetic are associated with blink rate increases, whereas daydreaming and object tracking decrease blink rate) as well as behavioral state (some report a higher blink rate during deception). Finally, blink rate seems stable within individuals across most of the day, only beginning to decrease after 8pm.

Related Posts:
Thinking about Thinking Harder: Pupil Dilation as an Index of Cognitive Workload

References:

Bacher L.F. & Smotherman, W.P. (2004). Spontaneous eye blinking in human infants: A review. Developmental Psychobiology, v44 issue 2 p95-102.

Dreisbach G, Muller J, Goschke T, Strobel A, Schulze K, Lesch KP, Brocke B. Dopamine and cognitive control: the influence of spontaneous eyeblink rate and dopamine gene polymorphisms on perseveration and distractibility. Behav Neurosci. 2005 Apr;119(2):483-90.

MacLean, W. E., Jr., Lewis, M. H., Bryson-Brockmann, W. A., Ellis, D. N., Arendt, R. E., & Baumeister, A. A. (1985). Blink rate and stereotyped behavior: Evidence for dopamine involvement? Biological Psychiatry, 20, 1321–1325.

Taylor JR, Elsworth JD, Lawrence MS, Sladek JR Jr, Roth RH, Redmond DE Jr. Spontaneous blink rates correlate with dopamine levels in the caudate nucleus of MPTP-treated monkeys. Exp Neurol. 1999 Jul;158(1):214-20.

Wallace, K. , Bacher, L. F., Norton, J. , Lewis, K. , Wynkoop, K. , Hubbard, L. and Zielinski, N. (2006, Jun) Spontaneous eye blinking: links to temperament and attention Paper presented at the annual meeting of the XVth Biennial International Conference on Infant Studies, Westin Miyako, Kyoto, Japan