The composer for Playdead's creepy side-scrolling game Inside has revealed that the title's haunting soundtrack was actually recorded using an actual human skull. In a post on Gamasutra, composer and sound designer Martin Stig Andersen wrote that he wanted to create different, yet familiar sounds for the game "as if they were happening inside your head".

"People are often shocked when they hear themselves recorded, because things sound totally different inside your head," Anderson explains. "Things sound much softer in there, more full, in a way. This is because a large part of what you hear is your voice resonating inside your body, in your jawbone for example. Try blocking your ears while you speak or sing; that's the sound I'm talking about.

"So I had the basic idea of trying to recreate sounds as they would sound if they were happening inside your head. That was the curious thought that led me to acquire a human skull and experiment with it."

Working with co-composer SØS Gunver Ryberg. Anderson created songs striving for a 1980s synth vibe. However, he said at the same time he "didn't really want to hear synth music in the game" since it felt "too much like a statement" which he felt was not appropriate for a game like Inside.

Playing the synthesised sounds through a skull, however, gave the music "another quality", he explained.

"During the development we acquired an intuition for what kind of soundscapes resonated well within the skull," he wrote. " I did some post-processing as well, of course, but even without it you can hear this kind of timbre, which inspire a very nuanced association with a synth soundtrack... The end result, after the post-processing, is generally a bit creepy and cold. The sound has almost a chill about it."

Released on 29 June to wide critical and popular acclaim, Inside is an eerie puzzle-platformer set in a dystopian, nightmarish world. The game is a spiritual successor to the Danish developer's Limbo that was released in July 2010.

IBTimes UK's glowing review of Inside called the game "a worthy success to Limbo, that improves upon it in many ways".

"Playdead has proven that its debut game was by no means a fluke, establishing itself as a unique developer brave enough to go places other developers would never dare," our review reads.

However, Anderson noted that working with a human skull comes with a few challenges, including finding the right contact microphone to pick up the sound properly, the necessary amount of post-processing to produce a decent result and the fragility of the skull.

"Eventually all the teeth fell out of the skull because of the vibrations, but while they were still there they created this small vibrating sound that I think was unsettling but also strangely familiar to people," he wrote.

Despite the successful result, Anderson admitted that he probably won't work with a human skull again.

"Usually with specific elements like this, I prefer to move on to the next thing," he said. "The use of the skull helped define the sonic quality of Inside, and now I think it will remain on the shelf of my studio for the foreseeable future."

Inside is available now for Xbox One, PS4 and PC.