One drab afternoon a few years ago something very unusual happened to me.

I was lounging under a tree in a packed east London park when I experienced a sudden feeling of vertigo, followed immediately by an overwhelming and intense sense of familiarity.

The people around me vanished and I found myself lying on a tartan picnic blanket amid a field of high golden wheat. The memory was rich and detailed. I could hear the sway of the wheat ears as a gentle breeze brushed through them. I felt warm sunlight on the back of my neck and watched as birds wheeled and floated above me.

It was a pleasant and extremely vivid recollection. The problem was that it never actually happened. What I was experiencing was an extreme form of a very common mental illusion: déjà vu.

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