It sounds like someone accidentally hit adjacent keys on ????? ?????? ??????? ????????a xylophone. The understated double ping hits me with a jolt of excitement, a swooping stomach, and even a bit of relief.

Whenever I hear the now-retired Facebook Messenger notification, I'm transported back to 2013, when I happily, gratefully, giddily got a message from someone I liked, who would later become my partner. Back then, we talked almost daily on, of all chat platforms, Messenger.

As devices, software applications, and apps become omnipresent, the User Interface (UI) sounds they emit — the pings, bings, and blongs vying for our attention — have also started to contribute to the sonic fabric of our lives. And just as a song has the power to take you back to a particular moment in time, the sounds emitted by our connected devices can trigger memories, thoughts, and feelings, too.

"The sounds that we have are adding to that tapestry," Will Littlejohn, Facebook's sound design director, said.

If you've had a particularly stressful job with a trigger-happy boss, perhaps you feel a churn of anxiety when a notification tells you you've received an email. Or if you grew up a child of AOL, maybe an intense, vivid memory of using AIM as a tween occurs if someone plays you the iconic doors opening and closing sounds. When distinct and repeated sensory stimuli, like UI sounds, are paired with feelings, moods, and memories, our brains build bridges between the two.

"Who we are is not just the neurons we have," Santiago Jaramillo, a University of Oregon neuroscientist who studies sound and the brain, said, referring to cells that transmit information. "It's how they are connected."

My partner and I began our casual courtship in 2013. For the next year, as we flirted, chatted, and became increasingly part of each other's lives, our preferred mode of communication was ... Facebook Messenger.

Facebook was already somewhat uncool by then — the days of painstaking album uploads had faded — but as young 20-somethings looking to chat during the day about nothing much, it worked for us. Somehow, texting felt too formal. But we weren't on the level of chatting daily over Gchat, like we both did with our friends. We were friends on Facebook, and Messenger was a way we could stay in constant communication, without the commitment or overt familiarity of other platforms.

In the intervening years, I didn't use Facebook Messenger much. But when I did, and when I received a sound notification when I wasn't expecting it, I noticed that the sound would immediately make me think of my partner. I would even get a sweep of relief and excited nervousness, like the person I was interested in had just sent me a message to say "hi," confirming that they liked me with a DM, all over again.

When I explained what happened with the Facebook Messenger sound, Jaramillo responded with a laugh: "You have been conditioned."

Pathways to the past

For the last 30 years, scientists have been using animals, like mice, to learn how sound becomes associated with a memory, thought, feeling, or state of being. They've discovered that your brain creates pathways connecting the parts that process sound with the parts linked to emotions and memories.

When your brain registers some sort of stimulus, like a sound, you can process it in a variety of ways. You might have an innate response, such as jumping when you hear a loud sound. You might also glean information from the sound: For example, the sound of an idling engine tells you someone is waiting outside.

In the most basic experiments that illustrate this, researchers shock a mouse every time it hears a particular sound. After a certain amount of time, just hearing the sound — without the shock — causes the mouse to jump as if it had just been shocked. What I was experiencing when I felt my own jolt of excitement at the Facebook Messenger sound was a more complex version of this same phenomenon, Jaramillo explained.

"It is through these changes and connections in the brain that you associate these sounds with these responses," Jaramillo said.

In the brain, a sound is never just the raw data of a sound wave: there's always something more to it. According to a study Jaramillo published in January, we associate sounds with memories at the first pitstop sound makes in our brains: the auditory cortex. As it gets digested in more complex regions of the brain, those associations only grow stronger.

This can have a domino effect throughout the brain, prompting powerful feelings.

"It's almost like a multiple step process," Jaramillo said. "Once you bring an association, that brings with it many other associations." 

Conditioning

I wondered why the Facebook Messenger sound prompted this reaction in me, while other sounds — like the Gchat notification sound — had no particular emotional effect. The Gchat sound is still coded with information (it's telling me I've got a chat!) — but that information isn't powerfully associated with a memory or feeling.

It turns out my partner and I had inadvertently created the perfect conditions for creating a strong neural pathway.

"To be effective for creating associations, a sound has to be clearly differentiable," Jaramillo said. "Then, if you have consistency and repetition, a strong association will be created."

