Would you notice if a gorilla started grunting during Beethoven’s Fifth?

Imagine listening intently to your favourite piece of music – counting to the beat, say, or trying to play close attention to the lyrics – when suddenly a gorilla starts grunting on the track. You'd definitely notice that, right?

Don't be so sure. In a recent paper in Attention, Perception, & Psychophysics, when people were busy counting the beats or words in popular and classical pieces, they regularly failed to notice animal sounds that the researchers had snuck into the music.

The new paper has its roots in a classic 1999 study on "inattentional blindness", which – on the small chance you haven't already heard of it - you can try for yourself here before you read on.

In this study, participants counted how many times basketball players passed a ball between themselves. A full 46% of participants didn't notice that, part-way through the video, a person dressed as a gorilla walks on, pounds their chest, then walks off again. In short, the study showed that when our attentional resources are occupied – in this case by counting the basketball passes – we can easily become "blind" to other, bizarre things going on.

A similar phenomenon also occurs when listening to, rather than watching, a scene. In a 2012 study, participants heard a recording of two conversations occurring simultaneously, one between a pair of men and one between a pair of women. When concentrating on the women's conversation, most participants failed to hear a new male voice that appeared part-way through the recording, repeating the phrase "I'm a gorilla".

In the new paper, Sandra Utz and colleagues from the University of Bamberg wondered whether this "inattentional deafness" also occurs when we're listening to music. They asked 36 participants to listen to 20 snippets of music, each around 46 seconds long on average. These included both classical pieces like O Fortuna and pop and rock songs, such as Nothing Else Matters. Unbeknownst to participants, however, half of these pieces had brief animal sounds inserted into them: for instance, Ain't No Sunshine included a wolf howl, while Beethoven's 5th symphony contained a gorilla sound.

Participants listened to each piece while performing a counting task relating to an aspect of the music. For Nothing Else Matters, for instance, they had to count the number of bass drum beats, while for Ain't No Sunshine they had to count how many times the phrase "I know" appeared. After each piece of music, they reported this number, and also indicated whether they had noticed anything odd in the piece. They also stated whether they knew the music, and rated how difficult the counting task was and how much they had concentrated on the task. Finally, they listened to all the pieces again, without counting, and again reported whether they heard anything odd in each one.

The researchers found that when doing the counting task, participants failed to notice about 30% of the animal sounds. These rates differed for different people: some heard every single animal sound, while others missed more than 80%.

You might expect that once participants had noticed an animal sound for the first time, they would realise what was going on in the task and would catch all of the animals in subsequent songs. Yet this wasn't the case: a third of animal sounds missed by participants occurred after they had already noticed an animal in an earlier song.

The study shows that inattentional deafness can indeed occur during music processing, and even for stimuli which seem completely out of place (you don't normally hear lions or geese in the middle of a song, after all).

The idea of a gorilla grunting away, unnoticed, during Beethoven's 5th is at least mildly amusing. But why does any of this matter? Well, there are cases where inattention to auditory stimuli can have drastic consequences. The researchers point to the example of pilots in a flight simulator study, who failed to hear an alarm going off while they were busy dealing with another crisis. It's clearly useful to figure out the situations in which people may be unaware of important stimuli – and what can be done to counteract this inattention.

One important factor seems to be how much of our attentional resources we are using for the task: in the new paper, the team found that when participants made more errors in the counting task (and so were presumably concentrating on it less), they had a greater chance of noticing the animal sound. However, people were no better at spotting the animals when they said they were concentrating less or when they felt the task was easier, so further research is needed to better understand how these kinds of things can influence inattentional deafness. And given that some people spotted every single animal sound while others missed almost all of them, it would be interesting to know why some people are more predisposed to the phenomenon than others.