By Helen Thomson | NewScientist
"I told my daughter her living room TV was out of sync. Then I noticed the kitchen telly was also dubbed badly. Suddenly I noticed that her voice was out of sync too. It wasn’t the TV, it was me."
Ever watched an old movie, only for the sound to go out of sync with the action? Now imagine every voice you hear sounds similarly off-kilter – even your own. That’s the world PH lives in. Soon after surgery for a heart problem, he began to notice that something wasn’t quite right.
"I was staying with my daughter and they like to have the television on in their house. I turned to my daughter and said ’you ought to get a decent telly, one where the sound and programme are synchronised’. I gave a little chuckle. But they said ’there’s nothing wrong with the TV’."
Puzzled, he went to the kitchen to make a cup of tea. "They’ve got another telly up on the wall and it was the same. I went into the lounge and I said to her ’hey you’ve got two TVs that need sorting!’."
That was when he started to notice that his daughter’s speech was out of time with her lip movements too. "It wasn’t the TV, it was me. It was happening in real life."
PH is the first confirmed case of someone who hears people speak before registering the movement of their lips. His situation is giving unique insights into how our brains unify what we hear and see.
It’s unclear why PH’s problem started when it did – but it may have had something to do with having acute pericarditis, inflammation of the sac around the heart, or the surgery he had to treat it.
Brain scans after the timing problems appeared showed two lesions in areas thought to play a role in hearing, timing and movement. "Where these came from is anyone’s guess," says PH. "They may have been there all my life or as a result of being in intensive care."
Disconcerting delay
Several weeks later, PH realised that it wasn’t just other people who were out of sync: when he spoke, he registered his words before he felt his jaw make the movement. "It felt like a significant delay, it sort of snuck up on me. It was very disconcerting. At the time I didn’t know whether the delay was going to get bigger, but it seems to have stuck at about a quarter of a second."
Light and sound travel at different speeds, so when someone speaks, visual and auditory inputs arrive at our eyes and ears at different times. The signals are then processed at different rates in the brain. Despite this, we normally perceive the events as happening simultaneously – but how the brain achieves this is unclear.
To investigate PH’s situation, Elliot Freeman at City University London and colleagues performed a temporal order judgement test. PH was shown clips of people talking and was asked whether the voice came before or after the lip movements. Sure enough, he said it came before, and to perceive them as synchronous the team had to play the voice about 200 milliseconds later than the lip movements.
The team then carried out a second, more objective test based on the McGurk illusion. This involves listening to one syllable while watching someone mouth another; the combination makes you perceive a third syllable.
Since PH hears people speaking before he sees their lips move, the team expected the illusion to work when they delayed the voice. So they were surprised to get the opposite result: presenting the voice 200 ms earlier than the lip movements triggered the illusion, suggesting that his brain was processing the sight before the sound in this particular task.
And it wasn’t only PH who gave these results. When 37 others were tested on both tasks, many showed a similar pattern, though none of the mismatches were noticeable in everyday life.
Many clocks
Freeman says this implies that the same event in the outside world is perceived by different parts of your brain as happening at different times. This suggests that, rather than one unified "now", there are many clocks in the brain – two of which showed up in the tasks – and that all the clocks measure their individual "nows" relative to their average.
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Read the full article at: newscientist.com
