A lot happens in the blink of an eye!

Do you have recurrent nightmares? I do, especially when I'm under a lot of stress. I often dream of missing a train. The setting, location, company and place I need to reach change every time, but the common factor is always the frightening sense of having missed the train and not being able to make it. Another recurrent nightmare I have is that the light is so bright I can't keep my eyes open. So I start blinking faster and faster but I can't see a thing and end up missing something very important.

Blinking seems to be such an important task that our body takes care of constantly, as important and essential as breathing. And yet we hardly ever think about it. Under normal conditions, we blink spontaneously 10 to 15 times a minute. We blink both eyes at the same time, which may seem obvious but (maybe you already knew this, I didn't!) it's unique to mammals. Birds, for example, blink one eye at the time and this prevents loss of visual information. In fact, blinking causes a momentary loss of vision that lasts lasts 100 to 150 milliseconds -- a mini black-out that happens constantly as we stare at things and yet we hardly ever notice it, in ourselves, or in others.

Another thing our eyes smoothly cope with is saccades, quick, simultaneous movements of both eyes. When I rapidly move my camera and press the shutter, not matter how fast the shutter speed, I get a blur. However, our eyes move all the time and yet what we perceive is a constant, flowing image.

We compensate these visual disruptions with two similar mechanisms: blinking suppression and saccadic suppression -- basically, visual sensitivity is suppressed immediately prior to and during both blinking and saccades. The two mechanisms are often coordinated in order to minimize downtime in visual processing. Unfortunately,

They are challenging to study because any brain-activity changes resulting from an extra-retinal signal associated with the blink motor command are potentially masked by profound neural-activity changes caused by the retinal-illumination reduction that results from occlusion of the pupil by the eyelid [1].

In order to measure the neural consequences of blinks on visual function one has to bypass the physical consequences of eyelid closure. In other words, you want to stimulate the retinae maintaining the eyes open. How? Via the mouth, as Volkmann et al. [2] showed in 1980: you insert a light probe in the mouth, the light passes through the palate and stimulates the retinae without forcing eyelid closure.

In [1], Bristow et al. use the same technique to stimulate both retinae while measuring brain activity through fMRI. They also used opaque goggles to ensure that retinal illumination remained constant throughout the experiment, whether the eyelids were open or closed. By doing this, they could see what parts of the brain were responding to the retinal stimulation independently from the change in illumination caused by eyelid closure. They find that

Whereas it might have been supposed that blink suppression is a purely low-level visual phenomenon, mediated solely by retinotopic visual areas, our whole-brain analysis surprisingly revealed that activity evoked by retinal stimulation in parietal and frontal cortices was also suppressed by blinking.

Thus, one possible interpretation of our findings is that the observed suppression of these parietal and prefrontal regions during blinking represents a neural mechanism underlying the lack of awareness of the changes in visual input that normally occur during a blink. Specifically, it may account for the lack of awareness of the percept of the eyelid descending across the pupil and the resulting reduction in retinal illumination.

Their experiment also proves the deep connection between saccade suppression and blinking suppression, as they conclude:

In summary, our data demonstrate that responses to retinal illumination are suppressed by blinking in retino-topic visual area V3 and in parietal and prefrontal cortices, whereas in the absence of retinal stimulation, we identified a positive blink-related signal in early visual areas LGN–V3. We propose that these findings represent a neural signature of blinking associated with the blink motor command and may go some way toward explaining both the neural mechanisms underlying the visual-sensitivity loss, known as blink suppression, that occurs during blinks, and why they go unnoticed. Our findings parallel recent observations of saccade-related changes in activity in visual cortex during saccades, suggesting that blink suppression and saccadic suppression may indeed share common neural mechanisms.

Finally, I'd like to mention a more recent paper by Bonfiglio et al. [3], which used EEGs to look at brain waves during blinking. Brain EEGs typically show oscillations that are classified based on their frequency. Spontaneous blinking modulates two oscillations in particular, alpha and delta, which are thought to be involved in the automatic mechanism of maintaining visual awareness. In their study, Bonfiglio et al. studied the alpha oscillations and postulated that

a) an early blink-related synchronization represents the short-term memory maintenance of the last visually perceived trace of the surroundings; b) the alpha blink-related desynchronization is associated with the comparison between the newly perceived image of the environment and its mnestic representation.

[1] Bristow, D., Haynes, J., Sylvester, R., Frith, C., & Rees, G. (2005). Blinking Suppresses the Neural Response to Unchanging Retinal Stimulation Current Biology, 15 (14), 1296-1300 DOI: 10.1016/j.cub.2005.06.025

[2] Volkmann FC, Riggs LA, & Moore RK (1980). Eyeblinks and visual suppression. Science (New York, N.Y.), 207 (4433), 900-2 PMID: 7355270

[3] Bonfiglio L, Sello S, Carboncini MC, Arrighi P, Andre P, & Rossi B (2011). Reciprocal dynamics of EEG alpha and delta oscillations during spontaneous blinking at rest: a survey on a default mode-based visuo-spatial awareness. International journal of psychophysiology : official journal of the International Organization of Psychophysiology, 80 (1), 44-53 PMID: 21238505

Photo: trees in downtown illuminated over the holidays. It was freezing cold, but the sky was so pretty and I just couldn't stop shooting. I could hardly feel my hands when I got home.