By Emma Young
If you have a couple of minutes, click through to this survey site of “noisy gifs” – brief silent movies that, for some people at least, evoke illusory sounds. If you hear a thwack when fists collide with a punchbag, or a yell while watching a man silently scream, then you’re experiencing a “visual-evoked auditory response” (vEAR), also called “hearing motion synaesthesia”.
Ten years after the first, preliminary journal paper on the phenomenon, Christopher Fassnidge and Elliot Freeman at City University, London, report – in a new paper in Cortex – that it’s remarkably common, affecting perhaps 20 – 30 per cent of us. Fassnidge and Freeman also investigated what induces vEAR, providing clues to what’s going on in the brain.
More than 4,000 volunteers, plus 126 paid participants, viewed 24, 5-second-long silent videos of real world scenarios and also more abstract images, such as shifting patterns. Using a scale of 0 to 5, they indicated how much auditory sensation they experienced for each video. They also answered a series of other questions, including about past experience of vEAR and of any other synaesthesias (when a perception via one sensory modality triggers a sensory perception in another modality).
As the unpaid volunteers were recruited via adverts with text such as “Do you experience ‘hearing motion?’”, it’s certainly possible that there was a self-selection bias. But, on enrolment, the paid participants did not know what the study was about, so, in theory, they should be more representative of the general population. Thirty-one per cent of this paid group (an even higher percentage, in fact, than in the bigger, unpaid group) reported past experience of vEAR.
When it came to the survey results, anyone who rated half of the videos at greater than or equal to 3 was identified as experiencing vEAR. Just over 20 per cent of the paid participants fell into this category. Taken with the self reports of past experience of vEAR, the findings suggest that the phenomenon is far from rare.
The higher-rated videos often depicted relatively familiar events that are reliably associated with particular sounds (like fists hitting a punchbag), suggesting that an understanding of what’s happening in the scene was involved in causing the illusory sounds.
However, in people who reported prior experience of vEAR, even videos containing abstract flickering or moving patterns could trigger sound perceptions.
Fassnidge and Freeman found that these videos had high levels of raw “motion energy” (imagine a flickering neon shop sign, for instance, compared with a silent video of a person screaming, in which case there’s very little movement or “motion energy”).
This specific sensitivity to motion energy suggests, the researchers write, that in some people, at least, visual motion – rather than the meaning of a scene – affects auditory perceptions directly.
Given that participants with prior experience of vEAR were also more likely to report other synaesthesias, including seeing flashes in reaction to hearing sudden sounds in the dark, it’s possible the two phenomena are related. Synaesthesia has been associated with higher than normal cortical “excitability”, for instance, and the researchers said cortical excitability might also be involved in vEAR.
This is something that could now be investigated, the researchers write, before concluding: “Given the prevalence of visual-evoked auditory sensations and our new ability to quantify and correlate them, a potentially broad class of related subjective audiovisual phenomena have now become more accessible to scientific study.”