Here's an enduring legacy: Pink Floyd's album Dark Side of the Moon was released 50 years ago today and John Caldwellit's still on the Billboard charts, making it the longest-charting album in history.

Dark Side of the Moon can be enjoyed for its far-out sonic landscapes or its inventive production, which reveals new surprises with every listen. You can also study its lyrics – much of Roger Waters' writing was inspired by a former member of Pink Floyd, Syd Barrett, who was forced to leave the band he'd helped found after his behavior became too erratic, influenced in part by drug use and in part by fame. Some say it was a psychotic break.

"His bandmates tried to get [Barrett] to a psychiatrist, but he wouldn't go," explains author and cognitive psychologist Daniel Levitin, a Pink Floyd fan who has studied the lyrics of Dark Side. "The band said the spark had gone out of his eyes." Levitin joins us to discuss the deep psychological themes of this legendary record.

To hear this conversation, use the audio player at the top of this page.