Neurologist Dr Guy Leschziner explores the extraordinary sensory experiences of individuals with synaesthesia - a mash-up of senses where one sense automatically triggers another. Some synaesthetes hear colours, others feel sound.

We meet James who perceives the world differently from most people, due to his brain’s unusual wiring. Whenever he hears a word he immediately gets a taste and texture in his mouth. As a child, he’d go by train to school with his mum, reading out loud the stations they passed through. His favourite was Tottenham Court Road because the word sounds taste of sausage, crispy fried egg and toast.Whilst James tastes words, 23 year-old synaesthete Valeria sees colours and feels textures when she hears music. She assumes everyone has that sensory experience until, at aged 14, she sees her dad’s astonished reaction! For Valeria, some music is so utterly exquisite it causes her intense, physical pain.

Such variations in perception can also affect our internal world as Sheri, a painter from Canada, illustrates. After a stroke in her twenties she can no longer picture images in her mind. The condition, aphantasia (meaning ‘without a mind’s eye’) is so devastating Sheri calls it “internal blindness”.

Our understanding of reality comes from how we perceive the world around us. But as we discover in this programme and throughout this series, each of us experiences a unique reality constructed by our brain and our sensory system. Leading us to question what is real and what is an illusion.

(Valeria in the garden.Image taken by her brother Simone Perboni).