Sparks used to be a property developer. 'I had a good business and a good life,' he said. 'But one night, I was pulled from my bed to a [space]craft.'

Extraterrestrials had abducted him hundreds of times since then, he said. They forced him to learn their languages and occasionally asked him to do 'embarrassing' things, such as give them samples of body fluids.

Some in the hall left midway through his talk, perhaps finding his claims hollow. But many more listened until the end and queued up for the 56-year-old market researcher's book and autograph.

Sparks was invited to speak in Hong Kong by Neil Gould, who says he, too, has been abducted by aliens. Gould heads the local branch of the Hawaii-based Exopolitics Institute, which studies extraterrestrial activities and has more than 70 members worldwide.

Gould, 56, is an electronics-parts trader who lives with his family on a luxurious boat moored off Aberdeen. He claims he has been in contact with aliens for 45 years.

'The first time was when I was nine,' he said. 'But it wasn't until a few years later that I had the courage to grab one, and accompanied them to places.'

His most recent experience was eight months ago, on his boat. 'A lady came into my room, looking completely human but was extraterrestrial. Blond hair, petite and her eyes were completely black. We had some dialogue. Afterwards, they gave my son electric shocks and when he got up he had a white triangle mark on his arm.

'I was very depressed after that,' he said. 'I like to meet them but I'd like it to be more two-way because the way I meet them is not always pleasant. Now it's all whenever they want.'

He said an experience in 2000 was a turning point: 'I was with 20 Chinese unloading cargo on Kowloon side, and four crafts about a kilometre wide flew over the IFC. They came over and circled the beam [from the top of the IFC tower], going round ... they literally vanished in front of our eyes after about seven seconds.'

He said he has had 'an incredible acceleration of mental abilities' since that time. He now understands complex physics without ever learning it. 'Way, way beyond normal human. Way, way, way beyond,' he said.

His family thought there was something wrong with him. Then, in 2007, while the Goulds were on a balcony in their then-Pok Fu Lam home, 'a huge craft flew in front of us and they all saw it. They were saying, 'My God. This is what he's been on about.' So now they know'.

Mental health professionals say people who claim to have interacted with aliens have a vivid imagination. They create false memories or experience sleep issues such as lucid dreams.

'People experience or see what they believe and what the cultures they are brought up in have them [see],' said Professor Lee Sing, a specialist in social psychiatry at Chinese University. That was why some abductees' accounts resembled science fiction stories, he said.

'To an extent, you may say aliens can be interpreted as one modern-day form of ghosts,' Lee said. In both cases, people are haunted by strange life forms.

Most scientists, on the other hand, do not even give these 'experiencers' the benefit of the doubt. Leo Blitz, an astronomy professor at the University of California, Berkeley, says extraterrestrial life almost certainly exists. But such alien abductions 'cannot be true'.

'It simply takes too much energy and too much time to get from another star to within the solar system,' he said. 'And after a journey of perhaps 10,000 years, why would they not go to the centre of government rather than picking some ordinary people off the earth? Why aren't there any unambiguous artefacts from an alien civilisation?'

Gould says aliens do not have to travel a long way to reach the earth because they have incredible technology. 'They take shortcuts through space and time,' he said. Unfortunately for those seeking evidence, 'they're not going to let you take photographs'. He believes crop circles are left behind by aliens.

Almost all claims of UFO sightings can be explained, said Alex Filippenko, astronomy professor at the University of California, Berkeley. The few that were not easily explained might still have a conventional explanation that we had not yet figured out, he added.

'The point is that, as [famed astronomer] Carl Sagan said, 'Extraordinary claims require extraordinary evidence',' he said.

All stories are edited versions of articles that appeared in the Sunday Morning Post on January 23, 2011