Hearing the sound of silence. In 2008 at Caltech, we discovered that about 1 in 100 people hear sounds when they see images flash or move, even when no physical sound is present. The sound is all their heads, but it’s not imagination. This experience is due to a cross-activation of sensory centers in the brain – a benign neurological trait called synaesthesia. People with “hearing-motion” synesthesia describe their sound perceptions with words like whirring, buzzing, wobbling, tapping, or beeping. It happens automatically, doesn’t require effort or attention, and is typically experienced since childhood – as far back as can be remembered. Someone with hearing-motion synesthesia may not even realize that he or she hears sounds that other people do not. Test yourself for hearing-motion synesthesia by viewing the video on the right.
How can we verify this? Published experiments from our lab provide objective evidence for hearing-motion synesthesia. We devised a task in which people with hearing-motion synesthesia would performed better than others if their sound perceptions were real. Normally, people find it easier to judge rhythmic temporal patterns like Morse code with sound (beeps) compared to vision (flashes). So, we asked subjects to judge whether pairs of Morse-like patterns of visual flashes were identical or not (in actuality 50% were identical, and 50% subtly different). For subjects without synesthesia this task was very hard, but people with synesthesia found it much easier. They reported that they heard a sound each time they saw a flash – transforming the difficult visual task into an easier auditory task. This synesthetic translation of vision to sound kept up with a rapid pace of visual flashes that were spaced as little as 150 ms apart.
Saenz and Koch (2008) The Sound of Change: Visually-induced auditory synesthesia. Current Biology. 18:650-651
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