12/11/2006

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

6 Comments:

Blogger Steve said...

hey chris... check out some of dave irwin's research on cognitive suppression during saccades... good stuff.

12/11/2006 11:58:00 AM  
Blogger MoonShadow said...

hmm....this may just be a fluke...but i noticed this while i was playing a game....

in this game, you can set the speed that you play it. i find that i had difficulty adjusting to higher speeds of gameplay. But, when adjusting, i seemed to blink and it would help me somehow readjust my processing speed to fit suit the speeds that the game was going at.

12/11/2006 12:10:00 PM  
Blogger Chris Chatham said...

thanks guys! i'll check out irwin's stuff. glad you enjoyed the post.

12/11/2006 03:00:00 PM  
Blogger Lantern Bearer said...

One of the first Neuro-liguistic techniques I ever used in learning situations was watching for eye blinks and and allowing for them to be the guide for any instructional process. I have used it in more recent work with dyslexic children coupled with the dePaul Techniques. I rarely have to ask a student if they understand the work we are doing. I keep adjusting and improvising until a good blink pattern sets up along with other subtle cues.

I came across your piece through reddit.

Lantern Bearer

12/11/2006 05:49:00 PM  
Anonymous www.Joansartwork@aol.com said...

thanks chris... found your blog & Resarch about " # of blinks = dopamine levels perhaps indicative of conditions" today web-surfing.
Mid-20th-Century Aldous Huxley wrote of blinking exercises... stated the need of blinking deliberately after staring to improve sight AND mood in "The Art Of Seeing" (his tribute to "The Bates Method" of eyesight exercises which restored his sight to near-function from near-blindness)...
I'm an artist & lately searching for some natural ways to restore & rejuvenate my own mental functions at this time on my own... I quit smoking "cold turkey" a month ago after a 40 year habit so i'm somewhat still a frazzled wreck sometimes its ok ... have done Huxley's program of Bates Eye Exercises often since college...
Would like to share my experience & concur & compliment your good research about Blinks & Dopamine which notices conditions the # of natural blinks could indicate, without postulating whether the # of blinks (increased or decreased) done consciously as an exercise could possibly affect these conditions in therapeutic ways.
I am here to tell you - Yes It Can! That's what blinking's been doing for me... blinking seems to be restoring my nicotine-exhausted myelin, dopamine transmitters etc.-
better than if I was not consciously blinking rapidly when I feel the need.
In my case - 1) My eyes have been tearing quite often this last month since stopping smoking which is a sign of healing and detox
2) Have come to realize recently = rapid blinking feels very good when experiencing "the blanks, etc." discomfort from nicotine withdrawal... I can restore a measure of energy & coherence to myself by doing so.
The Question Is:
Is this all in my mind - the power of positive thinking - or would this work for a majority ??? hmm?
My eyes tearing so much recently brought Huxley/Bates to mind, and then doing the exercise on my own the improvement in my mood, cognitive, and visual condition from rapid blinking was immediate. Was searching for nicotine/dopamine/etc. when your blog caught my interest, so I thought to share my own findings with you. Thank You, Again.
yepindeedy, some good stuff.

1/04/2007 12:36:00 PM  
Blogger Mark said...

have you considered the blink-like movements of REM sleep and dopamine levels?

on another topic, by concentrating for about 5-10 seconds i am able to give myself a rush of what i only know to be the broad field of "endorphins". 2 hits in the morning and i am unable virtually to repress my grin for about 30 mins. im not sure when it wears off but i am a very happy person thanks to doing whatever it is.

each "hit" takes time to recharge in between and feels borderline-sexual, a real rushing feeling of happy goodness...

apoligies for the poor wording but i have only just started looking into this stuff recently.

2/10/2008 07:39:00 AM  

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