This is my first clinical paper. Shortly after I arrived in Newcastle, Anya Hurlbert, our Institute director, put me in touch with Mike Clarke, a consultant ophthalmologist specialising in disorders of binocular vision. She figured that we’d have plenty to talk about and she was (as usual) quite right. Mike introduced me to a condition called intermittent exotropia. It’s a form of squint where the eye deviates outwards only occasionally, mainly when the person is tired and/or looking at faraway objects. What intrigued me about it was to learn that people with this condition generally retain their stereo 3D vision, the “gold standard” of binocular function, indicating that both their eyes can see well individually and can work together. And yet when their eyes are pointing in different directions, they don’t generally report any double vision, implying that input from one of the eyes has been temporarily turned off. Ignacio and I were intrigued that the brain can learn to turn the eyes’ input on and off in this way. We wondered what triggered the switch. Was it the eye movement itself, or was it the resulting change in retinal input – the fact that the two eyes’ images no longer matched up? Ignacio came up with a cool experiment to answer that question, and we spent a lot of time fine-tuning it to make it fun and enjoyable for child participants. For example, instead of dots or lines, we used child-friendly faces, such as Igglepiggle shown in the icon.
The experiment worked really well and we got a nice clear answer to our question. It turns out that eye movements are not required; it’s the retinal input which triggers the switch. If you grow up with intermittent exotropia, then your brain learns to look out for big offsets between the retinal images. That probably means that one of your eyes has turned outwards, and to avoid being troubled by double vision, your brain apparently switches to monocular mode.