I didn’t know this before reading an article about how our eyes and brain estimate motion, but we’re better at catching something that’s dropping towards us than something on an upward trajectory.
One argument goes that this reflects our familiarity with the effects of gravity. But a new paper by Takashi Hirata and colleagues in Japan has suggested a subtly different mechanism. Because we spend so much of our lives standing (or even sitting) upright, when things are descending they’re moving down our body’s long axis, consistent with gravity.
I say subtle, because both explanations rely on our familiarity with gravity – things fall down – and in the second explanation, it’s not the actual direction of gravity that’s important but the axis of our bodies.
We don’t just use our eyes to work out up from down. Our vestibular system, a metaphorical spirit level nestled in our inner ear, tells us where our bodies are in space. If Hirata is right, then that vestibular information is less important than visual cues.
How do you test between these two? Simple, get people to anticipate where an object is going while they’re lying down. Sure enough, hypothesis two wins; we’re better at estimating when an object is going to stop if it’s falling relative to our bodies. In fact, Hirata and mates also found that our eye movements are smoother when we’re tracking from head to feet rather than feet to head.
Ultimately, it’s still to do with what we’ve learned about gravity.
I find this interesting, but I’m not going to assume everyone will. It’s a slightly quirky finding, telling us more about how we work, but doesn’t have a lot of obvious day-to-day implications. Unless you find yourself in a zero-gravity situation, that is.
Another hot-off-the-press study using a visual task has some pretty big implications, but doesn’t tell us as much about how our “normal” brains work. Rather, Ahmet Begde and a group of scholars at Loughborough and Cambridge universities in the UK, have shown that being slower to detect changes in visual information may be an early warning sign of dementia.
Imagine you’re looking at a screen full of moving dots. Your job is to press a button as soon as you see a triangle forming somewhere on the screen. That’s the reaction time we’re talking about – how quickly you notice the triangle.
Begde’s research used the Epic-Norfolk prospective population-based cohort study. In this context, “prospective” means we’re trying to find things about people that predict things such as dementia before they happen. This is a powerful research design, particularly when we can’t conduct an experiment to see what causes what. What ethics committee will let you try to cause dementia?
In this particular slice of the study, more than 8000 participants completed the visual sensitivity test, and a bunch of other tests, between 2004 and 2011. By 2019, 537 of the original participants had received a diagnosis of dementia, and those people had shown poorer visual sensitivity 8-15 years earlier.
Why? We’ve known for a while that Alzheimer’s, probably the most easily-named form of dementia, affects vision, meaning patients don’t process visual information (say, faces) efficiently, and it’s harder to tell the difference between some colours or to detect the outline of objects.
This may be because the brain parts associated with vision are not only damaged by the build-up of toxic amyloid proteins, but also, as Begde’s research hints, may be affected earlier than other brain bits.
Interestingly, routinely doing things that involve moving your eyes is associated with better memory and lower risk of dementia. That’s right, I’m giving you licence to read books and watch TV.
