Sydney-born trombonist Michael Mulcahy joined the Chicago Symphony in 1989, and in 2000 was offered, by then music director Daniel Barenboim, the right to commission a trombone concerto by any composer in the world. Michael spent more than a decade weighing his options, and finally decided to offer me the incredible opportunity to create music for this magnificent trombonist, and for one of the great orchestras of the world.

In his book Hallucinations, the acclaimed neurologist Oliver Sacks chronicles a range of hallucinatory conditions reported by his patients throughout his illustrious career. I chose five cases as the conceptual backbone of this concerto, creating an imagined musical representation of each mental state.

Hallucinations are fascinating – instantaneous random inventions of our minds overlaid on the sensation of common reality and indistinguishable from it. Many of us will experience them in some way during our lives. When we sleep for example, we are aware that our brains are in free flight and their muddled dream scenarios are not real. On the edges of sleep however, we can confuse random mental impressions with reality, and are hallucinating.

Sufferers of neurological disorders or brain damage regularly hallucinate. Others without mental illness but under great stress...