The Facebook Messenger sound hit all of these criteria. It was a unique sound, that was consistently associated with a specific experience, repeated many times. Because I only ever really talked to my partner (and not other people) on Facebook Messenger, I associated the sound with him; because we talked a lot, the association became strong; because we repeated the experience almost daily for about a year, it became engrained — so engrained that years after the fact, an unexpected encounter with the sound rendered the emotional memory as if it were happening all over again.

"There are experiments where you don't present a sound for a long, long time," Jaramillo said. "But if you present it years later, you may still recall the memories. Some of the neurons keep those memories. Some of those seem to be very powerful in how long they last, and researchers are still trying to understand what are the mechanisms that allow you to have such a long, long memory."

Designing for life

Before notifications became a constant part of our lives, sound designers did not take as much care in their creation. Think of the grating early Nokia cellphone ring, or how annoying the AIM door shutting and opening could become if a friend got signed on or offline every time their computer went to sleep or woke up.

Today, sound designers are wiser, and more considerate, about how the sounds they design can be both useful, and — the holy grail of sound design — unnoticeable. A lot of that mission has to do with thinking through what emotions the sounds themselves might evoke.

"The best sound designers are not going to talk about the tools or the tech, they’re going to try to pull emotions out of people," Dallas Taylor, a sound designer who runs a popular podcast about sound, called Twenty Thousand Hertz, said.

These emotional considerations are something that sound designers consider at the highest levels.

When Littlejohn, Facebook's head of sound design, and his team design the sounds that populate Facebook, they try to create a sonic identity for the platform, while also presenting a neutral canvas.

"From the very beginning when we’re crafting sounds, we're making sure that the sounds are designed in such a way that they will not create negative emotions over time," Littlejohn said. "We're not trying to create sounds that are creating positive associations overtly, what we're trying to do is create sounds that have the potential to create great associations, if that's the context in which they're heard."

In other words, the sounds themselves don't create the emotions – the associations do. But the often repeated nature of UI sounds, and the social context in which they're used, makes them ripe for emotional connections.

"The sound itself can’t force a feeling," Taylor said. "It has to be the context that that sound is in."

Additionally, UI sounds themselves may be new — and specifically primed for association – but the phenomenon is just an extension of how our brains already process sound, whether created by the wind and the trees, or a buzz in our pocket.

"These cues are what help to bring context to what we’re experiencing with our other senses," Littlejohn said. "That’s how we interact with the world. Whether it’s being created through a device, and we design what’s emitted, or whether it comes from nature or something mechanical, I think that relationship has always been there. It’s now manifested in a new way through technology."

A leaking time capsule

Buried in an episode on UI sound design on Taylor's podcast was the Facebook notification noise that soundtracked the first six to 12 months of my relationship. Unlike the new, high-pitched, cheery "pop ding" notification, this one, which was used by Facebook prior to 2014, is more musical, yet muted. When I heard it, I knew instantly that this was the true store of my emotional memories about those early flirtations.

Beginning in 2013, I heard the clumsy xylophone — while for latter day chats, the pop ding. And I have a deeper connection to the first one. Unbeknownst to me, the memory had been mummified in my brain, ready to be re-awoken by the podcast.

"The memory is kind of there in the brain, latent and hidden," Jaramillo said. "If it gets associated again with the particular event, then it would reappear."

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Of course, the flip side of my sonic revelation is that the memories and emotions associated with the post-2014 sound are becoming diluted. I recently started using Messenger more to communicate with a group of colleagues. The butterflies in my stomach don't flutter as hard when I hear the pop ding these days. But they still do when I hear the vintage notes.

"When you hear the same sound, but you don't get exposed to the same thing, then the association can fade," Jaramillo said.

Thanks to a technological coincidence, I have a nostalgic jewel contained within a sound I am unlikely to hear, unless I seek it out. That's especially powerful for me today, six years later. But I can't go back to it all the time, or the association will become weaker.

But I don't need to, anyway; as my partner and I create new sonic associations I may discover in another six, 16, or 60 years that those first six months are still encased in neural amber. And that's enough to make my stomach flip whenever I choose to think about it.