Katy Perry expects all her songs to pass the "goose bump test".

The pop star is currently promoting her new single Unconditionally, from her latest album Prism. Katy has high expectations when it comes to choosing tracks for her records.

"There's this test that I do in the studio, after a song's complete and you've tracked some vocals. It's kind of like the goose bump test. If, every time you hear it, it gives you goose bumps, it's hitting a chord inside of you that's really important," she explained after premiering the video for Unconditionally on MTV. "It's an emotional chord inside of you. I love this song because not only can it be a romantic, intimate song, but it can be about the type of love that a mother has when she sees her first child, or best friends or partners ... It's a transcendent love, it's not just relationship status."

Katy felt inspired to sit down and write Unconditionally shortly after travelling to Madagascar with UNICEF. The 29-year-old star decided to pen lyrics about what it means to care for someone in the purest form.

"We live in such a world that is run by social status and material possession, and it can get really overwhelming and tiring to keep up with that - people are actually turned on by the amount of likes they have; it's crazy that our minds think that way these days," she explained.

"And so I went to the top of this mountain, I saw all these children, and the love that they had towards each other, they had no idea what Twitter or Facebook or any of that stuff was, they didn't care about it. It was this unconditional love between all of them. It was that kind of pure love that's unaffected by the trends of the world."

Katy was so affected by how the track turned out, she singles it out as her favourite on the album.