Following a stroke, Chris Birch's personality and sexuality altered dramatically. Now he is trying to rediscover who he is and why these changes may have happened.

"It's like looking at somebody else, but with my face only younger, and in all fairness, if I met myself I'd probably carry on walking."

Looking at past pictures of himself, 27-year-old Chris Birch struggles to remember or identify with his old self. He used to be a 19-stone, beer-swilling, party-loving rugby fan from the Welsh valleys, the life and soul of a party. He worked in a bank and loved sport and motorbikes.

After a freak accident in 2011, he says he underwent a big change to his personality. He believes that he has gone from being straight to gay.

"I was doing a forward roll down a grass bank one day and cut off the blood supply to my brain which caused a stroke to happen. It was from there, while I was recovering, that I realised I'd changed," says Birch, from Caerphilly.

"The Chris I knew had gone and a new Chris sort of came along. I came to the realisation that the stroke had turned me gay."

A stroke occurs when the blood, and therefore, oxygen supply to the brain is disrupted. Without oxygen, any part of the brain can be destroyed as brain cells die, leaving the brain to make new connections, which can affect how a person thinks, moves or feels.

Image caption Chris Birch does not recognise himself in photographs that were taken before the accident

Stroke patients have a 40% chance of suffering another stroke and Birch takes medication to prevent any such reoccurrence. He still has regular brain scans and cannot remember much of his life before the accident. He has also noticed physical changes to his body, for instance when he is tired, his left eye droops.

When Birch's story hit the headlines last year it sparked a media frenzy and the story went viral. However, some - including media organisations and those close to Birch - questioned whether a stroke could alter a person's sexual orientation.

There are few known cases of a stroke turning a straight person gay, and major personality changes in stroke sufferers are rare. Even Jak Powell, Birch's fiance, believes his partner may always have been gay.

"I've still got the same opinion that it was just something that was always there," says Powell.

"People grow up not knowing they are gay and have families and then they realise they are gay, but they don't have a stroke to realise that."

Find out more I Woke up Gay will be broadcast on Tuesday 17 April at 21:00 BST on BBC Three Or catch up on iPlayer

Yet Birch disagrees and is convinced that, neurologically, it was the stroke that altered his sense of self. The moment he realised his feelings towards men had changed was a scary period in his life.

"It was a sort of lonely time. It was a time I was afraid to tell anybody because that wasn't who I used to be, so it shouldn't be who I am now," he says.

"You're afraid to tell people, you're afraid to have that conversation or even talk about the possibility that I have even changed in some way, and I suppose I dealt with it by moving out of my family home by myself and having to realise who I was all over again."

A change in sexual orientation in a stroke sufferer is a controversial issue that can divide scientific opinion.

Dr Qazi Rahman of Queen Mary, University of London, an expert in human sexual orientation, has researched the neurological differences between gay and straight men and women.

He has tested hundreds of lesbian, gay and straight volunteers and discovered certain key patterns which reveal if a person might have been born gay or straight.

Stroke statistics Every five minutes someone in the UK has a stroke

Around a third of those who have suffered a stroke will recover

A third of stroke sufferers are left disabled or need further rehabilitation

And a third of stroke patients are likely to die within the first 10 days

Medical help should be sought as soon as a stroke is suspected to increase the chance of recovery BBC Health: Learn more about strokes

Rahman says the brains of gay men could be organised differently to those of straight men.

He invited Birch, who has swapped banking for hairdressing, to undergo the computer-based tests to see if he may, indeed, have been born gay. On half of the tests, Birch performed in the "expected direction" for a gay man, and for the other half was within the range of a straight man.

"The bulk of the evidence in the biological sciences of genetics and psychology and neuroscience suggest that sexuality is something you are born with and it develops later on through life," says Rahman.

"Sometimes it takes something like a neurological insult - which is what a stroke is - to make you reassess those feelings, perhaps that are lying dormant, and bring them into the front of your mind and it is possible that is what has happened with [Birch]."

Yet consultant neuro-psychiatrist Dr Sudad Jawad has worked with young people who have had strokes and has come across a similar case in his practice of a man whose sexuality changed from homosexual to heterosexual.

"Just like a stroke can change you as a person, your behaviour, your personality, the way you think, why not sexual orientation, it is part of the personality of the individual," says Jawad.

Birch's case brings to mind other examples of those whose personality has radically altered after a change in their medical history.

Tommy McHugh suffered a stroke in 2001 which unlocked his creative side. He used to be a builder and is now an artist, sculptor and writes poetry.

However before the stroke, McHugh had no interest in art apart from the tattoos on his arms.

I'm happier now than I ever have been, why would I want to change Chris Birch

In 2008, Cheryl Johnson claimed her personality and taste in literature had changed after a kidney transplant. She swapped popular novels for high-brow books by Dostoevsky.

Debbie McCann , a grandmother from Glasgow, suffered a stroke in 2011 and began speaking with an Italian accent, although she had never been to Italy.

One of the first recorded instances of a personality change after a head injury dates back to 1848, with the case of Phineas Gage . While working as a railroad construction foreman, his head was pierced by an iron bar propelled by an explosion. He survived the accident but suffered behavioural changes and was reported to have permanently lost his inhibitions.

Although science may never be able to reveal what happened after Birch's accident and the lasting effect of the stroke, he is continuing to rediscover himself and move on with his new life.

He has put away previous pictures of himself - and with them the "old Chris".

"I'm convinced more than ever looking at these photos that the stroke did turn me gay, because there is no way that I was gay before. I have photos as proof and I have friends as proof and now I have memories as proof."

"I'm happier now than I ever have been, why would I want to change?"