I’m signed up to receive internet alerts on various topics related to visual disorders. One is homonymous hemianopia.
For anyone who doesn’t know, homonymous hemianopia – HH – means blindness on one side, the result of damage to the visual cortex at the back of the brain. The commonest cause in older people is a stroke.
One of yesterday’s alerts was about HH and driving, a topic I’ve written about several time before. If you acquire HH then in most countries you are not allowed to drive. In some jurisdictions however it’s possible to regain one’s licence but one of the requirements is to demonstrate that you have learned to compensate for the visual field loss.
A new study, presented by a group of researchers well known in this area, examined how well HH drivers compensated. The resaerchers used a driving simulator to track how effectively subjects used their head and eyes in typical driving situations. One aspect measured was their ability to detect pedestrians at intersections.
The experiment involved 14 people with HH and a similar number with normal vision. The results are intriguing because they are a mixture of predictable and unpredictable.
In the UK traffic drives on the left. A common road-safety motto is: “Look right, look left, look right again”. Most countries in the world drive on the right however, so the maxim would be reversed: “Look left, look right, look left again”.
In the study, which was conducted in the US, the normally sighted subjects did just that, as expected. But subjects with right HH – loss of their right visual field –looked to the right first instead. Presumably they were compensating by checking their blind side first.
This is what we might expect. But the next finding isn’t. When HH suffers turned their head to the blind side they generally didn’t rotate it as far as those with normal vision did.
That’s correct. HH subjects had learned to scan to their blind side first but in spite of it being more important for them than for normally sighted subjects they usually did it less well.
The press release on the study has been bouncing around the internet and I’ve actually received the alert a number of times. But none of the sites I’ve read so far commented on this rather surprising aspect. Why should HH participants scan more narrowly and what does it tell us? According to the study the HH group did not suffer from “neglect” – the neurological condition in which a sufferer is inattentive to part of the field of observation. There must be standard test for neglect and neglect was not found amongst these participants.
Yet but… I would argue that all the same there was a form of neglect. The subjects were somehow less aware of their bind side. We none of us constantly turn around because of the blind spot called “the back of our heads”. If you have HH then that region of invisibility is extended but you don’t think about it. It’s migrated round the back of your head.
What can HH would-be drivers learn from this? It might seem simple: just make a conscious effort to scan further to the blind side. But perhaps they can’t just do that: the neglect may have been learned and what is out of sight then comes to be out of mind as well. But if I am right then it should be possible to unlearn it, although it would probably need some kind of systematic therapy.