Every time we blink, a wave of activity sweeps through our brain – and this could be a serious problem for some fMRI researchers.
French neuroscientists Hupé et al report on A BOLD signature of eyeblinks in the visual cortex. They found that spontaneous blinks are associated with a neural activation pattern over the occipital cortex areas responsible for processing vision.
In many ways this is not surprising – when you blink, everything goes dark, and then lights up again, all within a fraction of second, which means that blinks are a kind of very dramatic visual stimulus, equivalent to a big black object suddenly appearing and then vanishing again. However, it’s long been believed that blink suppression mechanisms in the eye and brain somehow block out the responses that would otherwise happen during a blink.
Don’t be so sure, say Hupé et al. In an elegant experiment, they showed volunteers a standard set of visual stimuli during fMRI scanning, while recording blinks using an eye tracking camera. Then they simply treated the blinks as events, and used standard analysis methods to find neural activation associated with them.
Blinks caused a significant BOLD response over a number of “visual” areas.
Compared to the “real” visual stimuli in the task, the blink signal was less extensive, but no less strong.
So what? The great majority of fMRI experiments don’t use eyetracking to measure blinks, so this study raises the scary possibility that blinks could lie behind some of the “stimulus-related” activations that we all know and love. It would be a problem if subject blinks were correlated with the stimuli or tasks, which they might be, because blink rate may vary with our psychological state.
I don’t think we should be too worried yet. The blink blobs were essentially confined to parts of the visual cortex. So any study that’s not focussed on vision is probably in the clear (although that’s just the average response: in some individual subjects, the activations were a lot wider.)
However, as the authors point out, there is a risk that alterations in blink rate, caused, perhaps, by emotional or cognitive stress, might be wrongly “found” to be causing visual cortex activation, which might call into question claims of “top-down” influences on early visual cortex… oh dear.
Hupé, J., Bordier, C., and Dojat, M. (2012). A BOLD signature of eyeblinks in the visual cortex NeuroImage DOI: 10.1016/j.neuroimage.2012.03.001